Symposium Organizers
Volker Schmidt Max Planck Institute of Microstructure Physics
LincolnJ. Lauhon Northwestern University
Takashi Fukui Hokkaido University
GeorgeT. Wang Sandia National Laboratories
Mikael Bjoerk IBM Research GmbH
Symposium Support
AIXTRON SE
FEI Company
IBM Research Zurich
JEOL USA
Veeco
EE1: Group IV Nanowires I
Session Chairs
Tuesday PM, April 26, 2011
Room 3001 (Moscone West)
9:00 AM - **EE1.1
Nucleation and Early-stage Vapor-liquid-solid Growth of Epitaxial Group IV Nanowires.
Paul McIntyre 1
1 , Stanford University, Stanford, California, United States
Show AbstractThis presentation will provide an overview of results both from our laboratory and from others on the nucleation and early-stage growth of metal-catalyzed Ge and Si nanowires (NWs). Given the importance of growth temperature for integration of nanowires in Si-compatible devices and systems, particular attention will be devoted to the topic of deep sub-eutectic vapor-liquid-solid (VLS) growth of Ge NWs. Attention will focus on capillary phenomena that cause non-uniformity of nanowire dimensions and defective growth (surface nanowire formation, early-stage kinking) during early-stage VLS growth in which epitaxy is used to control the orientation of NW arrays. The effects of substrate surface condition, catalyst diameter and catalyst phase (VSS vs. VLS) on defective nanowire growth will be discussed briefly.
9:30 AM - EE1.2
Growth of Ge Nanowires from Compositionally Controlled Au-Cu Alloy Nanoparticle Catalysts.
Justin Connell 1 , Zakaria Al Balushi 1 , Kwonnam Sohn 1 , Jiaxing Huang 1 , Lincoln Lauhon 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractCatalyst-mediated growth of semiconductor nanowires provides useful control over aspect ratio and doping, both of which are important for device applications. One drawback the vapor-liquid-solid process, however, is the “reservoir effect;” dopant junctions are not abrupt if a significant amount of the dopant is dissolved in the catalyst and continues to be incorporated into the nanowire even after the dopant precursor flow is stopped. The solubility of dopants should be lower in solid than liquid catalysts, leading to more abrupt junctions. The development of alloy catalysts with intermediate eutectic temperatures can provide opportunities to compare liquid and solid-catalyzed nanowire growth under comparable conditions, and can thereby serve as a useful platform to explore the influence of catalyst phase and chemistry on nanowire growth mechanisms. Towards that end, we developed a simple aqueous synthesis of Au-Cu2O core-shell nanoparticles to produce Au-Cu alloy nanoparticles of controlled size and composition by vacuum annealing. Colloidal Au nanoparticles were used as size-controlled seeds, and fine control of the Cu2O shell thickness enabled tuning of the resulting Au:Cu ratio to produce distinct populations of 1:1 and 1:3 Au:Cu nanoparticles. Annealing at 450°C revealed that both AuCu and AuCu3 alloy nanoparticles were formed, as expected from the distribution in nanoparticle sizes. The alloy nanoparticles were found to catalyze Ge nanowire growth in a low-pressure chemical vapor deposition environment. Energy-dispersive x-ray spectroscopy and electron diffraction analysis of catalysts at the tips of Ge nanowires confirmed that growth proceeds from an alloy and not simply the constituent metals. The growth temperature of 320°C is well below the Au-Ge and Cu-Ge eutectic temperatures, suggesting that the catalyst was solid during growth. Consistently, the nanowire growth rate for AuCu alloy nanoparticles was intermediate to that of Cu (slowest) and Au (fastest) nanoparticles under identical conditions. We conclude that synthesis of core-shell metal nanoparticles is a valid approach to alloy nanocatalysts of controlled size and composition, which in turn may provide enhanced control of doping and junction abruptness in the catalyst-mediated growth of semiconductor nanowires.
9:45 AM - EE1.3
In Situ Stability Study of Initially Coherent Ge-core/SiGe-shell Nanowires.
Shu Hu 1 , Irene Goldthorpe 1 , Shruti Thombare 1 , Marika Gunji 1 , Ann Marshall 2 , Paul McIntyre 1 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , Geballe Laboratory for Advanced Materials, Stanford, California, United States
Show AbstractGroup IV coaxial nanowire heterostructures can significantly improve the performance of nanoelectronic and nanophotonic devices. We have previously reported that by inhibiting surface roughening of a Si or SiGe shell around a Ge nanowire during growth, highly strained, dislocation-free, coherent core-shell nanowires are obtained. These as-grown nanowires with large Si composition gradient and core-shell lattice mismatch are not in thermodynamic equilibrium. The effects of thermal annealing on atomic motion leading to core-shell strain relaxation, including Si-Ge interdiffusion, stress-driven surface roughening, and associated dislocation formation, are not yet understood. As it is challenging to observe these phenomena during growth, we focus on in situ studies of core-shell nanowire surfaces during post-growth annealing in a transmission electron microscope (TEM).In this work, Au-catalyzed, chemical vapor deposited Ge nanowires are first synthesized, followed by heteroepitaxial deposition of a SiGe shell. These dislocation-free nanowires can be synthesized with controlled coherency strains in the core and shell, and with various core and shell radii. In situ TEM is employed to characterize the time evolution of nanowire surface roughness and dislocation density at temperatures as high as 700°C. Composition variation across the core/shell nanowires are quantified using energy-dispersive x-ray spectroscopy before and after in situ annealing. Results are compared to model predictions for stress-driven roughening, dislocation-mediated relaxation and interdiffusion in strained coaxial heterostructures.
10:00 AM - EE1.4
In-situ TEM Observation of the Growth of Si Nanowires by Catalysts of Ag-Au Alloy.
Yi-Chia Chou 1 2 , Cheng-Yen Wen 1 2 , Mark Reuter 2 , Eric Stach 1 3 , Frances Ross 2
1 Materials Science and Engineering, Purdue University , West Lafayette, Indiana, United States, 2 , IBM T. J. Watson Center, Yorktown Heights, New York, United States, 3 , Brookhaven National Lab, Upton, New York, United States
Show AbstractWe report the vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) growth of Si nanowires by catalysts of Ag-Au alloys, observed by in situ TEM. Recent experimental and theoretical studies have shown that the VSS growth mode is important because, in metal-catalyzed nanowire growth, sharp interfaces can be achieved by using solid catalysts that have low solubility for the growth species and hence do not act as a reservoir when switching between materials. Ag and Au have simple eutectic reactions with Si but different eutectic temperatures. Furthermore, Ag is completely miscible with Au. Thus, the eutectic temperature of an Ag-Au alloy with Si can be tuned by adjusting the Ag/Au ratio. This can be expected to determine whether the growth mode will be VLS or VSS, and to affect the growth rate of the nanowires at a given temperature. It also allows us to synthesize heterostructures such as Si/Ge/Si in a single nanowire by using the VSS growth mode. We will discuss in situ TEM observations of VLS and VSS growth of Si nanowires using disilane as the source and with different ratio of Au and Ag in the catalysts. Increasing the Ag proportion results in slower nanowire growth rates, perhaps because of the rise in the the eutectic temperature in the system with more Ag. Using a fixed catalyst composition of AgAu2, we find about four times faster growth for the VLS mode compared to the VSS. We will discuss transition temperatures and morphology changes for the Ag-Au catalysts and nanowires during the transition between VLS and VSS growth, and finally we will discuss Si nucleation in pure Ag and alloy particles above and below the eutectic temperature. We believe that alloy catalysts are a promising route towards control of nanowire and heterostructure growth and morphology.
10:15 AM - EE1.5
Novel In Situ Catalyst Alloying Enables Abrupt Interfaces without Kinking in VLS Growth of Si-Ge Axial Nanowire Heterostructures.
Daniel Perea 1 , Nan Li 1 , Amit Misra 1 , S. Tom Picraux 1
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractVapor-liquid-solid (VLS) grown axial semiconductor heterostructures have garnered much attention due to their potential for novel or enhanced optical, electronic, and thermoelectric properties compared to conventional planar technology. Of particular interest are Si-SiGe and Si-Ge axial superlattice heterostructures which are being explored for thermoelectric and tunnel field-effect transistor applications. However, it is commonly found that due to the VLS growth process, the interfaces are significantly broadened and also that a high percentage of Si-Ge heterostructured nanowires form undesired structural kinks by changing growth direction shortly after the change in composition. Here we report recent results in which the interfaces are much more abrupt and kinking is significantly reduced. The new process involves in situ alloying of the liquid Au catalyst with Ga by the introduction of trimethylgallium. This resulting novel liquid alloy catalyst significantly reduces the solubility of Si and Ge in the liquid which sharpens the interface, while at the same time reduces the growth rate at the higher precursor partial pressures favorable to nanowire growth, thus promoting a significant reduction in kinking. For example, considering a ~65nm diameter nanowire, for the transition from Ge to Si, the solubility of Ge is significantly decreased and the interface sharpness increases from a width of 45 nm to 22 nm for growth at 380 C using a Au0.67Ga0.33 alloy, as compared to growth from a pure Au catalyst. At the same time, the growth rate decreases from 2.8 nm/s for kinked Si segments grown from a pure Au catalyst, to 0.2 nm/s with a decrease from approximately 95% to <10% kinking, a 10X increase in unkinked Ge/Si heterostructure growth. While the underlying mechanisms for kinking which drive a nanowire to change growth direction requires additional study, it is known that this effect is sensitive to the kinetics of growth and becomes a significant issue when changing the growth precursors or temperature. In this presentation we will provide detailed results for this new approach to achieving high quality nanowire heterostructure interfaces, discuss the underlying mechanisms, and propose future directions for tailoring high quality VLS-grown compositional and doped interfaces based on this new understanding.
10:30 AM - EE1.6
Size Effects in Ni Catalyzed Germanium Nanowire Growth.
Shruti Thombare 1 , Ann Marshall 2 , Paul McIntyre 1 2
1 Materials Science Engineering, Stanford University, Stanford, California, United States, 2 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractThe great majority of literature studies of Ge nanowire growth have used Au as a catalyst. In most cases, growth is said to have occurred by the VLS mechanism. Gold has been a popular choice as a catalyst in part because of its ability to form a eutectic with Ge, allowing nanowire growth at a temperature below 400 C. However, Au induces trap levels deep in the Si and Ge bandgaps. In order to make deposited Ge nanowires compatible with silicon-based electronics and useful for photovoltaic applications, an electronically benign metal catalyst may be required. We report an investigation of low temperature Ge nanowire growth using Ni, which is electronically more benign than Au. Ni nanoparticles in colloidal solution were drop-cast on Ge (111) substrates. Ge nanowires were grown at temperatures as low as 375°C in a cold-wall CVD reactor with hydrogen diluted Germane as reactive precursor. Nanowire growth is expected to occur by the vapor-solid-solid (VSS) mechanism with a germanide of Ni as the catalyst phase, because the growth temperature is depressed by greater than 300° C relative to the lowest eutectic temperature in the Ni-Ge binary system. We observed a great difference in the morphology of the nanowires as a function of their diameter. Their length and preferred crystallographic orientation were strongly dependent on nanowire diameter. Nanowires with diameter greater than 25-35 nm are <111>-oriented and have a high density of grown-in defects such as twins and stacking faults and exhibit frequent kinking. Transmission electron microscopy showed that nanowires with diameter smaller than 25-35 nm, which grow preferentially in the <110> direction, appear to have no kinks despite having a substantial density of crystal defects. The observed size-dependence of Ge wire morphology will be discussed in terms of wire surface energies and the structure of the catalyst/nanowire growth facet interface.
10:45 AM - EE1.7
An in-situ Chemical Study of the Influence of Hydrogen on Si Nanowire Crystal Orientation, Catalyst Diffusion, and Faceting.
Naechul Shin 1 , Michael Filler 1
1 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractAn understanding of the atomic-scale chemistry that governs semiconductor nanowire growth is crucial for the precise control of nanoscale physical properties, especially in highly confined systems. Despite its importance, critical chemical information is currently lacking with respect to the factors that influence basic structural features such as crystal orientation and sidewall faceting. For example, hydrogen is prevalent during the hydride-based vapor-liquid-solid growth of semiconductor nanowires, but its role is largely unknown. To this end, we systematically studied the effect of hydrogen during the growth of Si nanowires and confirmed its influence on growth orientation, catalyst ripening, and sidewall faceting for the first time. In-situ transmission infrared (IR) spectroscopy was used to identify the presence and bonding of hydrogen on Si nanowires as a function of growth conditions. Si(111) substrates were cleaned via high temperature flashing to 1250oC in a ultra-high vacuum (UHV) chamber. After cooling to room temperature, a thin Au catalyst film was thermally evaporated in-situ. Si nanowires were subsequently grown via a two-step process: (1) brief nucleation at high temperature (550oC) and low pressure (5x10-5 torr) followed by (2) elongation under different conditions (400 - 500oC, 5x10-5 - 5x10-4 torr). Vertically-oriented epitaxial Si nanowires with uniform lengths were obtained with this method. In-situ IR data recorded in real-time reveals the evolution of surface Si-H stretching modes near 2090 cm-1 as a function of growth conditions. Our data indicates that surface-bound hydrogen is responsible for changes in crystal orientation even when nanowire diameter remains constant. More specifically, a clear increase of Si-H peak intensity is observed when the growth orientation shifts from <111> to <112>. The adsorption-desorption kinetics of hydrogen on the sidewall at each growth temperature and pressure will be discussed in detail. Furthermore, nanowire sidewalls frequently exhibit a rough region near the base due to conformal deposition of Si during extended growths. Observed increases of Si-H mode intensity at constant temperature are attributed to this sidewall roughening. This fundamental chemical understanding is an important step toward the rational design and controllable synthesis of semiconductor nanowires.
11:00 AM - EE1:Group IV Nan
BREAK
11:30 AM - **EE1.8
New Directions for Wire Arrays in High Efficiency Photovoltaics and Solar Fuel Synthesis.
Harry Atwater 1 , Dan Turner-Evans 1 , Morgan Putnam 3 , Michael Kelzenberg 1 , Nathan Lewis 2 , Emily Warren 3 , Chris Chen 4 , Adele Tamboli 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States, 3 Chemical Engineering, California Institute of Technology, Pasadena, California, United States, 2 Chemistry, California Institute of Technology, Pasadena, California, United States, 4 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractSemiconductor photovoltaic wire array technology has progressed rapidly in the last year, with demonstrated Si wire array photovoltaic efficiencies of 8% in array-fabricated cells and single wire measurements indicating efficiencies higher than 17% can be achieved. Beyond Si, multijunction wire array structures offer the possibility of achieving even higher photovoltaic efficiencies through epitaxial heterostructure synthesis of III-V/Si materials. Conformal GaP on Si wire array heterostructures have been synthesized by metallorganic chemical vapor deposition and offer an opportunity for a lattice-matched wide bandgap top cell for series-connected tandem multijunction wire array photovoltaics. The large surface area and high aspect ratio of Si and other semiconductor wire arrays also offers an attractive architecture for solar-driven photoelectrochemical reduction and oxidation of water and carbon dioxide to generate solar fuels. I will describe results for photoelectrochemical reduction of water to hydrogen and prospects for integrated photoreduction and photo-oxidation wire array heterostructures.
12:00 PM - EE1.9
Wafer-scale Growth of Silicon Microwire Arrays.
Adele Tamboli 1 , Christopher Chen 1 , Daniel Turner-Evans 1 , Michael Kelzenberg 1 , Harry Atwater 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractMicrowire arrays present a nearly optimal geometry for solar energy conversion in materials with limited minority carrier diffusion lengths, as they decouple the absorption length and minority carrier diffusion distance. This geometry allows for absorption of nearly all of the incident light while enabling efficient extraction of carriers. Previous reports have shown Si microwire arrays to be promising materials for photovoltaics[1,2] and solar fuel generation[3] using small (~1 cm2) samples grown in tube furnaces of limited size. For microwire array devices to be economically viable, however, the development of wafer-scale growth techniques will be necessary and growth yield will need to be high.We have grown Si microwires over entire 6 inch wafers using SiCl4-based CVD and vapor-liquid-solid (VLS) growth with a Cu catalyst. Our reactor uses RF induction heating to reach a growth temperature of 1000 C, above the Si-Cu eutectic temperature. This cold-wall geometry allows microwires to be grown reproducibly over 6 inch wafers, with growth rates of approximately 10 μm/min. We have grown more than one square meter or Si wire array material, demonstrating good growth yield and ability to scale up growth to commercially viable levels. We are also able to control growth parameters more precisely than previously possible, allowing for a more detailed understanding of the factors influencing growth. We have found that the presence of even small amounts of oxygen has a detrimental impact on microwire fidelity and growth rate control, while small amounts of BCl3 used for p-type doping dramatically improve growth fidelity and controllably increase growth rate. We have investigated the effects of susceptor angle on wafer uniformity to account for gas depletion along the length of the tube as well.After wire growth is complete, we can embed the wire arrays in a polymer, such as PDMS, and remove them from the substrate[4], resulting in a 6 inch diameter flexible array of Si microwires that can be used for flexible solar conversion devices, while the substrate can be re-used for further growths, recycling the most expensive component. For fabrication of high performance photovoltaics, microwire arrays also need to have controllable doping profiles and good electrical properties. Using BCl3, we can dope the microwires p-type with carrier concentrations up to 5*1019 cm-3. Diffusion doping then allows us to fabricated radial junctions in the wires. Using electrical and photoelectrochemical characterization of these large area arrays, we can compare their properties to earlier small-scale arrays of Si microwires and assess their viability as an emerging photovoltaic technology.References[1] M.C. Putnam et al., Energy & Environmental Science 3, 1037 (2010).[2] C.E. Kendrick et al., Appl. Phys. Lett. 97, 143108 (2010).[3] S.W. Boettcher et al., Science 327, 185 (2010).[4] J.M. Spurgeon et al., Appl. Phys. Lett. 93, 032112 (2008).
12:15 PM - EE1.10
Optical Absorption in Si Heterojunction Wire Arrays.
Andrey Poletayev 1 3 , Hal Emmer 2 3 , Michael Deceglie 2 3 , Daniel Turner-Evans 2 3 , Michael Kelzenberg 2 3 , Morgan Putnam 2 3 , Nathan Lewis 1 3 , Harry Atwater 2 3
1 Chemistry, California Institute of Technology, Pasadena, California, United States, 3 Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, United States, 2 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractWire array photovoltaics have demonstrated the potential to obtain efficiencies comparable to those of planar crystalline Si cells in flexible configurations. [1] However, higher open-circuit voltages are necessary to achieve these efficiencies in wire arrays. One method to obtain higher open circuit voltages would be to incorporate heterojunctions to amorphous Si, as has been done in planar HIT cells. [2] Amorphous silicon contributes to excellent junction passivation and near-record open-circuit voltages in HIT cells due to the favorable band edge alignment with c-Si. However, inclusion of a-Si carries the risk of photocurrent loss to parasitic absorption and photodegradation. Since the extinction coefficient of a-Si is much higher than that of c-Si, understanding the distribution of optical absorption in a-Si/c-Si wires will be critical for making efficient use of a-Si. In order to characterize optical absorption in a-Si/c-Si wire arrays, we have modeled c-Si/a-Si radial heterojunctions using 2D FDTD full-field electromagnetic simulations. Spatial carrier photogeneration profiles were calculated from electric fields throughout the simulation volume, integrated in wavelength and weighted by the AM 1.5G spectrum. The geometry of the amorphous shell was chosen to mimic the deposition of a-Si on c-Si microwires using PECVD, [1] and was optimized with respect to junction length, presence of light-trapping structures, and illumination direction. For wire arrays illuminated through the a-Si shell, over 40% of the photogeneration occurs in the a-Si. However, for illumination on the crystalline Si side, the incident light is directed into the crystalline part of the wire array, reducing the a-Si absorption to 0.1% of the total with only a 3.8% overall loss in photogeneration. Under such “inverted” illumination, the amorphous shell is sandwiched between the c-Si wire and the reflecting back contact. At wavelengths above the a-Si bandgap, our model predicts up to a ten-fold optical concentration in the exposed part of the crystalline microwires relative to the same structures under illumination through a-Si, attributable to increased light-trapping efficiency. Changing the wire end shape from circular to conical gives an additional minimization of reflection, and then we observe simulated photocurrent densities above 34.5 mA/cm2 in the c-Si part of the wire arrays. We will also discuss the fabrication and optical characterization of a-Si/c-Si heterojunction wire arrays, as well as device-physics modeling used to estimate the energy conversion efficiency in a-Si/c-Si wire photovoltaics based upon the calculated absorption profiles. [1] M.D. Kelzenberg et al, D.B. Turner-Evans et al, submitted.[2] Y. Tsunomura et al, Solar Energy Mater. and Solar Cells, 93, 6, 670. (2009)
12:30 PM - EE1.11
Radial Junction Silicon Microwire Photocathodes.
Emily Warren 1 , Shannon Boettcher 1 2 , Michael Walter 1 , Harry Atwater 2 , Nathan Lewis 1
1 Division of Chemistry and Chemical Engineering, Caltech, Pasadena, California, United States, 2 Divison of Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States
Show AbstractSilicon microwires are promising materials for driving the cathodic (hydrogen producing) component of the overall photoelectrochemical water-splitting reaction. The radial geometry of microwire arrays decouples the direction of light absorption and carrier collection, enabling the use of materials with shorter minority-carrier diffusion lengths than would be acceptable in a planar geometry liquid junction. Unfortunately, bare silicon has band positions that are strongly influenced by pH, which limits the attainable photovoltage from p-Si/aqueous systems. Introduction of an n
+-doped emitter layer, to create a “buried junction”, should decouple the band banding, and thus the photovoltage, of the photoelectrode from the energetics of the semiconductor/liquid contact. The ability to decouple the photovoltage of a Si photocathode from the pH of the contacting aqueous solution should increase the versatility of these materials for use in photoelectrochemical fuel-forming systems.The effects of introducing a doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (MV
2+/+), the pH dependence of the band-edge positions of Si in contact with aqueous electrolytes yielded open-circuit voltages, V
oc, for both planar and wire array Si photoelectrodes that varied as the pH of the solution was changed. Increases in the pH of the electrolyte produced a decrease in V
oc by approximately -44 mV/pH unit for both planar electrodes and Si microwire array electrodes. In contrast, introduction of a highly doped, n
+ emitter layer produced V
oc = 0.56 V for planar Si electrodes and V
oc = 0.52 V for Si microwire array electrodes, with the photoelectrode properties in each system being essentially independent of pH over six pH units (3
+ emitter layer not only improved the photovoltages for planar and Si microwire array photoelectrodes, but decoupled the band energetics of the semiconductor (and hence the obtainable photovoltage) from the value of the redox potential of the solution.1 The formation of radial junctions on Si microwire arrays thus provides an approach to obtaining high-photovoltage Si-based photoelectrodes that can be used for a variety of photoelectrochemical processes, regardless of the intrinsic barrier height and flat-band properties of the Si/liquid contact. This presentation will also report on progress that has been made on integrating hydrogen evolution catalysts onto n+-p Si microwire arrays towards the fabrication of photocathodes for the overall water-splitting reaction. [1]S. W. Boettcher et al., “Energy-Conversion Properties of Vapor-Liquid-Solid-Grown Silicon Wire-Array Photocathodes,” Science, 327(5962), 185-187 (2010).12:45 PM - EE1.12
Measurement of Minority Carrier Diffusion Lengths in VLS-grown p-n Junction Silicon Nanowires.
Aditya Mohite 1 2 , Daniel Perea 1 , Sanjeev Singh 1 2 , Shadi Dayeh 1 , Samuel Picraux 1 , Han Htoon 1 2
1 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractVLS-grown semiconductor nanowires have emerged as a viable prospect for future solar energy applications. Despite tremendous efforts invested in all areas of nanowire research, the fundamental processes critical to the functioning of a nanowire photovoltaic device such as charge dissociation, collection and recombination processes of photo-generated carriers in NWs is still poorly understood. A direct measurement of the minority carrier diffusion lengths and quantitative assessment of the impact of doping, diameter, operating bias, virtual gating and heterostructing is of critical importance in understanding and designing efficient 1D photovoltaic devices. To date, there have been a few published studies which report minority carrier diffusion lengths and carrier lifetimes in semiconductor nanowires1,2, however to our knowledge no measurement has been reported for a VLS grown axial p-n junction nanowires. Here, we report a scanning photocurrent microscopy study of VLS-grown p-n junction silicon nanowires to measure the minority carrier diffusion length, mobility and lifetime of the photo-generated carriers.Photocurrent measurement studies were performed on in-situ doped Si nanowire p-n junctions and devices with Ni ohmic contacts probed separately to the p- and n–segments of the devices for p-n junction assessment as well as the field-effect properties of either segments of the junction. The measured photoresponse shows an exponential increase in the photocurrent by an orders of magnitude as the laser spot (λ=532 nm, spot size ~400 nm) is scanned across the p-n junction with a peak photocurrent at the center of the junction and an exponential decrease on either side as the laser spot departs from the junction. With increasing reverse bias, the central photocurrent plateau width increases indicative of increased depletion width. By fitting the function to the exponential decay in the photocurrent for a 40 nm diameter nanowire, we calculated a minority carrier diffusion length of Ln=1.842 µm and Lp=1.45 µm for electrons and holes, respectively. Such relatively long minority carrier diffusion lengths are indicative of low dopant incorporation for the gas ratios we used, consistent with LEAP analysis and suggest that the measured lengths scale with doping concentration despite the impact of surface states for 1D systems. We will further discuss the dependence of the minority carrier diffusion length, mobility and lifetime as a function of diameter, doping concentrations, and back-gating. 1)Quantitative measurement of electron and hole mobility-lifetime products in semiconductor nanowires Yi Gu et al. Nano Letters 6, 948, (2006)2)Photovoltaic measurements in single-nanowire silicon solar cells; Michael D. Kelzenberg et al, Nano Letters 8, 710-714, (2008).3)Direct measurement of dopant distribution in an individual vapour–liquid–solid nanowire Daniel E. Perea et al., Nature Nanotechnology 4, 315, (2009).
Symposium Organizers
Volker Schmidt Max Planck Institute of Microstructure Physics
LincolnJ. Lauhon Northwestern University
Takashi Fukui Hokkaido University
GeorgeT. Wang Sandia National Laboratories
Mikael Bjoerk IBM Research GmbH
Symposium Support
AIXTRON SE
FEI Company
IBM Research Zurich
JEOL USA
Veeco
EE6: Poster Session: III-V and II-IV (Non-Oxide) Nanowires
Session Chairs
Wednesday PM, April 27, 2011
Salons 7-9 (Marriott)
EE5: III-V (Non-Nitride) Nanowires I
Session Chairs
Wednesday PM, April 27, 2011
Room 3001 (Moscone West)
2:30 PM - **EE5.1
Periodic Nanowire Structures.
Erik Bakkers 1 2 3 , Rienk Algra 1 4 , Marcel Verheijen 1 3 , Lou-Fe Feiner 1 , Elias Vlieg 4 , Willem van Enckevort 4 , Moira Hocevar 2
1 , TU Eindhoven, Eindhoven Netherlands, 2 , TU Delft, Delft Netherlands, 3 , Philips Research, Eindhoven Netherlands, 4 , Radboud University, Nijmegen Netherlands
Show AbstractWe show recent advances on inducing periodicity on both intra- and interwire level, such to obtain 3 dimensional position control. The nanowire position is determined by that of the catalyst particle. We have developed a generic soft nano-imprint lithography process to fabricate arrays of metal particles. From these structures nearly defect free arrays of InP and GaP nanowires have been grown. This approach gives in-plane periodicity. Next, we demonstrate control of the crystal structure of indium phosphide (InP) and gallium phosphide (GaP) nanowires by impurity dopants. More importantly, we demonstrate that we can, once we have enforced the zinc blende crystal structure, induce twinning superlattices with long-range order in the z-direction in the nanowires. The spacing of the superlattices is tuned by the wire diameter and the zinc dopant concentration. These findings have been quantitatively modelled based on the cross-sectional shape of the zinc-blende nanowires.
3:00 PM - EE5.2
Combinatorial Approaches to Understanding Polytypism in III-V Nanowires.
Jonas Johansson 1 , Jessica Bolinsson 1 , Martin Ek 2 , Kimberly Dick 1 2
1 Solid State Physics and the Nanometer Structure Consortium, Lund University, Lund Sweden, 2 Polymer & Materials Chemistry, Lund University, Lund Sweden
Show AbstractMetal particle seeded III–V semiconductor nanowires are currently being investigated for a wide range of applications in electronics, photonics, and life sciences. Many of these applications require a well-defined crystal structure. However, nanowire growth at arbitrary conditions often results in a poorly defined crystal structure, which can be described as a mixture of twinned zinc blende structure and wurtzite structure with stacking faults.Existing models of polytypism (the occurrence of different crystal structures that differ in stacking sequence only) in nanowires are strongly focused on explaining the occurrence of wurtzite (2H) in materials that have zinc blende (3C) as the bulk stable crystal structure. Recent observations of higher polytypes, such as 4H and 6H, motivates the consideration of more polytypes than just 2H and 3C when predicting the influence of experimental and materials parameters on the crystal structure of III–V nanowires.In this investigation we first calculate the limits for phase purity in materials where the inter-layer interactions are limited to next nearest neighboring layers only. Using combinatorics, we express the formation probabilities for the 2H, 4H, 6H, and 3C polytypes as functions of the nucleation probabilities for hexagonal and cubic type stacking. Next, we relate these nucleation probabilities to supersaturation and we show that for low supersaturation the 3C polytype dominates. As the supersaturation is increased, 6H starts to dominate and the probability for 4H reaches its maximum. If the supersaturation is further increased, 2H dominates more and more. This combinatorial approach is new and offers unexpected insights into the polytypism in III–V nanowires.We also consider longer range inter-layer interactions by applying an Ising model, which has been used to explain the polytypism in SiC, in our classical nucleation framework. Considering up to third nearest neighbor, this approach enables us to express the nucleation probabilities for the 3C, 4H, and 2H polytypes. We demonstrate that formation of the 4H polytype can dominate in a small supersaturation interval even if it is not the energetically favorable phase.We discuss both the short range and the longer range models in light of recent nanowire growth experiments, both our own and from the literature. In some nanowire materials systems, for instance GaAs, it seems very hard to control the crystal structure. In other nanowires, typically containing antimony, it seems to be easier to achieve pure phases. The former class of materials shows good qualitative agreement with the short range model, whereas the latter class agrees well with the longer range model. Finally, we briefly discuss possible reasons for the apparently different inter-layer interaction ranges during nanowire growth.
3:15 PM - EE5.3
Unit Cell Deformations of Hexagonal Polytypes in III-V Semiconductor Nanowires.
Dominik Kriegner 1 , Christian Panse 2 , Bernhard Mandl 1 3 , Kimberly Dick 3 4 , Mario Keplinger 1 , Johan Persson 5 , Philippe Caroff 6 , Daniele Ercolani 7 , Lucia Sorba 7 , Friedhelm Bechstedt 2 , Julian Stangl 1 , Guenther Bauer 1
1 Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Linz Austria, 2 Institut fuer Festkoerpertheorie und -optik, Friedrich-Schiller-Universität Jena, Jena Germany, 3 Solid State Physics, Lund University, Lund Sweden, 4 Polymer & Materials Chemistry, Lund University, Lund Sweden, 5 Center for Electron Nanoscopy, Technical University Denmark, Kgs Lyngby Denmark, 6 IEMN, UMR CNRS 8520, Villeneuve d'Ascq France, 7 NEST, Scuola Normale Superiore and CNR-INFM, Pisa Italy
Show AbstractIn contrast to bulk crystals and 2D layers, where only the zinc blende (ZB) structure is stable, in InAs and InSb nanowires (NWs) the formation of hexagonal wurtzite (WZ) and 4H segments is a common phenomenon, which strongly affects their electronic and optical properties.Only recently, control over the different NW crystal structures [1,2,3] has been achieved. As this offers a new degree of freedom for NW device design, like polytypic multi quantum well structures, an understanding of the structural properties of different polytypes is urgently needed.Here we present precise systematic studies of the variations of the unit cell parameters of InAs and InSb NWs for ZB, WZ, and 4H crystal structure in NWs. We employ high-resolution synchrotron x-ray diffraction (XRD) studies on NW samples with controlled crystal phases, which were selected by transmission electron microscopy (TEM) investigations. We compare the obtained lattice parameters for the InAs and InSb polytypes to results of density functional theory (DFT) calculations. Experiment and theory consistently show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c-axis, and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, which is defined as the fraction of bilayers with hexagonal character within one unit cell. Quantitative agreement between experimental and theoretical results is found. The DFT calculations not only reveal the change of the unit cell dimensions, but describe also deformations of the bonding tetrahedra, which lead to displacements of the atom positions within the unit cell. Therefore the comparison with numerical data contributes a better understanding of the geometric variations as a function of the bilayer stacking.Our analysis shows that the often used simple geometric conversion of the ZB bulk lattice constants in order to obtain those of the hexagonal polytypes is not correct since this procedure neglects the distortions of the bonding tetrahedra in the hexagonal polytypes. Furthermore our precise results will enable accurate calculations of the band structure of the hexagonal polytypes as well as of the corresponding band alignments in InAs and InSb nanowires.[1] Caroff, P.; et al. Nature Nanotech. 2009, 4, 50-55.[2] Dick, K. A.; et al. Nano Lett. 2010, 10, 3494-3499.[3] Kitauchi, Y.; et al. Nano Lett. 2010, 10, 1699-1703.
3:30 PM - EE5.4
Growth and Characterization of InSb Nanowires by Chemical Beam Epitaxy.
Alexander Vogel 1 , Johannes de Boor 1 , Joerg Wittemann 1 , Samuel Mensah 1 , Peter Werner 1 , Volker Schmidt 1
1 , Max Planck Institute of Microstructure Physics, Halle (Saale) Germany
Show AbstractThere is growing interest in antimony based III-V semiconductor materials, due to their intriguing physical properties. For example, InSb is very promising candidate for high-speed, low-power electronics due to the extremely high bulk electron mobility of 77000 cm2V-1s-1. Also, InSb has a good hole mobility of 850 cm2V-1s-1.However, heteroepitaxial growth of InSb is not easily achieved due to the large lattice constant (a0 = 0.648 nm) of InSb as compared to other semiconductor materials. A large lattice mismatch exists between InSb and typical semiconductor substrate materials like InAs (7%), GaAs (15%) and Si (19%). Compared to thin films, nanowires can potentially relax part of the strain energy by elastically deforming its shape. Concerning the growth of InSb nanowires, not much is known about such relaxation mechanisms.We are going to present detailed growth studies and structural characterization of InSb nanowires grown directly on InSb and InAs substrates using Chemical Beam Epitaxy. CBE has some decisive advantages over other epitaxial growth techniques like MOCVD or MBE. In general, using CBE one is able to grow at lower temperatures compared to MOCVD, which is particularly important when it comes to the growth of materials with very low melting points like InSb. Trimethylindium and triethylantimony were used as precursors to grow InSb nanowires. Both Au and Ag containing seed particles were used for nanowire growth.We identified two very different growth regimes. In the low temperature growth regime, around 350°C, nanowire growth was promoted by a (most likely liquid) indium-rich alloy. In the high temperature growth regime, around 430°C, post-growth characterization suggested that an AuIn2 alloy promoted nanowire growth.(HR)TEM investigations showed that wires grown at lower temperatures exhibited a large number of stacking faults and twin-planes. However, the stacking fault density could be heavily reduced by gradually increasing the growth temperature. Completely stacking fault-free InSb nanowires were grown at temperatures above 415°C. Those nanowires had a length of up to 2,8 µm with diameters as small as 35 nm.By combining CBE nanowire growth and laser interference lithography ordered arrays of InSb nanowires were grown. Those arrays have good homogeneity over a large area with a density of up to 8 wires per square micron. Temperature dependant electrical characterization (150K to 300K) was conducted by embedding the nanowires in polyimide and contacting those using Cr/Au top contacts. The InSb nanowires investigated showed good electrical conductivity and the temperature dependence suggested intrinsic behavior.
3:45 PM - EE5.5
Growth of Antimony-containing Heterostructure Nanowires by Molecular Beam Epitaxy: Crystal Phase Control, Surfaces and Interfaces.
Philippe Caroff 1 , Tao Xu 2 , Kimberly Dick 3 5 , Sébastien Plissard 1 4 , Thanh Nguyen 2 , Bruno Grandidier 2 , Xavier Wallart 1
1 , Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Villeneuve d'Ascq France, 2 ISEN, Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Villeneuve d'Ascq France, 3 Solid State Physics / the Nanometer Structure Consortium, Lund University, Lund Sweden, 5 Polymer & Materials Chemistry, Lund University, Lund Sweden, 4 Department of Applied Physics, Photonics and Semiconductor Nanophysics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractGrowth of III-V nanowires has now gained some maturity and fundamental understanding of the growth mechanisms has dramatically progressed. However, there are only three families of III-Vs which have been extensively explored: nitrides, arsenides and phosphides. The growth and understanding of binary or ternary antimonides is only emerging now. The main motivation for developing antimonide nanowires is to link the intrinsic advantages of the nanowire geometry (possibility to relax strain very efficiently without creation of dislocations, quantum confinement, control of crystal phase), with the exceptional bulk properties of these semiconductors, such as small bandgaps, huge electron (InSb) or hole (GaSb) mobilities, the largest Landé g factor (InSb), a high thermoelectric figure of merit (InSb) and vast possibilities of bandstructure engineering, both in type I or type II/III alignments.Here we present advanced nanowire structures containing antimony and grown by molecular beam epitaxy. Structural characteristics are analyzed using scanning electron microscopy, transmission electron microscopy, and scanning tunneling microscopy. Nanowire heterostructures have been grown by a vapour-liquid-solid mechanism, using either gold seed particles or a gold-free “self-catalyzed” approach. We report the achievement, for the first time, of InSb nanowires by molecular beam epitaxy. We then show ternary GaAs/GaAsxSb1-x, InAs/InAsxSb1-x abrupt nanowire heterostructures, of controlled composition. It is demonstrated that crystal structure of ternary nanowires can be tuned with high control from defected to perfect phases, by inclusion of just a few atomic percent of antimony. Due to an overgrowth process, both wurtzite and zinc-blende InAsSb sidewall facets occur, and are revealed by low temperature STM with atomic resolution.
4:00 PM - EE5: III-V
BREAK
4:30 PM - **EE5.6
III-V Semiconductor Nanowires on Si: Selective Area MOVPE and Their Device Applications.
Katsuhiro Tomioka 1 2 , Junichi Motohisa 1 , Shinjiroh Hara 1 , Kenji Hiruma 1 , Takashi Fukui 1
1 Graduate School of Information Science and Technology, and Research Center for Integrated Quantum Electronics (RCIQE), Hokkaido University, Sapporo Japan, 2 PRESTO, Japan Science and Technology Agency (JST), Saitama Japan
Show AbstractIntegration of III-V nanowires on Si has attracted much attention for further advancing Si-based CMOS and optoelectronics. Recent advances in epitaxial techniques such as Vapor-Liquid-Solid (VLS) and selective-area growth (SAG) have enabled us to integrate III-V nanowires on Si without considering mismatches in lattice constant and thermal expansion coefficient. The nm-scaled heteroepitaxy reveals specific properties in III-V/Si heterojunctions, which have been screened with degradations due to the mismatches. We report on the systematically controlled growth of III-V compound semiconductor nanowire arrays by catalyst-free selective area metalorganic vapor phase epitaxy on partially masked Si(111) substrates. First, we present the growth of III-V (GaAs, InAs, and InP) nanowires on partially masked (111) oriented substrate, and demonstrate core-multishell nanowire-based devices, such as GaAs nanowire-based light-emitting diode (LED), InP nanowire-based solar cell, light-pumped laser operation from GaAs/GaAsP core-shell nanowire. Next, we present the SAG of vertical InAs and GaAs nanowire on Si (111) substrates by modifying initial Si (111) surface. We used the specific growth sequence to align the growth directions into vertical direction. We explain the growth of InAs, GaAs, and InGaAs nanowires on Si substrate. Cross-sectional transmission electron microscope images revealed that misfit dislocation with local strains was accommodated in InAs/Si interface, while no misfit dislocation was observed in GaAs/Si interface in case of thin nanowire diameter. Then, we demonstrate integration of III-V nanowire-based optical devices on Si for solid-state-lightning.Finally, we demonstrate vertical surrounding-gate FETs (VSGFETs) using as-grown InAs and InGaAs NWs on a Si substrate. After the growth, Hafnium alminate was deposited as high-k gate dielectric, followed by the deposition of tungsten by plasma sputtering for gate metal. Then, drain and source metals were evaporated on the top of NWs and backside of the substrate, respectively. Fabricated VSGFET contained single NW in the channel. In case of the InGaAs-based VSGFET, we observed n-type enhancement-mode FET behavior in ID-VDS and ID-VG characteristics. The performances are threshold voltage ~ 0.2 V, Gm,max = 0.96 S/mm, Ion / Ioff ~ 106, subthreshold swing, SS = 120 mV/decade. In addition, we present device concept for tunnel FET using III-V nanowire/Si heterojunctions.
5:00 PM - EE5.7
Control of Nanowire Morphology by Choice of Precursor Chemistry.
Omid Salehzadeh 1 , Simon Watkins 1
1 Physics, Simon Fraser University, Burnaby, British Columbia, Canada
Show AbstractMetalorganic vapour phase epitaxy (MOVPE) is a key technology for the growth of III-V nanowires (NW) by the vapour liquid-solid (VLS) mechanism. The ability to grow axial and core shell heterostructures is an important requirement for NW device applications. The selection of shell growth over axial growth is usually achieved by using higher growth temperatures where the lateral growth is favoured over VLS growth. In this work we show that enhanced lateral growth can be achieved by using lower temperature precursors. Most prior work on GaAs NW growth using the VLS mechanism has involved the growth precursor trimethylgallium (TMGa) as the group III source. In this work we compare the growth of GaAs NWs grown using the ethyl-based precursor triethylgallium (TEGa), with TMGa growth over the temperature range of 360-480°C. Gold nanoparticles were formed by evaporation of thin Au films (0.5-2nm) followed by annealing under As vapor. Detailed measurements of the nanowire morphology were realized using scanning electron microscopy (SEM) and transmission electron microscopy. TEGa decomposes at approximately 100°C lower than TMGa based on the measurement of growth rates for planar films on (100) GaAs substrates. Growth with TEGa results in highly tapered NWs for growth temperatures above 370°C. Under these conditions, the catalyst free planar growth rate is still mass transport limited. NWs grown with TEGa are highly tapered due to the fact that planar growth on the substrate and nanowire sidewalls competes with the VLS mechanism for incoming Ga flux. As a result, as wires increase in length, the sidewall portion of the wire grown at earlier times receives a larger Ga flux. In contrast, TMGa decomposes very little below 450°C in the absence of Au catalyst. The Au catalyst permits the growth of nontapered GaAs NWs using TMGa at temperatures as low as 370°C, where the planar growth rate is essentially zero. Little tapering is observed using TMGa until approximately 460°C under the present growth conditions. The use of carbon dopants such as CBr4 results in a similar reduction in tapering for TEGa growth of GaAs NWs. At high carbon flows, planar growth is also suppressed by surface reactions. As a result NWs grown in the presence of large amounts of carbon become untapered and have greatly enhanced axial growth rates compared with undoped wires grown with TEGa. In contrast, NWs grown using TMGa and carbon are unchanged since the planar growth is already strongly suppressed at low growth temperatures.NWs grown under conditions which suppress tapering have predominantly zincblende structure with a low density of stacking faults, while tapered wires have high densities of stacking faults and domains of wurtzite phase material. These conclusions are supported by Raman measurements showing pure zincblende lattice modes for the nontapered wires, and mixtures of zincblende and wurtzite modes for the tapered ones.
5:15 PM - EE5.8
Measuring Surface Energies and Chemical Potential during Nanowire Growth.
Marcel Verheijen 1 4 , Rienk Algra 3 4 5 , Lou-Fe Feiner 4 2 , George Immink 4 , Willem van Enckevort 5 , Elias Vlieg 5 , Erik Bakkers 2 4
1 Plasma & Materials Processing, Eindhoven University of Technology, Eindhoven Netherlands, 4 , Philips Research Laboratories, Eindhoven Netherlands, 3 , Materials Innovation Institute, Delft Netherlands, 5 Institue of Molecules and Materials, Radboud University, Nijmegen Netherlands, 2 Photonics and Semiconductor Nanophysics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractFor all applications and to fully use the potential of nanowires it is important to understand and control their structural properties. The defect density and crystal structure of nanowires grown by the vapor-liquid-solid (VLS) growth mechanism can be tuned by impurity atoms, temperature, diameter, III-V ratio or by a combination of these external parameters. It has been argued that these parameters can affect the chemical potential in the droplet, Δμ, the nanowire surface energy, γSV, and the solid-liquid or the liquid-vapor interface energies, γSL or γLV,which in turn drive the formation of planar defects and/or specific crystal structures. However, these energies have not been determined experimentally and their effect on nanowire growth is not clear. Here we present an approach to quantitatively determine the variation of Δμ, γSL, and γLV upon variation of the group III partial pressure by using GaP twinning superlattice (TSL) nanowires. We show that γLV is the main quantity determining the twin density, its influence being more than five times stronger than the combined effect of Δμ and γSL. This unexpected result implies that surfactants could be used during nanowire growth to engineer the nanowire defect structure and crystal structure.
5:30 PM - EE5.9
The Role of In Droplets in the Spontaneous Formation of InAs Nanowires on Bare Si(111).
Emmanouil Dimakis 1 , Jonas Laehnemann 1 , Uwe Jahn 1 , Steffen Breuer 1 , Maria Hilse 1 , Lutz Geelhaar 1 , Henning Riechert 1
1 , Paul-Drude-Institut für Festkörperelektronik, Berlin Germany
Show AbstractProbably the most popular approach to grow semiconductor nanowires (NWs) is the vapor-liquid-solid mechanism which relies on tiny droplets of a foreign metal, typically Au. However, a drawback of this technique is that Au may be incorporated into the NWs. Recently, we showed that the internal quantum efficiency of GaAs NWs fabricated without Au is two orders of magnitude higher than that of their Au-induced counterparts. Thus, for optoelectronic applications it is essential to pursue alternative approaches to NW synthesis that do not rely on Au or any other foreign metal. InAs NWs can be fabricated without Au by employing masks, coatings, or thin oxide layers, but so far the details of the underlying mechanism are unclear. The crucial question is whether the formation of these NWs is induced by In droplets or not. Ex-situ measurements are inconclusive as In droplets might evaporate or be consumed by As after nominal growth termination. Here, we investigate the spontaneous growth of InAs nanowires by molecular-beam epitaxy (MBE) on Si(111). The effect of substrate inhomogeneities such as oxide openings is excluded by the chemical removal of the native oxide. InAs nanowires form under a wide range of As-rich growth conditions. Electron backscatter diffraction (EBSD) measurements on dispersed NWs show that the NWs grow in the wurtzite structure and are enclosed by {11-20} sidewall facets. Both results are surprising since Au-free InAs NWs are typically zinc-blende and wurtzite InAs NWs normally exhibit {1-100} facets. After growth the tip of our NWs is a smooth facet. In order to elucidate the growth mechanism the substrate temperature, the In flux and the As flux are varied systematically. When the surface diffusivity of In is increased by raising the substrate temperature or reducing the In flux, fewer and thicker NWs form. Hence, NW nucleation depends on the local accumulation of In on the substrate surface. The axial growth rate of the NWs is higher than the impinging In flux but lower than the impinging As flux. Thus, NW growth is fuelled by In diffusion along the NW sidewalls to the NW tips. Moreover, the NW growth rate is independent of the As flux but increases with increasing In flux at constant V/III flux ratio. Therefore, during NW growth the local conditions at the NW tip must be As-rich, i.e. there cannot be any In droplets. However, a series of growth runs with different growth durations shows that during the first minute the NW growth rate is higher than both the impinging In and As flux. The most likely explanation is that in this stage In droplets are present at the NW tip during growth and are consumed by the continued growth of InAs after shutter closing. In conclusion, the following growth mechanism emerges: NW nucleation is induced by the local accumulation of In into droplets on the Si surface. During the first stage of NW growth, these droplets are consumed, and subsequently NWs continue to grow without any droplet at the tip.
5:45 PM - EE5.10
Transient Supersaturation Effects of Au VLS Catalysts on the Growth of Low-dimensional Axial GaAs/GaP Heterostructure Nanowires for Optoelectronic Applications.
Jonathan Boulanger 1 2 , Ray LaPierre 1 2
1 Engineering Physics, McMaster University, Hamilton, Ontario, Canada, 2 Center for Emerging Device Technologies, McMaster University, Hamilton, Ontario, Canada
Show AbstractThe impact of Au-assisted vapor-liquid-solid (VLS) nanowire growth on low-dimensional, lattice mismatched heterostructures is explored in the gallium arsenide / gallium phosphide (GaAs/GaP) system. The supersaturation of the liquid Au-III-V alloy droplet during MBE growth was found to be of utmost importance and was observed to have a profound impact on both the crystal phase, growth rate, and interface abruptness of low-dimensional nanowire heterostructures. In particular, growth interruptions for group V gas switching at hetero-interfaces were found to cause transient supersaturation effects. Among these effects was the appearance of a previously unreported 4H GaP crystal phase. Experimental observations indicate the 4H GaP segment occurred during a minimum in the Au-III-V droplet supersaturation level. Furthermore, the transient supersaturation level was also linked to an effective slowing of the axial VLS growth rate. This phenomenon is proposed to be the result of Ga purging from the Au droplet during growth interruptions. Both crystal phase and growth rate changes were observed to behave differently for GaAs and GaP, underlining the important role of group V elements in Au-III-V droplet supersaturation and thus VLS nanowire growth. Finally, interface abruptness was found to be negatively impacted by Ga purging during group V dimer switching. Extensive transmission electron microscopy (TEM), high angle annular dark field (HAADF), and energy dispersive x-ray spectroscopy (EDS) studies are presented for several different axial GaAs/GaP heterostructure nanowires. GaAs and GaP segments approaching thicknesses of one or two monolayers are demonstrated. A series of 10-period GaAs/GaP superlattice nanowire structures demonstrate improved dimensional control over initial attempts. These structures also serve as examples of potential optoelectronic devices such as quantum well infrared photodetectors (QWIPs), multiple quantum well solar cells (MQWSCs), or intermediate bandgap solar cells (IBSCs).
EE6: Poster Session: III-V and II-IV (Non-Oxide) Nanowires
Session Chairs
Thursday AM, April 28, 2011
Salons 7-9 (Marriott)
9:00 PM - EE6.1
Microstructural Characterization of GaAs-AlxGa1-xAs Core-shell Nanowires Grown by Au-catalyst Assisted MOVPE.
Davide Altamura 2 1 4 , Ilio Miccoli 2 , Paola Prete 3 , Nicola Lovergine 2 , Leander Tapfer 1
2 Dip. Ingegneria dell'Innovazione, Università del Salento, Lecce Italy, 1 UTTMATB, ENEA, Brindisi Italy, 4 IC, CNR, Lecce Italy, 3 IMM, CNR, Lecce Italy
Show AbstractNanowires (NWs) of III-V semiconductors are of particular interest for energy conversion (photovoltaics), electronic, optoelectronic and sensor applications. High-resolution TEM techniques are usually employed for a full microstructural characterization of single NWs, but as-obtained informations are intrinsically local. In this respect, high-resolution X-ray diffraction (HRXRD) and reciprocal space mapping (RSM) may be very powerful, non-destructive investigation techniques to obtain statistically relevant (i.e. averaged over millions-to-billions of NWs) information on the NW structural properties and correlate them with growth conditions.In this work, we report on a detailed structural and morphological characterization of III-V based NWs epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted MOVPE. As-grown dense (108-109 cm-2) arrays of almost vertically-aligned (i.e. parallel to the <111> crystallographic axis) GaAs, AlxGa1-xAs and core-shell GaAs-AlxGa1-xAs NWs were investigated, carrying out HRXRD measurements on different (hkl) reflections, in symmetric and asymmetric geometrical configuration, and by recording RSMs around the materials (111) reciprocal lattice points (relps). We show that NW diffraction peaks are visible in the RSM by means of a characteristic halo. In the case of the GaAs NWs, the halo is located at the (111) relp indicating that the NWs are grown along the <111> direction and parallel to the <111> axis of the GaAs substrate. On the contrary, for AlxGa1-xAs NWs or core-shell GaAs-AlxGa1-xAs NWs the halo is displaced (along qz) with respect to the GaAs (111) relp due to the elastic lattice strain associated with the chemical composition, i.e. the Al molar fraction in the AlxGa1-xAs alloy. Our experiments allow us to determine the strain state within the NWs, separating the radial from the axial strain contribution. The experimental strain data can be compared with the theoretical values obtained by using a strain model within the validity of the continuum elasticity theory. Here, it should be also noted that the NWs are randomly distributed across the (111) GaAs substrate surface. This means that there is only a very weak (if any) in-plane correlation between NWs, and consequently they diffract incoherently without interference: the observed diffracted intensity of the NW arrays is the diffracted intensity of a single NW multiplied by the number of irradiated NWs (i.e., the algebraic sum of all NW diffracted intensities). The widths along qx of the NW halo in the RSM allow thus to estimate the average diameter of present NWs, whose values are in excellent agreement with those obtained by FE-SEM observations.
9:00 PM - EE6.12
Binary Data Transmission Performance of Sub-20 nm Indium Antimonide Nanowires.
Ali Guvenc 1 , Miroslav Penchev 1 , Jiebin Zhong 2 , Cengiz Ozkan 2 3 , Mihrimah Ozkan 1
1 Electrical Engineering, University of California, Riverside, Riverside, California, United States, 2 Mechanical Engineering, University of California, Riverside, Riverside, California, United States, 3 Materials Science and Engineering , University of California, Riverside, Riverside, California, United States
Show AbstractWe investigated the data transmission performance of indium antimonide (InSb) nanowires (NWs) synthesized on InSb (100) substrate using chemical vapor deposition (CVD) having diameters below 20 nm. The results indicate that the data transmission performance of NWs suffer from low mobility values on the order of 10-15 cm2V-1s-1 because of the scattering due to their small diameters, crystal defects and oxidation occurs during growth and cooling. The 20 nm NWs can sustain data rates up to 10 mega bits per second (Mbps) without any impedance matching and de-embedding of the parasitic parameters coming from the measurement system. The data rate is directly proportional to the diameter of the NWs. Improving the mobility to higher values and introducing de-embedding and impedance matching to the measurements and analysis could carry the bandwidth beyond the gigabits per second level (Gbps).
9:00 PM - EE6.13
Surface Electronic Properties of GaAs Nanowires for Solar Applications.
Olivier Demichel 1 2 , Martin Heiss 1 , Joel Bleuse 2 , Henri Mariette 2 , Anna Fontcuberta i Morral 1
1 , EPFL, Lausanne Switzerland, 2 , CEA-Grenoble, Grenoble France
Show AbstractSemiconducting nanowires (NWs) are a topic of intense research as they offer the opportunity to explore properties of one-dimensional electronic systems.
Their electronic properties are promising for applications in nanoelectronics or photodetection as well as for sensing applications.
The understanding and the mastering of the electronic properties of such one-dimensional systems are essential to achieve high efficiency devices.
In particular, with the increase of the surface/volume ratio, the electronic properties of NW based devices become strongly dependent on the surface electronic states.
Indeed, surfaces electronic states can appear in the electronic band gap, altering the NW electronic properties. Two main effects are reported in literature: surface states can act
as recombination centers for free carriers or as surface charged traps. We has recently quantified this first effect by an original optical method for silicon NWs [1-2].
The surface charge traps induce a pinning of the Fermi-level at the surface and a depletion shell appears: the electronic canal available for carriers is decreased.
Here, we present our results on the influence of {110} surfaces on GaAs NWs measured by low temperature micro-photoluminescence (μ-PL) spectroscopy. We compare unpassivated NWs with those which were capped
(passivated) with a shell of Al0.4Ga0.6 As. NWs were obtained by molecular beam epitaxy have a prismatic cross section [3].
Moreover, we take advantage of the tapered shape of the NWs to understand the role of diameter and surface/volume ratios in the luminescence efficiency. We demonstrate that
capping directly modifies the recombination velocity and passivated NW electronic properties are governed by surface recombinations whereas unpassivated NWs are strongly depleted by the
Fermi-level pinning at the surface induced by charged surface trap states. Finally, we measured a surface recombination velocity of 3.103 cm.s-1 for passivated NWs:
one order of magnitude lower than values previously reported for {110} GaAs surfaces. And the surface charged trap density of uncapped NWs (~1012 cm -2)
is in agreement with values of surface charged trap density of native oxidized GaAs surfaces. Photo-courant measurements were then performed and we demonstrate the increase of the photo-voltage
conversion efficiency once NWs are passivated. This work will serve as a guidance for the passivation of the NW surfaces which is of crucial importance for applications in laser and solar cell
technology.
[1] O. Demichel, V. Calvo, N. Pauc, A. Besson, P. Noé, F. Oehler, P. Gentile, and N. Magnea, Nano Lett. 9 (7), 2575 (2009).
[2] O. Demichel, V. Calvo, A. Besson,
P. Noé, B. Salem, N. Pauc, F. Oehler, P. Gentile, and N. Magnea, Nano Lett. 10 (7), 2323 (2010).<br>[3] A. Fontcuberta i Morral, D. Spirkoska, J. Arbiol, M. Heigoldt, J. R.
Morante, and G. Abstreiter, Small 4 (7), 899 (2008), ISSN 1613-6829.
9:00 PM - EE6.14
P-doping Mechanism in Catalyst-free MBE Grown GaAs Nanowires.
Carlo Colombo 1 , Joseph Dufouleur 2 , Tonko Garma 2 , Bernt Ketterer 1 , Emanuele Uccelli 1 , Anna Fontcuberta i Morral 1
1 , EPFL, Lausanne Switzerland, 2 , TUM, Muenchen Germany
Show AbstractSemiconductor nanowires have stimulated a lot of interests in the nanotechnology field thanks to the opportunity to make extraordinary progress in several fields such as single molecule sensing, batteries, solar cells and thermoelectric devices. In order to realize all the above mentioned possibilities it is of primary importance to understand and control the doping mechanism during the nanowires growth. In this work this was done in the case of catalyst free Molecular Beam Epitaxy (MBE) grown GaAs nanowires doped with Silicon. To analyze the spatial distribution of dopants in the nanowire, two different approaches have been used simultaneously to the identical nanowire: Raman spectroscopy and electrical measurements. The former has been realized scanning the nanowire with a Ar+Kr+ laser focused at the diffraction limit. The spatial variation in Silicon content has been obtained measuring the evolution of the Local Vibrational Mode peak of silicon in GaAs. At high silicon concentration, evidences of doping compensation have been obtained. The intensity of this peak can be correlated to the concentration of Si atoms in the crystal. Electrical measurements have been performed fabricating multiple metal contacts along the nanowire by Electron Beam Lithography. In order to enable a simultaneous measurement of the regions between contacts with Raman Spectroscpy, the distance between two contacts was kept larger than the size of the illumination spot. The resistance has been evaluated between each couple of contacts in a 4 points configuration to avoid the influence of the contact resistance.The measurements have shown the presence of two competing mechanisms for the doping incorporation, the first one occurring in a Vapor Liquid Solid manner trough the catalyst droplet and the second one in a simpler Vapor Solid way at the nanowire facets. Hole concentrations of at least 2.4x1018 cm-3 have been achieved, which to our knowledge is the largest p doping range obtained up to date.To better characterize the carriers transport properties, scanning photo-current (SPCM) and electron beam induced (EBIC) measurements have been performed. In this way estimations of the carrier diffusion length have been obtained as well as indications about the nanowire crystal quality and structure. This work opens the avenue for the use of doped GaAs nanowires in advanced applications and in mesoscopic physics experiments. [1] C. Colombo, D. Spirkoska, M. Frimmer, G. Abstreiter, A. Fontcuberta i Morral, Phys. Rev. B, 77, 155326 (2008); [2] J. Dufouleur, C. Colombo, T. Garma, B. Ketterer, E. Uccelli, M. Nicotra, A. Fontcuberta i Morral, Nano Lett. 10(5), (2010);[3] B. Ketterer, E. Mikheev, E. Uccelli and A. Fontcuberta i Morral, Appl. Phys. Lett. Asap.
9:00 PM - EE6.16
Negative Differential Resistance in Photocurrent in Core-shell Semiconductor Nanowires.
Guannan Chen 1 , Eric Gallo 1 , Paola Prete 2 , Nico Lovergine 3 , Jonathan Spanier 1
1 Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 , Consiglio Nazionale delle Ricerche, Lecce Italy, 3 Department of Innovation Engineering, University of Salento, Lecce Italy
Show AbstractWe report on negative differential resistance (NDR) observed in the photocurrent in individual GaAs/AlxGa1−xAs (x = 0.30) core-shell nanowires (NWs) grown without dopant via metalorganic vapor phase epitaxy. Two- and three-terminal electron transport measurements of the NWs were collected in the temperature range of 4K < T < 350K under linearly-polarized laser excitation, lamp illumination, and under dark conditions. In addition to small (<1 pA) dark currents at room temperature, we observe invariance of the magnitude of photocurrent from 4K to 160K, suggesting that the transport of the photo-excited carriers in these devices is not limited by the usual sources of scattering. Several candidate mechanisms were considered. On the basis of our observation and analysis of tuning of the NDR feature in photocurrent via either electrostatic and optical means we propose that the feature can be explained by real space transfer of photo-excited electrons from core to shell. We discuss the functional aspects and implications of our prototype nanowire optoelectronic device for applications in highly-sensitive and gate-tunable low-power photodetection. Work supported by NSF-DMR.
9:00 PM - EE6.19
Microstructure Evolution and Development of Annealed Ni/Au Contacts to p-GaN Nanowires.
Andrew Herrero 1 , Paul Blanchard 1 , Devin Rourke 1 , Aric Sanders 1 , Matthew Brubaker 1 , Chris Dodson 1 , Norman Sanford 1 , Kristine Bertness 1
1 Optoelectronics Division, NIST, Boulder, Colorado, United States
Show AbstractThe development of Ni/Au contacts to p-GaN nanowires is examined. p-GaN nanowires, grown by catalyst-free MBE on Si(111), were randomly dispersed onto SiO2/Si substrates and subsequently contacted with a Ni/Au scheme to form 2-terminal test structures. Current-voltage (I-V) measurements of these p-GaN nanowire devices frequently exhibit a strong degradation after annealing in UHP N2/O2 at a temperature of approximately 550C for 10min. Control experiments performed using the same Ni/Au contact scheme and annealing conditions applied to MOCVD grown p-GaN films resulted in ohmic contacts. The difficulty in creating ohmic contacts and the degradation in the I-V response of the p-GaN nanowires is proposed to originate from the poor wetting behavior of Ni and Au on SiO2 and the three-dimensional morphology of the nanowires. Well-characterized n-type GaN nanowires were processed and tested alongside of the p-GaN nanowires in order to help determine if the cause of degradation was electrical or mechanical. The morphology of the annealed Ni/Au contacts on SiO2 and the p-GaN films was investigated using SEM, AFM, and XRD measurements. Variations in the Ni/Au thickness and surface pretreatment prior to metal deposition are examined with respect to their effect on the Ni/Au morphology and adhesion to SiO2. XRD measurements of the Ni/Au layers on p-GaN films and SiO2 before and after annealing at different temperatures showed that the phase formation and evolution of the crystallinity of the metal films are dependent on the substrate material. SEM of the Ni/Au films on SiO2 after annealing shows roughening and delamination of the metal films, the severity of which is dependent on the Ni/Au thickness. Use of different adhesion layers of Ti/Al/Ti and Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition eliminated the delamination of the Ni/Au films after annealing yet some degradation in the I-V response of the nanowires still occurred. In order to improve the physical contact of the Ni/Au film to the p-GaN nanowire, several devices that had degraded after annealing had platinum selectively deposited over the contact areas using a focused ion beam (FIB). The Pt deposition improved the contacts such that semi-linear I-V curves were observed when the p-GaN nanowire devices were tested. The I-V curves were linear at low voltages and displayed nonexponential I-V characteristics with the relationship I∝V2 at higher voltages. This behavior is characteristic of space-charge-limited conduction, which has been observed in GaN nanowires and is consistent with the nanowire morphology and device geometry used in this study. Plausible mechanisms will be discussed to explain the observed results.
9:00 PM - EE6.2
Inner Composition, Defects and Morphology of AlGaAs Nanowires Grown by Au-catalyzed MOVPE.
Paola Prete 1 , Benjamin Buick 2 , Pasquale Paiano 3 , Eugen Speiser 2 , Dorin Geiger 4 , Daniel Wolf 4 , Nico Lovergine 3 , Wolfgang Richter 2
1 Institute of Microelectronics and Microsystems (IMM), CNR, Lecce Italy, 2 Dept. of Physics, University of Rome 'Tor Vergata', Rome Italy, 3 Dept. of Innovation Engineering, University of Salento, Lecce Italy, 4 Triebenberg Laboratory, Institute of Structure Physics, Technische Universität Dresden, Dresden Germany
Show AbstractWe report on the inner compositional structure, defects content and morphology of AlGaAs nanowires grown by Au-catalyzed MOVPE. To date, few studies have been published on the properties of AlGaAs nanowires grown by either MBE [1,2] or MOVPE [3,4]. AlGaAs nanowires are difficult to growth because of the complexity of the (Al-Ga-Au) phase diagram driving the Vapor-Liquid-Solid (VLS) process; moreover, the melting temperatures of the Au-Ga and Au-Al eutectics (339°C and ~500°C, respectively) are very different. These facts, along with the different solubility and diffusion of Ga and Al atoms in liquid Au, affect the composition and homogeneity of AlGaAs nanowires [3].Few-micron long AlGaAs nanowires were grown on (111)B-GaAs wafers using trimethylgallium, trimethylaluminum (TMAl), and tertiarybutylarsine (TBAs). Au nanoparticles with 60-70 nm diameters were used as catalysts. Growth runs were performed in H2 under 50 mbar pressure at growth temperatures between 450°C and 575°C.Nanowires grown with an Al-fraction in the vapor x=0.50 turns kink-free with their major axis normal to the substrate surface and a tapered morphology. Nanowires grown under x=0.68 are often kinked, likely a result of Al-induced instabilities at the Au-catalyst/nanowire interface.The phonon spectra of single AlGaAs nanowires were measured by non-resonant Raman scattering with micrometer spatial resolution. The nanowire alloy stoichiometry was determined based on the composition-dependent frequencies of the GaAs- and AlAs-like transversal (TO) and longitudinal (LO) optical phonon peaks, with a <10% uncertainty on Al-content [5]. Systematic analyses of the Raman spectra indicated that Al incorporation into the nanowire material is obtained above 475°C and increases with growth temperature. Correspondingly, the nanowire axial growth rate increases well beyond that expected for GaAs nanowires [6]. Measurements at different positions along the axis of single nanowires revealed a two-fold compositional variation within the nanostructure. This evidenced that the nanowires possess an inner compositional structure, namely an Al-rich alloy core surrounded by a lower Al-content AlGaAs shell, a result ascribed to the coexisting axial (via VLS) and sidewall (via conventional vapor–solid) growth mechanisms. With increasing the Al-content the nanowire section monotonously changes from hexagonal (as for GaAs [6]) to triangular. High resolution TEM observations demonstrate that the nanowires are twinned, the twins leading to a characteristic zigzagged faceting of nanowire sidewalls; as result, 30°-rotations of entire nanowire portions around the [111] growth direction often occur.[1] Z.H. Wu, et al, Appl. Phys. Lett. 85 (2004) 657.[2] C. Chen, et al, Nano Lett. 7 (2007) 2584. [3] O. Ouattara, et al, Nano Lett. 7 (2007) 2859.[4] S. K. Lim, et al, Nano Lett. 8 (2008) 1386.[5] B. Buick, et al, Phys. Status Sol. B 247 (2010) 2027. [6] P. Paiano, et al., J. Appl. Phys. 100 (2006) 094305.
9:00 PM - EE6.20
Photoconductivity in Single AlN Nanowires by Subbandgap Excitation.
Ruei-San Chen 1 2 , Hsuan-Ming Huang 2 3 , Hsin-Yi Chen 3 4 , Li-Chyong Chen 4 , Kuei-Hsien Chen 2 4 , Ying-Jay Yang 3
1 Graduate Institute of Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan, 3 Graduate Institute of Electronics Engineering, National Taiwan University, Taipei Taiwan, 4 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan
Show AbstractPhotoconductivity of individual aluminum nitride (AlN) nanowires has been characterized using different subbandgap excitation sources. It is interesting that both positive (under 1.53 and 2.33 eV excitations) and negative (under 3.06 and 3.81 eV excitations) photocurrent responses are observed from the wide bandgap nitride nanowires. The negative photoconductivity, which is attributed to the presences of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to be the reason resulting in the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum. Understanding of the diverse photoconductivity and its molecular effect is of great importance on the development of energy-selective and highly sensitive nanowire photodetector of AlN in the visible and ultraviolet ranges.
9:00 PM - EE6.21
Axial Modulation of the Diameter within Individual GaN Nanowires.
Samuel Crawford 1 , Sung Keun Lim 1 , Silvija Gradecak 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractNanowires composed of III-V semiconductors are promising materials for a variety of optoelectronic applications due to the versatility they offer for device design and integration, including tunable bandgaps from infrared to ultraviolet and the potential for diverse architectures with axial and radial junctions. The ability to controllably modulate the diameter along the length of a nanowire further expands the realm of possible nanowire device architectures, such as nanowires containing isolated segments of controlled length with radial quantum confinement. Using Au-particle-mediated nanowire growth via metal-organic chemical vapor deposition, we demonstrate the ability to modulate the diameter of GaN nanowires along the nanowire length. The diameter of each segment is determined by the size and wetting angle of the Au seed particle, which we control here by adjusting the flows of both source and carrier gases, while the length of each segment is controlled by the growth time. Using scanning electron microscopy and transmission electron microscopy coupled with energy dispersive x-ray spectroscopy, together with modeling of the diameter-dependent growth rates of individual nanowire segments, we elucidate the fundamental processes governing the nanowire growth and describe the effects of both source and carrier gases on nanowire diameter and growth kinetics. The group-III source increases the supersaturation of Ga in the seed, while the group-V source increases the consumption rate of Ga from the seed, and the carrier gas composition affects the thermodynamic equilibrium. The conditions favorable for achieving significant diameter fluctuations are explained and are used to achieve diameter ratios up to a factor of two within a single nanowire. This ability to axially modulate the diameter of nanowires opens up new opportunities in nanowire device design, and the description of the mechanism provides a fundamental understanding of how such diameter fluctuations can be controlled.
9:00 PM - EE6.22
Effect of AlN Buffer Layer and Nitrogen Plasma Conditions on the Growth of GaN Nanowires by Plasma Assisted Molecular Beam Epitaxy.
Matt Brubaker 1 3 , Albert Davydov 2 , Igor Levin 2 , Devin Rourke 1 , Kris Bertness 1
1 Optoelectronics Division, Precision Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado, United States, 3 Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado, United States, 2 Metallurgy Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractGaN nanowires were grown under N-rich conditions on Si (111) substrates coated with a thin epitaxial AlN buffer layer. The morphology of GaN nanowires was observed to depend significantly on the underlying AlN buffer layer, which was grown at 615○C with III-V flux ratios spanning N-rich to Al-rich growth conditions. The fractional surface coverage, diameter, and irregularity of the nanowire cross sections increase with the Al/N flux ratio during the buffer layer deposition, particularly near the stoichiometric condition. A companion set of thicker AlN samples with nominally identical Al/N flux ratios was prepared for polarity sensitive etching in hot phosphoric acid. AFM images of the etched AlN samples reveal slow etching regions that also increase in surface coverage, diameter, and irregularity with the Al/N flux ratio, similar to the associated nanowire samples. The slow etching regions are typically assigned to Al-polar AlN and are embedded in a matrix of fast etching material. In some cases this fast etching material reveals pyramidal structures that are consistent with N-polar AlN. Based on these results we tentatively propose that the AlN buffer layers exhibit mixed polarity and that the GaN nanowires grow preferentially on the Al-polar regions. GaN nanowire samples were also fabricated using various rf plasma powers and substrate temperatures. The relative concentrations of active nitrogen species in the plasma source and at the substrate were measured with Optical Emission Spectroscopy (OES) and Quadrupole Mass Spectrometry (QMS), respectively. At higher atomic nitrogen fluxes the growth rate of the surrounding matrix layer could be suppressed far below that of the nanowire growth rate. This result is consistent with reports of decomposition limited growth of N-polar GaN at high temperatures and high atomic nitrogen fluxes, presuming that GaN growth proceeds with the same polarity as the underlying AlN buffer layer.
9:00 PM - EE6.23
Effect of Ti/Al Contact Alloying on GaN Nanowire Resistance.
Paul Blanchard 1 , Kris Bertness 1 , Todd Harvey 1 , Mary Rowe 1 , Aric Sanders 1 , Norman Sanford 1
1 , NIST, Boulder, Colorado, United States
Show Abstract Because they are essentially free of defects and strain, GaN nanowires (NWs) grown by catalyst-free molecular beam epitaxy (MBE) are ideal candidates for efficient nanoscale optoelectronic devices. In order to optimize such devices, reliable and well-characterized ohmic contacts are essential. Over the past few years, NW 4-point and transmission line test structures have been reported. However, interpreting 4-point measurements on MBE-grown GaN NWs is challenging, because the limited NW device length means that the sections of NW under the contacts become important. It has been reported that annealed Ti-based contacts on GaN can cause increased n-type doping under the contact due to nitrogen out-diffusion. Hence, a significant ambiguity in GaN NW 4-point measurements is the question of whether the NW resistivity under the contacts is altered by the contact anneal.In this report, we investigate the effect of Ti/Al ohmic contact alloying on the resistance of GaN NWs in 4-contact structures. The NWs were Si-doped (ND ~3-7×1017 cm-3) GaN grown by MBE (lengths ~15-20 μm, diameters ~200-400 nm). NWs were dispersed onto a SiO2 substrate before 20 nm Ti/200 nm Al end contacts (C1 and C4) were deposited and annealed at 500 oC for 60 s in 5% H2/Ar. Next, two additional 20 nm Ti/200 nm Al contacts (C2 and C3) were deposited between C1 and C4. C2 and C3 were annealed at 500 oC for 60 s in 5% H2/Ar, then selectively etched away via HCl-based wet etchant while C1 and C4 were preserved by a photoresist mask.The effect of annealing C2 and C3 was tested by measuring the resistance R14 between C1 and C4 three times: before annealing C2 and C3 (R14A), after annealing C2 and C3 (R14B), and after removing C2 and C3 via wet etch (R14C). For these NW devices, R14A was between 30 and 120 kΩ, and R14B was consistently 20-25% lower than R14A. Two-contact NW control structures without the presence of C2 and C3 showed no such decrease in R14 after this anneal. Interestingly, in the four-contact structures, we found that R14C differed from R14A by less than ±5%; that is, etching away C2 and C3 caused R14 to rise back to almost exactly its value from before the anneal of C2 and C3. Control structures subjected to the etch without the presence of C2 and C3 showed no increase in R14C relative to R14B.Attempts to extract NW resistivity and contact resistivity from 4-point measurements of these devices are ongoing, but preliminary modeling shows that current shunting through C2 and C3 could account for the decrease in R14B relative to R14A, even if the NW resistivity is unaltered. These results suggest that any changes in the NW resistivity under the alloyed Ti/Al contact are limited to a tiny fraction of the total diameter of the NW, and do not affect the bulk resistivity. Ongoing work with NWs of varying diameters could help to illuminate any near-surface resistivity changes that do occur.
9:00 PM - EE6.25
V-L-S Growth of CdTe Nanowires on Mo Foil for Solar Cell Applications.
Benjamin Williams 1 , Ken Durose 1
1 Physics, Durham University, Durham United Kingdom
Show AbstractThis work presents an experimental study of CdTe nanowire growth aimed at generating structures for use in solar cells based on flexible metal foil substrates. V-L-S growth of Au catalysed CdTe wires on Mo foil substrates has been successfully demonstrated. For this study the vapour source was provided in the ‘close space sublimation’ geometry which is widely used in conventional CdTe solar cell fabrication. An important design feature of a nanowire solar cell is that the junction partner layer to the nanowires should not form a short circuit by direct contact with the substrate. Conditions encouraging the formation of a continuous coating on the substrate (in addition to nanowires) were therefore sought.Mo foil substrates were coated with ~4 nm evaporated Au and annealed under N2 for 30 mins. CdTe was deposited on such substrates held in the range 480 – 520°C under 10 – 150 Torr of N2 for times of up to 60 mins. The influence of these conditions on the morphology of the resulting layers, structures and nanowires as determined by FEG-SEM is discussed. In particular the transition from film growth to nanowire growth as the N2 pressure was reduced is described. Nanowires having diameters in the range 100 – 500 nm and length of up to 100 µm were generated. These were further characterised by TEM, PL and optical reflectance measurements. Prospects for incorporating these layer/nanowire combination structures into flexible II-VI heterojunction and organic/inorganic hybrid solar cells are discussed.
9:00 PM - EE6.28
Features of Topological Transitions and Size Effects in the Strain Dependences of Thermopower and Resistance in Nanowires Based on Bi and Its Alloys.
Albina Nikolaeva 1 2 , Leonid Konopko 1 2 , Gheorghe Para 1 , Anna Tsurkan 1 , Oxana Botnary 1
1 , Institute of Electronic Ingineering and Industrial Technologies, Chisinau Moldova (the Republic of), 2 , International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw Poland
Show AbstractIn this work, we studied the strain dependences of the resistance and thermopower of nanowires of Bi and its alloys with Sn and Te in conditions of an anisotropic elastic strain with record values of 2-3% elongation.Glass-coated wires with diameters ranging from 50 nm to a few microns were prepared by liquid phase casting; they were single crystals with a strictly cylindrical shape with diameters ranging from 50 nm to 1 μm. The change in the Fermi surface was recorded using Shubnikov de Haas oscillations.It was shown that anomalies in the strain dependences of thermopower are not observed in pure Bi wires; they are observed only in doped Bi wires in electronic topological transitions (T < 10 K), which is attributed to the appearance of a selective scattering channel (interband scattering in the heavy T-band). It is known that the interband scattering in pure bismuth at temperatures below 43 K is negligible. In the Lifshits theory [1], the selective carrier scattering can occur in new cavities that appear at a certain critical value. The partial removal of degeneracy at higher temperatures leads to a reduction in the thermopower anomalies. It is shown that the selectivity of electron scattering in wires of Bi-Te and Bi-Sn alloys is removed also by the rise of the Fermi level above the band bottom, which corresponds to the exit from the anomaly in the range of high concentrations.It is found that the nonmonotonic dependence of resistance on stretching R(ξ) is attributed to topological transitions, and its fundamental difference from similar curves for diameters less than 80 nm is explained by the manifestation of a quantum size effect.An anomalous behavior of the transverse and longitudinal magnetoresistance and the magnetothermopower is found in quantum wires under elastic strain; it is associated with both classical and quantum size effects.This work was supported by the SCOPES project no. IZ73Z0_1 27968 and no. 09.819.05.05F1. I.M. Lifshits, Zh. Exp. Teor. Fiz., vol. 38, no. 5, pp. 1569-1576, (1906).
9:00 PM - EE6.29
Emergent Semiconductor Properties in Selenium Nanowire Arrays Fabricated by Direct Size Reduction Technique.
Erol Ozgur 1 , Mecit Yaman 1 , Ozan Aktas 1 2 , Mehmet Bayindir 1 2
1 UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara Turkey, 2 Physics Department, Bilkent University, Ankara Turkey
Show AbstractElemental selenium has interesting thermomechanical, electrical and optical characteristics such as the convenience of thermal drawing to obtain fibers, phase dependent electrical conductivity, light induced crystallization, and photoconductivity. Therefore it is an excellent contender for novel phase change memory, optoelectronics and photodetection purposes, provided that the fabrication process is carefully designed and established according to the demand of the particular application. We have fabricated selenium nanowire arrays by direct size reduction technique, which is an innovative nanoscale fabrication method recently developed in our laboratories, enabling us to produce millions of indefinitely long, ordered, and hermetically encapsulated nanowire and nanotube arrays of various materials including glasses, metals and polymers, by iterative thermal drawing. After each fiber drawing step, we obtain an array of parallel selenium nanowires with shrinking size and increasing number of individual wires, all within a tight polymer envelope. The fabrication practice, bridging the macroscopic world to nanoscale, allows the observation of size dependent alterations in the material characteristics besides supplying state-of-the-art tools for the semiconductor field.We compared selenium micro- and nanowire arrays in terms of their semiconductor properties, particularly of photoconductivity. Following crystallization of selenium by heat or organic solvent treatment and verification of crystallization via XRD and TEM, we measured the transient electrical conductivity changes regarding light exposure. We observed a three order of magnitude increase in conductivity of the nanowire array concurring with the light exposure, which is the highest value reported. The change in photoconductivity is shown to correlate with the reduced diameter. The relaxation time after each individual light exposure is reduced, producing faster switching response towards light. Selenium nanowire arrays fabricated by direct size reduction technique; therefore, might lead to unprecedented applications in photodetection and optoelectronics areas.
9:00 PM - EE6.3
Diameter Dependent Current-Voltage Characteristics of InSb Nanowires.
Miroslav Penchev 1 , Jiebin Zhong 2 , Mihri Ozkan 1 , Cengiz Ozkan 2
1 Electrical Engineering, University of California, Riverside, Riverside, California, United States, 2 Mechanical Engineering, University of California, Riverside, Riverside, California, United States
Show AbstractSingle crystalline Indium Antimonide (InSb) nanowires were synthesized by chemical vapor deposition (CVD) technique, using Au particles as catalyst, via a vapor liquid solid mechanism. Structural properties of the as-grown InSb nanowires were investigated by AFM, SEM and TEM analysis. Nanowire Field Effect Transistors (NWFETs) were fabricated in back-gate configuration using SiO2 as gate insulator, and top-gate configuration using high-k dielectric. The diameter of InSb nanowires used in the fabricated NWFETs varied from 10-60 nm. Current-voltage measurements were conducted to determine the dependence of NWFETs parameters on the InSb nanowire diameter. InSb NWFETs were n-type with electron carrier concentration of ~1019, and exhibited very high Ion/Ioff current ratio of ~106. Carrier mobility was shown to decrease with decrease of nanowire diameter. Temperature dependant current-voltage measurements were also conducted to determine the effect of operating temperature on the InSb NWFET device performance.
9:00 PM - EE6.30
Enhanced Field-emission and Red Lasing of Ordered CdSe Nanowire Branched Arrays.
Guohua Li 1 2 , Tianyou Zhai 3 , Yang Jiang 1 , Yoshio Bando 3 , Dmitri Golberg 3
1 Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, China, 2 Dept. Chem. Eng., UC Berkeley, Berkeley, California, United States, 3 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractHighly ordered CdSe nanowire branched arrays were designed and synthesized while merging two particular structural features within a single nanomaterial. This novel CdSe nanostructure combines a branched structure and highly ordered single-crystalline character. The stems and branches consist of wurtzite CdSe single crystals. The low turn-on field, 4.3 V µm-1 for the current densities of 10 µA cm–2, high field-enhancement factor (1160), and long emission stability indicate that these CdSe novel nanostructures could potentially be used as field emitters. The decent FE performance is due to the unique morphology of CdSe, e.g. high structural order, branched structure, perfect single-crystallinity, and tapered nanotips.In addition, lasing in a red range, 700 to 720 nm, of the ordered CdSe nanowire branched arrays were demonstrated. The resonant emission features correlate with longitudinal Fabry-Perot modes, which result from quasi-cylindrical cavity geometry of the single-crystalline CdSe nanowires.
9:00 PM - EE6.32
Magnetic Quantum Oscillations from Surface States of Bi Nanowires.
Leonid Konopko 1 2 , Tito Huber 3 , Albina Nikolaeva 1 2
1 , Institute of Electronic Engineering and Industrial Technologies, Chisinau Moldova (the Republic of), 2 , International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw Poland, 3 Department of Chemistry, Howard University, Washington, District of Columbia, United States
Show AbstractIn this work, we discuss our results of studying the transverse magnetoresistance (MR) of single-crystal Bi nanowires with a diameter d < 80 nm. The nanowire samples with glass coating were prepared by an improved Ulitovsky technique; they were cylindrical single crystals with the (1011) orientation along the wire axis. Semimetal bismuth has electrons and holes with extremely low effective masses; as a result, electronic quantum confinement effects induce a semimetal-to-semiconductor transformation for wires with diameters below 50 nm. Angle-resolved photoemission spectroscopy studies of planar surfaces of Bi showed that it supports surface states (with carrier densities Σ of around 5×1012 cm-2) with strong spin-orbit interactions. The surface carriers become majority in nanowires with diameters below 100 nm at low temperatures. At that point, the Bi nanowire should effectively become a conducting tube. Nonmonotonic changes of transverse MR that are equidistant in the direct magnetic field were observed at low temperatures in a wide range of magnetic fields up to 14 T. The period of oscillations ΔB≈Φ0/S=(h/e)/(πd2/4) depends on the wire diameter d as for the case of longitudinal MR. There are equidistant oscillations of MR on a direct magnetic field under conditions where the magnetic flux through the cylinder Φ = 0. The period of oscillations in a wide range of angles (~50o) keeps a constant value. The amplitude of oscillations depends on the angle θ between the direction of applied magnetic field B and the C3 nanowire axis, increasing in intensity with increasing angle θ. At angles θ > 50o the nonmonotonic changes of transverse MR lose their periodicity in a direct magnetic field. Earlier [1, 2], we observed the oscillations of longitudinal MR of Bi nanowires with two periods proportional to h/e and h/2e. From B ~ 8 T down to B = 0 the extrema of the h/2e oscillation was shifted up to 3π at B = 0, which was the manifestation of the Berry phase shift due to electron moving in the nonuniform magnetic field BΣ=B+BSO, where BSO is the Zeeman-like effective magnetic field. The fact that the h/2e oscillations exhibit a phase shift only in one direction means that, like in topological insulators, the surface states of Bi nanowires have only one spin degree of freedom. Probably, transverse MR oscillations arise due to the topological nature of Bi nanowire surface states.This work was partially supported by STCU Grant no. 5050 and SCOPES Grant no. IZ73ZO_127968.1.A. Nikolaeva, T. Huber, D. Gitsu, and L. Konopko, Phys. Rev. B 77, 075332 (2008).2.L. Konopko, T. Huber, and A. Nikolaeva, J Low Temp. Phys. 158, 523 (2010).
9:00 PM - EE6.33
In-fiber Chalcogenide Nanowire Arrays for Nonlinear Photonics Applications.
Tural Khudiyev 1 , Reha Ozalp 1 , Ekin Ozgur 1 , Mecit Yaman 1 , Mehmet Bayindir 1 2
1 , UNAM-Institute of Materials Science and Nanotechnology,Bilkent, Ankara Turkey, 2 Physics, Bilkent, Ankara Turkey
Show AbstractUtilizing common promises of the nano scale such as low power consumption, high speed and high density packing, chalcogenide nanowires (S, Se, Te containing compounds) have found wide applications in non-volatile phase change memory. On the other hand, because these glasses generally have exceptionally high nonlinear optical constants, they have great potential in nonlinear photonics applications such as optical switching and supercontinuum generation. Fabricating complex nanowire structures of the chalcogenide glasses can help improve their performances and result in novel functionalities. Here, we report first fabrication of the ordered long arrays of chalcogenide nanowires by thermal drawing that is well suited for extreme supercontinuum generation. The structures offer high power throughput and broad spectrum generation. Supercontinuum generation is mainly sought to be used in photodynamic therapy, optical tomography, telecommunication, spectroscopy, optical frequency metrology and for military applications. Our fiber embedded nanowire structure is thermally drawn from a composite, that has a arsenic triselenide core, an isolation polymer layer (poly vinyl fluoride) and a wrapping second jacket polymer (poly ether sulphone). Arsenic triselenide is a chalcogenide glass with a very high nonlinear index (80,000x that of silica). After multiple thermal drawing steps, hundreds of sub-micron arsenic triselenide nanowires are obtained in well-ordered geometry and for extended lengths. Specifically, we obtained a 30 meter fiber that has a minimum diameter of 35 micron embedded with 360 nanowires with 65 nm diameter to maximum 400 micron diameter embedded with 360 nanowires with 950 nm diameter. In this way, sub-micron nanowires are obtained such that a customized polymeric layer reduces mode coupling between nanowires to below 0.1%, and distortion of pulse shape due to dispersion is minimized which are critical parameters for super-continuum generation. SEM images of the fiber indicate that by controlling temeperature, and other fiber drawing parameters it is possible to produce extended lengths of ordered optically nonlinear nanowire arrays in polymer fibers. Our simulations indicate that, within the fiber embedded nanowires, a spectral broadening of 900 nm to 3 micrometer width, can be achieved around 1.55 micrometer pump wavelength with 150 fs, 50 W peak power laser pulses. For the simulations we used 50 micrometer to 2 cm nanowires. Fiber embedded geometry is a novel structure that probably has great potential for nonlinear nanowire photonics. In-fiber nanowire array enable, high power, wavelength scalable supercontinuum generation, and easy handling and coupling to the nanowires.
9:00 PM - EE6.36
Single Nanowire Absorption Spectroscopy: Breaking through the Limitations of Ensemble Measurements.
Jay Giblin 1 , Masaru Kuno 1
1 Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana, United States
Show Abstract Molecules and nanoparticles exhibit unique properties such as emission intermittency (i.e. blinking) and spectral wandering which are only observable at the single molecule level. Until more recently, single molecule detection has been mainly restricted to fluorescent entities. Now techniques such as photothermal imaging and spatial modulation allow single molecules and nanoparticles to be detected, regardless of their ability to emit light. This opens up the possibility of discovering new absorptive properties masked in ensemble measurements. Here we present the use of spatial modulation techniques to obtain single CdSe NW extinction spectra across the visible. The CdSe NWs used in this study were synthesized via solution-liquid-solid (SLS) growth. The preparation yields highly crystalline, straight wires. Ensemble NW radii were varied between 2.5-5 nm and lengths readily exceeded 1μm. Single NW extinction spectra were obtained on a homebuilt system assembled off of a commercial inverted microscope. The excitation source was the output of a supercontinuum laser dispersed by a homebuilt double prism monochromator. The position of a single NW was modulated in a focused beam using a piezoelectric stage. The transmitted light was collected using a second objective and was detected by an auto-balanced photodiode. The output signal from the photodiode was then fed into a lock-in amplifier. Extinction spectra of single CdSe NW were recorded from 500 nm to 750 nm along with their accompanying emission spectra. Resulting data reveal structure in the extinction. These measurements also reveal (apparent) single NW Stokes shifts on the order of 20 meV. Because the direct extinction of NWs is measured we can also report direct extinction cross sections. Values are on the order of 10-10-10-11 cm2μm-1 for parallel excitation. This is in good agreement with predictions of both local field approximations and more rigorous Poynting vector calculations. In addition, apparent bleaching of the band edge is seen at large excitation intensities. Little or no bleaching is seen for wavelengths to the blue of the band edge. This study demonstrates the possibilities of direct absorption techniques for studying the absorptive properties of single nanostructures.
9:00 PM - EE6.37
O2 –Manipulated Synthesis of CdS Nanorods Inspired by Nanoseparation.
Xiaoming Sun 1 , Xiuju Ma 1 , Lu Bai 1 , Junfeng Liu 1 , Zheng Chang 1 , Xue Duan 1 , Joseph Chiang 2
1 , State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing China, 2 , Department of Chemistry and Biochemistry, SUNY- Oneonta, Oneonta, New York, United States
Show AbstractCdS nanorods can be sorted by length using density gradient ultracentrifuge rate separation method. The fractions containing longer rods showed relatively stronger oxygen related surface trap emission, while the shorter ones had dominant band-edge emission. Inspired by this, different synthetic environments (N2, air and O2) was applied to tailor the length distribution. It is surprising to our finding the phases and photoluminescence properties of CdS nanorods could also be manipulated simultaneously, in addition to the rod length. Increasing the oxygen partial pressure significantly changed the growth behavior of CdS nanorods by converting the zinc-blende phase to wurtzite and improving the anisotropic growth from 3D to 1D. A very simple Empty-Ratio-Control method was thus developed based on above results.
9:00 PM - EE6.39
Nucleobase-mediated Synthesis of CdS Nanostructures.
Kiran Challa 1 , Hun Hoe Heo 1 , Eui-Tae Kim 1
1 Material Science & Engineering., Chungnam National University., Daejeon, -, Korea (the Republic of)
Show AbstractIt is widely recognized that nucleic acids have the ability to control the growth and morphology of inorganic nanoparticles. Nowadays, the use of nucleic acids and the biopolymers as templates for the synthesis of nanostructures such as nanoparticles, nanowires and nanotubes has become an active area of study. The synthesis process of inorganic materials by the formation of a precursor complex via specific interactions between metal or metal ion and a bio template is called biomineralization process. As an important II-IV semiconductor, CdS has been considered as a perspective optoelectronic material for utilization in nonlinear optical devices, light-emitting diodes, because of its intermediate bandgap energy (2.42 eV) and lower work function value (4.2 eV). Here, we report a simple biomineralization process for the synthesis of CdS one-dimensional nanostructures and nanoparticles by using aqueous solutions of cadmium acetate and thiourea with the introduction of RNA related nucleobase at nearly room temperature. These synthesized CdS nanostructures have the high order of fluorescence characteristics and aqueous solubility. We also investigated the influences of experimental parameters including the precursor chemical concentrations; solution pH, reaction time, and reaction temperature for propose of the formation mechanism of these CdS nanostructures. Empty orbital of the metal cations can accept electrons from the nucleobase moieties forming coordination complexes and acts as capping agents and templates for the formation of CdS nanostructures. Variations in pH have been shown to affect the formation of nanostructures and also on the emissive properties. These developed nanostructures were systematically characterized by electron microscopes, X-ray diffraction, photoluminescence, and absorbance spectroscopy techniques for better understanding the growth mechanism. Due to of having fluorescence characteristics, these synthesized CdS nanostructures hoping to be considered as perspective material for utilization in light emitting diodes. Another advantage of nucleobases as ligands is the use of in vitro evolutionary techniques to identify suitable templating sequences. We will further discuss the deep information about the synthesis, formation mechanism and application of these synthesized CdS nanostructures
9:00 PM - EE6.4
Growth Behaviors and Characterizations of Epitaxial InSb Nanowires by Chemical Vapor Deposition: Substrate, Diameter and Temperature Effects.
Jiebin Zhong 1 , Jian Lin 1 , Miroslav Penchev 2 , Maziar Ghazinejad 1 , Mihri Ozkan 2 , Cengiz Ozkan 1
1 Mechanical Engineering, University of California Riverside, Riverside, California, United States, 2 Electrical Engineering, University of California Riverside, Riverside, California, United States
Show AbstractIndium antimonide (InSb) nanowires (NWs) are attractive nano-building blocks for high-speed and low-power electronics, infrared detectors and thermal-electric (TE) devices. In this report, we experimental investigated the growth behaviors and characterizations of Au-assisted epitaxial InSb NWs by Chemical Vapor Deposition (CVD) technique, influenced by substrate, diameter and growth temperature history. First of all, InSb NWs were grown on various III-V substrates such as InSb (001), InSb (111), InP (001), InAs (001) and ITO/glass substrates by CVD. Analyzed by AFM, SEM and TEM, InSb NWs preferential growth direction and morphology were affected by NWs diameter, growth temperature and temperature history. More importantly, by optimizing the growth parameters, preliminary results of in situ planar growth of InSb NWs on InSb (001) substrate was presented and possible growth mechanism was discussed based on the growth behaviors. Finally, the NW had been fabricated as single NW-Field Effect Transistor (NWFET), to study the electrical property and optical properties were studied by UV-Vis-NIR spectrophotometer.These experimental findings provide comprehensive review of control over the growth of InSb NWs and open opportunities for semiconductor NWs array based nano-electronics, photonic applications and thermal-electric devices.
9:00 PM - EE6.5
Gallium Phosphide Nanowires for Photoelectrochemical Solar Energy Conversion.
Wen Wen 1 , Stephen Maldonado 1
1 Chemistry, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractSplitting water with a photoelectrochemical cell requires semiconductors with a bandgap large enough to overcome the thermodynamic and kinetic barriers (>1.7 V) in hydrogen evolution and water oxidation. Gallium phosphide (GaP) has a bandgap of 2.26 eV and is a candidate material for constructing solar-powered fuel-forming systems. However, the short carrier diffusion length, relative to the depth of light absorption in GaP is a key disadvantage of utilizing the traditional planar structure as an effective photoelectrode. In this poster presentation, we explore GaP nanostructures with high aspect ratio to decouple the optical and electrical collection vectors. Specifically, we show data for nanowire (NW) films that operate in photoelectrochemical systems as either photoanodes or photocathodes. GaP NWs were grown from a solid source on the GaP (111) substrate by chemical vapor deposition (CVD). The activity of as-prepared materials as photoelectrodes is demonstrated in aqueous and non-aqueous regenerative photoelectrochemical cell. To further enhance the photoresponse in the visible region beyond the bandgap limit of 550 nm, we show data for GaP NW films after incorporating nitrogen. The incorporation of a significant amount nitrogen into GaP NWs lattice without severely simultaneous destruction of the crystallinity is achieved by careful high temperature treatments in ammonia rich ambients. Data are presented from Raman spectroscopy, X ray diffraction (XRD), X-ray photon spectroscopy (XPS), scanning electron microscopy (SEM) transmission electron microscopy (TEM), steady-state photoelectrochemical analyses.
9:00 PM - EE6.6
Insights into Growth Mechanisms of Self-catalyzed GaAs Nanowires Grown by Molecular Beam Epitaxy.
A. Mazid Munshi 1 , Dheeraj L. Dasa 1 , Jelena Todorovic 2 , Antonius T. J. van helvoort 2 , Bjørn-Ove Fimland 1 , Helge Weman 1
1 Department of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim Norway, 2 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway
Show AbstractSemiconductor nanowires (NWs) grown by Au-catalyzed vapor-liquid-solid (VLS) growth mechanism have attracted a great deal of interest due to their potential in nanoscale electronics and optoelectronics devices. However, in many cases, it has been realized that the extrinsic metal catalyst particle is undesirable as it might get incorporated in the NW during growth, resulting in unwanted deep level traps, thereby deteriorate the optical and electrical properties. Furthermore, not all III-V NW heterostructures with technological potential can be grown by Au-catalyzed VLS technique. Such systems would require changing the composition of catalyst particle [1]. One of the alternatives to avoid these problems is to use a self-catalyzed growth method [2]. This method has the added advantage of a complete ceasing of the axial growth of the NW while growing in the radial direction. To exploit such NWs for future device applications it is essential to understand the growth mechanism and to have control over the morphology and phase purity. Here we demonstrate the growth of self-catalyzed GaAs NWs on p-Si(111) substrates using molecular beam epitaxy (MBE). It has been observed that the surface preparation method, growth temperature and V/III ratio determine the growth rate of the 2D layer on the substrate, as well as the orientation, morphology, density and crystal phase of the NWs. A detailed systematic study was performed by varying these growth parameters to understand the underlying mechanism. We demonstrate that by adapting a two-condition growth technique we are able to control the density and morphology of the NWs and the growth rate of the 2D layer on the substrate, at the same time. The effects of these growth conditions on the evolution of the NWs will be discussed. A dedicated growth series has been designed to gain insight in the role of group V adatoms on the NW growth. It has been found that the group V adatoms is an important parameter that affects the crystal phase of NWs and needs to be understood for band gap engineering via crystal phase controlled growth. [1] K.A. Dick, S. Kodambaka, M.C. Reuter, K. Deppert, L. Samuelson, W. Seifert, F.M. Ross, Nano Lett. 7 (2007) 1817.[2] Eric A. Stach, Peter J. Pauzauskie, Tevye Kuykendall, Joshua Goldberger, Rongrui He, Peidong Yang, Nano Lett. 3 (2003) 867.
9:00 PM - EE6.8
Scalable MOCVD Nanowire Growth of Heterostructured Nanowires for Solar Energy Conversion.
Anuj Madaria 1 , Maoqing Yao 2 , Chung-Yung Chi 2 , Daniel Dapkus 2 , Chongwu Zhou 2
1 Material Science, USC, Los Angeles, California, United States, 2 Electrical Engineering, USC, Los Angeles, California, United States
Show AbstractDuring the past decade, nanomaterials have emerged as a building block for constructing next generation of photovoltaic devices. Nanowire based semiconductor solar cells, among other candidates, have shown potential to produce high efficiency solar cells. Here we report a scalable method of fabricating vertical III-V semiconducting nanowires using Selected Area Metal-Organic Chemical Vapor Deposition (SA-MOCVD) technique which can find application in various optoelectronic devices. We used both nanosphere lithography and diblock copolymer patterning techniques to obtain highly ordered pattern for SA-MOCVD and demonstrate wafer-scale fabrication of GaAs and InAs nanowires nanowire on different substrates. We also compared various optical and electrical properties of these nanowires with the nanowires grown by using E-beam patterned substrates which is a more conventional but expensive and slow techniques. In case of nanosphere lithography technique, the packing density and the diameter of these nanowires can be easily tuned by changing the diameter of the nanospheres and the etching times and, in case of diblock copolymer, it can be changed by the type of copolymer used. These heterostructured nanowires can also be used for solar energy conversion which we demonstrated.
Symposium Organizers
Volker Schmidt Max Planck Institute of Microstructure Physics
LincolnJ. Lauhon Northwestern University
Takashi Fukui Hokkaido University
GeorgeT. Wang Sandia National Laboratories
Mikael Bjoerk IBM Research GmbH
Symposium Support
AIXTRON SE
FEI Company
IBM Research Zurich
JEOL USA
Veeco
EE9: Poster Session: Oxide Nanowires
Session Chairs
Thursday PM, April 28, 2011
Salons 7-9 (Marriott)
EE7: III-V (Non-Nitride) Nanowires II
Session Chairs
Thursday PM, April 28, 2011
Room 3001 (Moscone West)
9:00 AM - **EE7.1
III-V Nanowire Array Solar Cells.
Knut Deppert 1 , Frank Dimroth 2 , Peter Kailuweit 2 , Martin Magnusson 3 , Guenther Bauer 4 , Julian Stangl 4 , Rafal Dunin-Borkowski 5 , Jakob Wagner 5 , Bernd Witzigmann 6 , Lars Samuelson 1 , Magnus Borgstroem 1
1 Solid State Physics, Lund University, Lund Sweden, 2 , Fraunhofer Institute for Solar Energy Systems, Freiburg Germany, 3 , SOL Voltaics AB, Lund Sweden, 4 Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Linz Austria, 5 Center for Electron Nanoscopy, Technical University of Denmark, Lyngby Denmark, 6 Center for Interdisciplinary Nanostructure Science and Technology, University of Kassel, Kassel Germany
Show AbstractSolar radiation is the most plentiful renewable energy source. Harvesting some portion of this enormous renewable energy source would offer to significantly reduce the burning of fossil fuel. The breakthrough for solar energy technology implementation has however been hampered by two issues: the conversion efficiency of light into electricity, and the cost for solar panel production. A route to improve efficiency was demonstrated by using multi-junction cells, where several junctions at different band-gaps lead to a better exploitation of the solar spectrum. However, these structure need to be grown on expensive Ge or GaAs substrates, violating the demand for reduced cost. The use of III-V compound nanowires (NWs) in photovoltaics allows to respond to both demands at the same time: They offer efficient light absorption and significant cost reduction. These low dimensional structures can be grown epitaxially in dense NW arrays directly on silicon wafers, which are abundant and cheaper than. For planar structures, lattice matching poses a strong restriction on growth. III-V nanowires, in contrast, offer the possibility to create highly efficient multi-junction devices since multiple materials can be combined to match the solar spectrum without the need of tightly controlled lattice-matching. At the same time less material is required for NW-based solar cells than those based on planar architecture. This approach has the potential to reach more than 50 % in efficiency.The structures discussed in this paper ultimately aim to surpass the performance of traditional multi-junction photovoltaics (MJPV), while at the same time reducing manufacturing cost. The work is largely done within the EU-project AMON-RA, where the partners combine NW growth, device processing development, characterization and electro-optical modeling. The authors are working towards a tandem-junction NW device based on III-V materials on silicon substrates. The aim are NW containing two junctions, absorbing different parts of the solar spectrum, connected in series via a tunnel diode. We will demonstrate the growth of controlled doped GaAs and InP nanowires and the successful creation of both pn- and tunnel diode structures; necessary ingredients for efficient PV devices. Further, we will demonstrate total control of sidewall growth by in-situ etching in order to minimize leakage. First results of fully processed vertically integrated nanowire solar cells will be presented.
9:30 AM - EE7.2
Growth and Characterization of InAsSb Nanowires.
Mattias Borg 1 , Kimberly Dick 1 2 , Lars-Erik Wernersson 1 3
1 Department of Solid State Physics, Lund University, Lund, 0, Sweden, 2 Polymer & Materials Chemistry, Lund University, Lund Sweden, 3 Department of Electrical and Information Technology, Lund University, Lund, 0, Sweden
Show AbstractInAsSb, with a direct band gap tuneable from 3 to nearly 12µm, is a promising replacement for HgCdTe in infra-red detectors and light sources. These can be used for detection of environmentally important gases such as CO2, CO, CH4, N2O and O3. So far, the lack of lattice-matched substrates has been a major concern for the development of InAsSb optoelectronic devices. The lattice-constant of InAsSb varies considerably with composition, from 6.05Å (InAs) to 6.49Å (InSb). The choice of substrate is thus not trivial, and a high density of misfit dislocations is often present in films with significant Sb content. For this reason the vertical nanowire geometry is attractive, since it allows for strain relaxation without dislocation formation, even for very high lattice-mismatch. InAsSb nanowires may thus have greatly improved optical properties compared to InAsSb epitaxial layers. Here, we demonstrate for the first time the growth of Au-nucleated InAsSb nanowires by MOVPE. By varying the precursor V/III-ratio and TMIn molar fraction, the composition of the InAs1-xSbx nanowires is controlled over a wide range, between x=0.04 to 0.80 as measured by High Resolution X-ray Diffraction (XRD). High resolution XRD characterization using a lab-setup XRD system also indicates that Sb incorporation is radically enhanced in the nanowires compared to the simultaneously grown epi-layer. Further characterization of the InAsSb nanowires is performed by Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy. Both the morphology and crystal structure of the nanowires is investigated, and finally the solid composition is evaluated using a thermodynamical model.
9:45 AM - EE7.3
Structure, Mechanics, and Conductivity of III-V Semiconductor Nanowires Using Scanning Tunneling Microscopy.
Rainer Timm 1 , Martin Hjort 1 , Alexander Fian 1 , Magnus Borgstroem 1 , Jesper Wallentin 1 , Edvin Lundgren 1 , Lars Samuelson 1 , Anders Mikkelsen 1
1 The Nanometer Structure Consortium, Department of Physics, Lund University, Lund Sweden
Show AbstractFree-standing III-V semiconductor nanowires offer tremendous possibilities for device application in energy and information technology [1]. Recently, we have obtained first atomically resolved images of the interior crystal structure [2] as well as of the surface [3] of semiconductor nanowires by applying scanning tunneling microscopy (STM) to cleaved nanowire samples and to the side surfaces of suspended nanowires, respectively. Here, we combine STM and scanning tunneling spectroscopy (STS) measurements to simultaneously study the surface structure and local electronic properties of nanowire heterostructures, and we further extend the setup of the STM experiment to measure vibrational mechanics and conductivity of individual up-right standing nanowires. We will present STM and STS results on InP p-n-junction nanowires. STM images reflect a structural change of the nanowire at the interface, between that of a zincblende structure with periodic twinning (Zn p-doped) and a predominantly wurtzite structure with stacking faults (Sn n-doped). Corresponding STS spectra show a clear shift of the band edges for the n-type part, while the band alignment of the only weakly doped p-type part is similar to that of an undoped reference sample. Detailed STM imaging along several micrometers on InAs nanowires will be presented, revealing details on both wire atomic scale structure and long range surface morphology.While STM is usually performed on nanowires suspended on a suitable substrate, we will also show results obtained on free-standing wires. Although objects with such an extreme aspect ratio are challenging for STM, it is possible to image the nanowires. Furthermore, the STM is used to measure mechanical properties of vibrating InAs nanowires of several micron length under resonant high-frequency excitation, studying resonance frequencies and oscillation modes of individual nanowires [4].Finally, we will show initial conductivity measurements on individual free-standing nanowires. By establishing a point contact between the STM tip and the gold particle on top of the nanowire within ultrahigh vacuum the problems in contacting single nanowires in conventional setups can be overcome. This technique is demonstrated to study I-V characteristics of simple InAs and InP nanowires with different doping levels as well as for nanowire heterostructures.[1] Y. Li et al., Mater. Today 9 (10), 18 (2006).[2] A. Mikkelsen et al., Nature Mater. 3, 519 (2004); L. Ouattara et al., Nano Letters 7, 2859 (2007).[3] E. Hilner et al., Nano Letters 8, 3978 (2008). [4] A. Fian et al., Nano Letters 10, 3893 (2010).
10:00 AM - EE7.4
Surface Photovoltage Study of InP Nanowire Networks.
Andrew Lohn 1 3 , Jin-woo Han 2 , Nobuhiko Kobayashi 1 3
1 Baskin School of Engineering, University of California Santa Cruz, Santa Cruz, California, United States, 3 Nanostructured Energy Conversion Technology and Research (NECTAR), Advanced Studies Laboratories of NASA Ames Research Center, Moffett Field, California, United States, 2 Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California, United States
Show AbstractA wide range of physics related to single nanowires and multiple independent nanowires have been greatly explored in the past decade. Feasible device applications based on these two material building blocks are currently being sought extensively. Our unique vehicle within the context of implementing nanowires as a part of solid-state devices is an ensemble of group III-V compound semiconductor nanowires that are randomly-oriented and intersecting each other. These intersecting nanowires naturally form a three-dimensional network (nanowire network) through which mobile carriers travel. The nanowires are epitaxially integrated on a layer of non-single-crystal hydrogenated silicon prepared on an electrically insulating substrate, therefore, mobile carriers in the nanowire network find multiple paths during the process of spreading spatially, in particular, along the direction parallel to the substrate. Surface photovoltage is a well-developed but somewhat lesser-known technique for studying surface and bulk phenomenon. Surface states at a semiconductor interface create a space-charge region and a resulting surface voltage. Photoexcited carriers near the surface cause a redistribution of charges in the space charge region and a change in the surface potential; the change in the surface potential is termed surface photovoltage. This technique has been used to study a variety of properties and phenomena including minority carrier diffusion lengths and lifetimes, bulk-like defects, surface and interface states, surface heterogeneity and chemical changes at the surface. We applied an optical excitation technique by which surface voltage is measured over the nanowire network made of indium phosphide nanowires using a commercial apparatus with novel capabilities provided by its dual-electrode operation. A chopped excitation beam is shone through a transparent electrode that is surrounded by a separate electrode. The two electrodes are independently monitored to provide additional information about lateral diffusion of photogenerated carriers which when combined with varying chopping frequency gives temporal information relating to the ensemble-effects of nanowires.We observed substantial differences between the surface voltage collected from the nanowire network and that from the reference hydrogenated silicon surface both in terms of direct surface photovoltage effects and lateral diffusion effects. In this paper, we assess the origin of the difference in the surface voltage we observed within the context of mobile carrier diffusion through the nanowire network. Understanding the complex carrier transport in these three-dimensional nanowire networks will open new avenues in designing solid-state devices.
10:15 AM - EE7.5
Surface Passivation of Gallium Arsenide Nanowires.
Andrew Chia 1 , Ray LaPierre 1
1 Engineering Physics, McMaster University, Hamilton, Ontario, Canada
Show AbstractA numerical model of surface depletion in a gallium arsenide (GaAs) nanowire was constructed by solving Poisson's equation. This model demonstrated that nanowires of typical dimensions and doping densities are completely depleted by surface states. This, along with the presence of surface recombination, stresses the need for a wide bandgap passivation layer to be grown on GaAs nanowires for future optoelectronic applications. To address this need, n-doped GaAs nanowires with intrinsic aluminum indium phosphide (AlInP) shells were grown to form a Type-I, straddling heterojunction. The nanowires were then characterized by electron microscopy. Scanning electron microscopy (SEM) of the nanowire array was used to examine the dimensions and density of nanowires, while transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDX) of broken nanowires were used to study the AlInP shell thickness uniformity along the length of the wires, the AlInP shell composition and defects in the nanowire. Once morphological characterization was complete, nanowire resistor devices were fabricated and their current-voltage characteristics were examined. By comparing the resistances of the nanowires covered in AlInP with those without AlInP, the effect of this passivation layer was analyzed.
10:30 AM - EE7.6
Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires.
Aaron Katzenmeyer 1 , Francois Leonard 1 , A. Alec Talin 2 1 , Ping-Show Wong 3 , Diana Huffaker 3
1 , Sandia National Laboratories, Livermore, California, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 , UCLA, Los Angeles, California, United States
Show AbstractBulk GaAs crystals typically contain a large concentration of deep traps with EL2 being the most frequently observed defect in MOCVD grown GaAs with high As/Ga ratio. However, the effect of these traps on charge transport is usually masked in bulk devices by the high free carrier concentration. In nanowires, surface states can fully deplete the free carriers, thus allowing the traps to dominate the electronic transport [1]. Here we report in situ electrical characterization of individual GaAs nanowires using a field-emission scanning electron microscope retrofitted with a nanoprobe. The nanowires, grown by catalyst-free MOCVD, are inordinately resistive despite notable Si doping that leads to high conductivity in thin films. By measuring high aspect ratio nanowires we decouple the role of the contacts from bulk transport and show that carrier transport through the nanowires is trap-mediated. At lower electric fields we observe Poole-Frenkel transport that crosses over to phonon-assisted tunneling at higher fields when the trap barrier is sufficiently thinned. Electron beam techniques are used to affirm the presence of traps and to reveal the nature of the contacts - which consist of one Ohmic and one Schottky contact to the nanowire. A novel technique, electroconductivity, is developed to reveal the trap-activated behavior and to determine the Schottky barrier height. [1] A. M. Katzenmeyer, F. Léonard, A. A. Talin, P.-S. Wong, D. L. Huffaker, Nano Letters (accepted).
10:45 AM - EE7.7
Consecutive Photoluminescence and Transmission Electron Microscopy Studies of Single GaAs/AlGaAs Core-shell Nanowires with a GaAsSb Insert.
Fervin Moses Anthonysamy 1 , Jelena Todorovic 2 , Thomas Karlberg 1 , Antonius van Helvoort 2 , Dheeraj Dasa 1 , Phillip Olk 1 , Bjørn-Ove Fimland 1 , Helge Weman 1
1 Institute of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim, Sør-Trondelag, Norway, 2 Department of Physics, Norwegian University of Science and Technology, Trondheim, Sør-Trøndelag, Norway
Show AbstractWe report on a correlation between the optical properties of single semiconductor nanowires (NWs) and their crystal structure, including growth related defects, by consecutive micro-photoluminescence (µ-PL) and transmission electron microscopy (TEM) characterizations. The optical properties of a single NW are affected by structural defects such as stacking faults (SFs) in wurtzite (WZ) crystal structure. The density of structural defects may differ for NWs from the same growth batch. Therefore, in the presented work, the low temperature (10 K) power dependent µ-PL followed by TEM studies were performed on the same individual NWs in order to obtain a correlation between the PL properties and the local variation of SFs in the NW. The radial and axial heterostructured NWs studied in this work were grown by Au-assisted molecular beam epitaxy (MBE) [1, 2]. The NWs consist of a ~80 nm diameter GaAs WZ dominated core with zinc blende (ZB) GaAsSb inserts (~50 nm long). The NW core surface is passivated by a radial AlGaAs shell and thin GaAs capping layer.In this contribution we focus on comparing power dependent µ-PL spectra of individual NWs with different density of GaAs SFs in the vicinity of the ZB-GaAsSb insert. Conventional type-I recombination of carriers in the ZB GaAsSb insert is expected to occur when the insert is enclosed by SF free WZ GaAs core. In this case, band filling i.e. shift of the insert PL peak towards higher energy should be observed as the excitation intensity increases. A SF near the insert could lead to type-II recombination. However, we observed some NWs which require other parameters to be considered in order to explain their observed PL behavior.[1] D. L. Dheeraj, G. Patriarche, H. Zhou, T. B. Hoang, A. F. Moses, S. Grønsberg, A. T. J. van Helvoort, B.O. Fimland, and H. Weman, Nano Lett., 8 4459 (2008).[2] T. B. Hoang, A. F. Moses, L. Ahtapodov, H. Zhou, D. L. Dheeraj, A. T. J. van Helvoort, B. O. Fimland, and H. Weman, Nano Lett., 10 2927 (2010).
11:00 AM - EE7: III-V
BREAK
11:30 AM - **EE7.8
Ga-assisted MBE Grown GaAs Nanowires and Related Quantum Heterostructures for Solar Applications.
Anna Fontcuberta i Morral 1
1 , EPFL, Lausanne Switzerland
Show AbstractNanowires represent model systems for studying a variety of low dimensional phenomena as well as building blocks for the future generation of nanoscale devices. The most exploited nanowire growth technique is the vapor-liquid-solid method, which very often employs gold as a seed for the growth. We grow GaAs nanowires by molecular beam epitaxy (MBE) without using gold as a catalyst. We will show how MBE offers a unique possibility for obtaining high purity and high quality materials. Additionally, it gives a great flexibility for the fabrication of many types of nanowire heterostructures such as quantum wells, wires and dots in the nanowire axial and radial direction. We will present here how this combination can be beneficial for application in third generation solar cell designs. The optical and transport properties will be elucidated by means of luminescence, Raman spectroscopy, current-voltage and microscopy experiments realized on the same single nanowire.
12:00 PM - EE7.9
Realization of Hole Quantum Dots in GaSb Nanowires.
Bahram Ganjipour 1 , B.Mattias Borg 1 , Lars-Erik Wernersson 1 , Lars Samuelson Samuelson 1 , Honqgi Xu 1 , Claes Thelander 1
1 Division of Solid State Physics, Lund University, Lund Sweden
Show AbstractHole quantum dots were realized using GaSb segments of GaAs/GaSb heterostructure nanowires. The nanowires were grown from 30 or 40 nm gold aerosol particles which were deposited on a GaAs(111)B substrate. The sample consists of undoped GaAs/GaSb nanowires grown with MOVPE, where the precursors used were TMGa, TMSb and AsH3. The nanowires were transferred from the growth substrates to highly doped Si substrates with a 100-nm-thick SiO2 capping layer. Ti/Au contacts with a varying contact separation were defined to the GaSb nanowire segments of selected nanowires. The fabricated devices were characterized by low temperature 1.5K transport measurements, where periodic conductance oscillations due to Coulomb blockade were observed in the measurements with a charging energy of 5 meV. The conductance showed the typical back-gate voltage behavior expected for hole transport, as verified by biasing the device outside the Coulomb blockade regime, and from measurements at non-cryogenic temperatures. The measurements further showed that quantum levels of the GaSb quantum dots have small g factors, with absolute values up to 0.51.
12:15 PM - EE7.10
Stranski-Krastanov InAs Quantum Dots on the Facets of GaAs Nanowires.
Jordi Arbiol 1 , Emanuelle Uccelli 2 3 , Joan Ramon Morante 4 5 , Anna Fontcuberta i Morral 2 3
1 , ICREA and Institut de Ciencia de Materials de Barcelona, CSIC, Bellaterra, CAT, Spain, 2 Laboratoire des Materiaux Semiconducteurs, Ecole Polytechnique Federale de Lausanne, Lausanne Switzerland, 3 Physics Department, Walter Schottky Institut, Garching Germany, 4 Departament d’Electronica, Universitat de Barcelona, Barcelona, CAT, Spain, 5 , Catalonia Institute for Energy Research, Barcelona, CAT, Spain
Show AbstractMastering of the nanowire morphology, composition and structure has shown to be key factor for the development of new advanced applications in nanotechnology. The formation of heterostructures within the nanowire and/or the combination of nanowires with other nanostructures adds advanced functionalities. In particular, the use of core-shell or axial heterostructures can be used to create QWs which can be used to modulate the nanowire light emission [1,2]. On the other hand, the use of different polytypes such as combination of zinc-blende and wurtzite in the same nanowire could be also used as an important tool for bandgap engineering [3,4].The particular geometry of nanowires has enabled optical experiments that were unthought-of with classical thin film structures. If thick enough, they can act as extremely small waveguides. By inserting quantum dot (QD) structures in the nanowire waveguides, the brightness of the QD can be increased up to an order of magnitude higher compared to self-assembled QD on a planar substrate. Finally, QDs could be embedded in the intrinsic region of radial p-i-n devices to optimize the absorption in nanowire based solar cells. In the present work, InAs quantum dot arrays are obtained on GaAs nanowire facets by molecular beam epitaxy [5]. The GaAs nanowires are first grown by the gallium-assisted catalyst free method. Decoration of the nanowire facets with InAs quantum dots is achieved only when the facets are capped with an ultra-thin AlAs layer, as demonstrated by atomic force and high resolution electron microscopy [5]. A detailed HRTEM analysis is performed in order to corroborate the role of this layer for compensate the growth conditions onto {110} GaAs facets that prevent the formation of InAs quantum dots. 3D animated atomic models [6] of the QDs showing the epitaxial relationship between the different heterostructures as well as the detailed faceting properties will be also presented. A detailed strain analysis will be performed on the QDs and directly correlated to its optical properties. Photoluminescence spectroscopy on single QDs is realized and the excitation of single and double excitons demonstrated. This new type of heterostructures opens a new avenue to the fabrication of highly efficient single photon sources, novel quantum optics experiments, as well as the realization of intermediate-band nanowire solar cells for third generation photovoltaics.References[1]A. Fontcuberta i Morral et al. Small, 4, 899 (2008)[2]M. Heigoldt et al. J. Mat. Chem, 19, 840 (2009)[3] D. Spirkoska et al., Physical Review B, 80, 245325 (2009)[4] I. Zardo et al., Physical Review B, 80, 245324 (2009)[5]E. Uccelli et al., ACS Nano, 4, 5985 (2010)[6]http://www.icmab.cat/gaen/research/121
12:30 PM - EE7.11
InGaAs Nanopillar Lasers Monolithically Grown on MOSFET-Si.
Fanglu Lu 1 , Thai-Truong Tran 1 , Wai Son Ko 1 , Kar Wei Ng 1 , Roger Chen 1 , Connie Chang-Hasnain 1
1 Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California, United States
Show AbstractScalable integration of optoelectronic devices with CMOS-based electronic circuits can address the growing needs for low power consumption optical interconnects, communications and signal processing. However, mismatches of lattice constant and thermal expansion coefficient have fundamentally limited monolithic integration of III-V lasers onto silicon-based CMOS circuits. Most importantly, the high growth temperature of III-V materials places a critical roadblock on wafer-scale integration. Nanostructures are promising because they can be synthesized at low temperatures on lattice-mismatched substrates while maintaining high crystal quality. Here, we report novel InGaAs nanopillar lasers monolithically grown on a Si substrate containing metal-oxide-semiconductor field effect transistors (MOSFETs). The growth is done catalyst free and at low-temperature (410°C), to maintain the MOSFET performance, by metal-organic chemical vapor deposition (MOCVD).The growth process was based on a novel nanoneedle on (111)-Si we reported earlier [1,2]. In this work, we used a typical (100)-Si substrate consisting of MOSFETs with their contact metals removed. We deposited silicon nitride protection layer onto half of the chip by PECVD. The growth of InGaAs/GaAs nanopillars is carried out by MOCVD. Subsequently, silicon nitride layer is removed and metal contacts are deposited on MOSFETs. Optical and electrical characterizations are conducted on the nanopillars and MOS transistors to confirm that both are functional on the same chip.As-grown nanopillars are found on both gate region (polycrystalline Si) and source/drain region ((100)-Si) on every transistor. The nanopillars have a hexagonal base and six slanted sidewalls. The hexagonal shape originates from the single wurtzite-phase crystalline structure, as confirmed by TEM analysis. With 1.5 hour growth time, a typical nanopillar has a base diameter of ~800nm and a typical height of ~4µm. The nanopillar is a natural resonator supporting a mode that helically propagates through the pillar. As a result of this mode, sufficient feedback is achieved despite of a very low refractive index difference between the pillar and poly-Si or Si. Lasers are achieved by optical pumping with 120fs Ti:sapphire pulses at temperatures ranging from 4K to 150K. Emission at 880nm (1.41eV) dominates nanopillar emission spectrum after the onset of lasing, with a 20dB background suppression ratio. Typical threshold pump power is ~0.23mW, which increases with temperature. Speckle patterns are also observed, attesting to the high coherence of laser output. The MOS transistors show standard transistor performance after MOCVD growth. This result demonstrates the CMOS-compatibility of our nanopillar laser growth for the first time, serving as a proof-of-concept that such integration can be extended to more complicated CMOS integrated circuits.[1]M. Moewe et al Appl. Phys. Lett. 93 (2008),023116[2]M. Moewe et al Opt. Express 17 (2009),7831
12:45 PM - EE7.12
InAs/GaAs Core-shell Nanowires Grown by MBE.
Ronit Popovitz-Biro 1 , Palle von Huth 1 , Andrey Kretinin 2 , Hadas Shtrikman 2
1 Electron Microscopy Unit, The Weizmann Institute of Science, Rehovot Israel, 2 Braun Center for Submicron Research, The Weizmann Institute of Science , Rehovot Israel
Show AbstractOn route to the fabrication of III/V semiconductor nanowires that will be good candidates for ballistic transport measurements, InAs nanowires were embedded in situ in a GaAs shell, using the gold-assisted VLS-MBE process. The aim of such shell is to provide a robust passivation of surface states and keep the oxide layer and any adhered impurities away from the conductive channel. InAs nanowires having the wurzite structure, grown on InAs (111)B substrate under low temperature and slow growth-rate conditions, were coated by a 5-30nm thick GaAs shell, under similar conditions. The coating appears rather uniform along a 5-6µm long nanowire as seen by transmission electron microscopy (TEM) imaging. Energy dispersive spectrometry (EDS) across a single nanowire confirms the presence of Ga at the periphery. Electron diffraction spots from such coated nanowires were found to be split, corresponding to the two adjacent lattices. Due to the large mismatch of 7%, it is expected that the GaAs shell will relax beyond 1-2 monolayers. Indeed, the presence of two overlapping lattices is manifested by the appearance of a strain related moiré fringe pattern in the TEM images, along the central part of the nanowire. High resolution transmission electron microscopy (HRTEM) revealed a semi periodic array of edge dislocations at the interface between the InAs core and the GaAs shell. Structural investigation carried out on the cross-section, prepared by focused ion beam (FIB), show hexagonal morphology with strain induced contrast at the interface between core and shell. Elemental mapping by energy filtered transmission electron microscopy (EFTEM) of a cross section taken from the center of such nanowire further confirms the different composition of the core and the shell. Split electron diffraction spots and misfit dislocations were observed in this orientation as well, indicating that strain relaxation occurs both along the nanowire axis and around the core. In spite of the larger mismatch relative to the more commonly used InP (3.2%) shell, GaAs can be used as a shell for coating InAs nanowires. Furthermore, it has the advantage of being part of most III-V MBE systems and the large band gap of GaAs with respect to InAs can provide stronger carrier confinement. In addition to that, field-effect-transitor (FET) devices were fabricated using the InAs/GaAs core/shell nanowires which exhibit transistor characteristics similar to those, obtained from bare InAs nanowire FETs.References1. M. Paladugu , J. Zou , Y. N. Guo, X. Zhang , H. J. Joyce, Q. Gao, H. H. Tan, C. Jagadish , Y. Kim . Nanoscale Res Lett , 2009, 4, 846.2. K. L Kavanagh, Semicond. Sci. Technol. 2010, 25, 024006.
EE9: Poster Session: Oxide Nanowires
Session Chairs
Friday AM, April 29, 2011
Salons 7-9 (Marriott)
9:00 PM - EE9.1
UV Activated Electrochemically Grown Zn (Ni, Cu, Co) O Nanorod Ethanol Sensor on Flexible Substrate.
Hosang Ahn 1 , Seon-Bae Kim 1 , Minseo Park 2 , Dong-Joo Kim 1
1 Materials Eng., Auburn Univ., Auburn, Alabama, United States, 2 Physics, Auburn University, Auburn, Alabama, United States
Show AbstractNanostructured ZnO with good semiconducting and electronic properties has drawn a lot of interest in microdevices, such as thin film transistors, gas sensor applications, and spintronics. ZnO nanorods have been investigated for gas sensors because nanorods have a high surface to volume ratio, which can maximize the gas-sensing properties. To enhance sensing characteristics by carrier density, a UV light with higher photonic energy can be used as an alternative energy source in metal oxide-based gas sensors. Furthermore, doping of transition metals in ZnO is reported to have a photoluminescence effect under UV illumination, which makes it possible to sense gases at room temperature. However, a systematic study on the effects of transition metals, such as nickel, copper, and cobalt, on morphological and gas-sensing properties has rarely been done. In addition, the use of flexible substrates for integrating sensing components and investigating sensing mechanisms are not well understood. In this study, in-situ doping of Ni, Cu, and Co during ZnO nanorod synthesis was conducted and their electrical properties for gas sensing were characterized. For ZnO synthesis, thermolysis assisted chemical solution deposition was used to grow ZnO nanostructure. ZnO nanorods were grown at 85°C for 4hrs from 0.01mol of Zinc nitrate hexahydrate and HMT (Hexamethyltetramine) solution. An 80nm thick ZnO seed layer was deposited by R.F. sputtering on polyimide film prior to solution growth. The dopant concentration of transition metal in a solution, for example, nickel, ranged from 0.0002 mol to 0.001 mol. In the case of approximately 2 at% ~ 6 at% of nickel doped ZnO, substitutional doping was confirmed by EDS, XRD, and Raman spectroscopy without forming a secondary oxide phase. Improvement in both response time and sensitivity were observed. However, a secondary phase such as NiO was observed when nickel ions exceeded 6 at%, and such a secondary phase deteriorated gas sensitivity. In addition, evaluation by UV illumination was systematically performed to explain the effects of transition metal ions on ZnO nanorods. A mechanism including photoluminescence effects and gas sensitivity is proposed.
9:00 PM - EE9.12
Phase Shift Lithography Based Nanopatterning for the Fabrication of Large Scale Si and ZnO Nanowire Arrays.
Kittitat Subannajui 1 , Firat Guder 1 , Margit Zacharias 1
1 , IMTEK, Uni-Freiburg, Freiburg Germany
Show AbstractIn recent years, semiconductor nanowires have attracted significant attention due to their novel and superior properties as compared to bulk materials. As semi-1D structures, they are considered as a feasible alternative for future electronic devices in the post-CMOS era. In standard micro fabrication processes, a series of sequential steps are performed that are aligned with the previously processed layers. Therefore, a deterministic approach is mandatory for the positioning of semiconductor nanowires in order to integrate these structures into an electronic circuit. However, the exact positioning of nanowires still remains to be a challenging task. In the past, we demonstrated the use of nanosphere lithography and interference lithography for patterning catalysts for vapour-phase nanowire growth[1]. Then again, these methods are limited to small areas and are not suitable for integration into wafer scale device processes. In this work, we focus on a more up-scalable process based on near field contact phase-shift lithography using a homemade borosilicate glass mask[2]. This method offers the ability to precisely control the nanowire positions on the substrate and is compatible with wafer scale fabrication. It can be used for both catalyst and mask patterning to realize VLS grown as well as etched nanowire arrays, respectively. Based on the generated patterns vertical arrays of etched Si nanowires and epitaxially grown ZnO nanowires were fabricated on an 11 cm2 area on a 4-inch wafer with nanowire diameters down to 100 nm. The produced nanowires show excellent uniformity and are suitable for future device integration. As an example, by combining phase shift lithography with atomic layer deposition, we produced and electrically characterized vertical nanowire diode arrays to exhibit the versatility of this patterning strategy for future device fabrication[3].[1] Zacharias et. al., Physica Status Solidi B 2002, 247 2305–2314.[2] Du et. al., Microelec. Eng. 2002, 61, 265-270.[3] Subannajui et. al.. 2010 (submitted)
9:00 PM - EE9.13
Catalyzed Vapor-liquid-solid Oxidation: Germanium Oxide Nanowires.
Marika Gunji 1 , Shruti Thombare 1 , Shu Hu 1 , Paul McIntyre 1 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractGermanium oxide (GeOx) nanowires (NW) may find application in future optical communication devices. Photoluminescence (PL) study of GeOx nanocrystals has shown that GeOx can exhibit blue or green luminescence. Several synthesis methods for crystalline GeOx nanowires have been reported including laser ablation, thermal deposition of germanium dioxide (GeO2) powders, and metal catalyzed growth in a germanium- and oxygen- containing ambient. Here we report a novel method for directed synthesis of GeO2 wires by thermal oxidation of gold (Au) catalyst-tipped Ge <111> single crystal nanowires. During oxidation at temperatures greater than the Au-Ge binary eutectic temperature (T > 361°C), one-dimensional oxidation of as-grown Ge NWs occurs by Ge diffusion through Au-Ge catalyst droplet driven by the free energy of oxidation of Ge dissolved in the droplet in the presence of oxygen. Elongated GeOx wires form at the catalyst tip, consuming the adjoining Ge nanowire as they grow. The oxide nanowire diameter is dictated by the catalyst diameter, and their alignment generally parallels that of the initial Ge nanowires, although some viscous flow of the oxide, causing GeOx wire bending, is evident. Transmission electron microscopy and scanning electron microscopy were used to investigate the kinetics of VLS oxidation induced GeO2 nanowire growth.
9:00 PM - EE9.15
Multi-junction ZnO Nanowires for Enhanced Surface Area and Light Trapping Solar Cells and Room Temperature Gas Sensing Applications.
Ghim Wei Ho 1 , Moe Kevin 1 , Wei Li Ong 1 , Zhihan Lim 1
1 Electrical & Computer Engineering, National University of Singapore, Singapore Singapore
Show AbstractA maskless method of employing polymer growth inhibitor layers is used to modulate the conflicting parameters of density and alignment of multi-junction nanowires via large-scale low temperature chemical route. This low temperature chemical route is shown to synthesize multi-junction nanostructures without compromising the crystal quality at the interfaces. The final morphology of an optimized multi-junctions nanowire arrays can be demonstrated on various substrates due to substrate independence and low temperature processing. Here, we also follow-up on device demonstrations whereby p-n junction are created by exposure of secondary nanowires to ammonia plasma, converting them to p-type characteristics and also the density modulated multi-junction nanowires were tuned to infiltrate nanoparticles to create a hybrid hierarchically-structured nanowire/nanoparticles solar cell. The fabrication of hierarchically-structured nanowire/nanoparticles composites presents an advantageous structure, one that allow nanoparticles to provide large surface areas for the dye adsorption, whilst the nanowires can enhance the light harvesting, electron transport rate, and also the mechanical properties of the films. This work can be of great scientific and commercial interest since the technique employed is of low temperature (< 90 C) and economical for large-scale solution processing, much valued in today’s flexible display and photovoltaic industries. In addition, ZnO nanostructures for gas and optical sensing will be presented.
9:00 PM - EE9.16
In2O3:Sn-TiO2 Core-shell Nanowire Array Architecture for Dye-sensitized Solar Cells.
Hyun Soo Han 1 , Jun Hong Noh 1 , Sangwook Lee 1 , Ju Seong Kim 1 , Hyun Suk Jung 2 , Kug Sun Hong 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractThis three-dimensional (3-D) transparent conducting oxide (TCO) array is expected to have superior charge collection properties in dye-sensitized solar cell (DSSC) compared to the conventional two-dimensional (2-D) TCO layer. Herein, we successfully synthesized vertically aligned In2O3:Sn (ITO) nanowire (NW) array with diameter of ~100 nm and length of ~20 µm on glass substrate by vapor transport method in a horizontal tube furnace. However, ITO NW itself is not able to operate as a working electrode due to severe back-electron transfer at the interface between highly conductive ITO NW arrays and the electrolyte. For use as a working electrode of a DSSC, an ITO NW surface should be coated by an appropriate semiconducting oxide shell layer such as TiO2. Therefore, we designed ITO-TiO2 core-shell NW array architecture and chose TiCl4 treatment to deposit a TiO2 shell layer on ITO NW surface, which is a commonly used technique during fabrication of DSSCs. The TiO2 shell layer which is composed of agglomerated anatase TiO2 nanocrystals (NCs) with a size of ~2 nm is conformally deposited on the ITO NW by TiCl4 treatment with a thickness of ~70 nm. However, the cell performance of using as-prepared ITO-TiO2 core-shell NW array revealed remarkably low power conversion efficiency (PCE) resulting from low short-circuit photocurrent density (Jsc) due to insufficiency of dye adsorption and low open-circuit voltage (Voc) due to recombination at the interface between working electrode and the electrolyte. In order to obtain higher PCE, ITO-TiO2 core-shell NW array should have annealing process with proper temperature. ITO-TiO2 core-shell NW array cells are annealed in air respectively at 120 °C, 300 °C, 450 °C and 550 °C and the cell performances are shown in Table 1. The solar cell of 450 °C annealed ITO-TiO2 core-shell NW arrays present the best performance with a (Jsc) of 9.978 mA cm−2, a (Voc) of 0.689 V and a overall power conversion efficiency (PCE) of 3.769 % as shown in Table 1. We discuss the phase, morphology and surface condition of TiO2 shell layer as a function of annealing temperature and their influences on cell performance in this study.
9:00 PM - EE9.17
Low-voltage Operating SnO2 Nanowire Active-matrix Devices on a Flexible Polyimide Substrate.
Sahngki Hong 1 , Jeong Sook Ha 1
1 Department of Chemical and Biological Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractNext generation display industry requires the establishment of flexible device technology. Conventional Si based semiconductor fabrication process has a limit to be applied to flexible devices due to high process temperature and the lack of flexibility. Therefore, there have been extensive efforts to overcome such limitation by using various organic semiconductors. However, it was found difficult to commercialize the organic thin film devices due to their low mobility and high operating voltage. On the other hand, nanowires (NWs) can be an alternative to realize the transistors with high mobility, low operating voltage, transparency and flexibility. In this paper, we have demonstrated the fabrication of low-voltage operating active-matrix with aligned SnO2 NWs on Si/SiO2 and flexible polyimide substrates. SnO2 nanowires grown by chemical vapor deposition were transferred onto the desired position by sliding transfer method to obtain uniform and aligned nanowire networks, providing uniform electrical performances over the large area. 8×8 arrays of AM consisting of 1 capacitor and 3 SnO2 NW transistors with cross-linked polyvinyl phenol as top-gate dielectric have been fabricated, where 100 NWs were contained per one transistor with a channel length and width of 3 and 50 μm, respectively. The transistor yields are estimated to be ~ 91%. The on-current of SnO2 NW AM was ~ 22 μA, considered sufficient to operate OLED at VSCAN = 2 V, VDATA = 3 V, VDD = 1 V and its bending stability was also investigated.
9:00 PM - EE9.18
Decoration of TiO2 Nanowires with Various Metal Nanoparticles and Their Comparative Photocatalytic Properties.
Yung-Jung Hsu 1 , Yu-Chih Chen 1
1 MSE Department, National Chiao Tung University, Hsinchu Taiwan
Show Abstract Recently, one-dimensional (1-D) nanostructures of TiO2 such as nanotubes and nanowires have attracted intensive research interests due to their potential use as active components and interconnects in nanoscale electronic devices. The pronounced delocalization of charge carriers in 1-D crystals can significantly lower the probability of e--h+ recombination [1]. Consequently, improved photoelectric conversion efficiency such as the photocatalytic activity was regularly observed in 1-D nanostructures when compared with their spherical or particulate counterparts. In this work, we successfully synthesized single-crystalline anatase TiO2 nanowires (NWs) with a modified alkaline hydrothermal approach by using commercial P-25 TiO2 powder as starting material. To extract the photoexcited electrons from TiO2 NWs for participation in photocatalytic reaction, we introduced an electron-acceptor, metal nanoparticles (Ag, Au or Pt), at the surfaces of NWs. TEM measurement clearly reveals the interface between the surface-coated metal particle and NW body, demonstrating the successful decoration of metal on TiO2 by using our approach. The photocatalytic performance of the three metal-decorated NW samples was then compared through the photodegradation of methylene blue (MB). As compared to the Ag- and Au-decorated samples, Pt-decorated NWs exhibited superior photocatalytic efficiency toward MB photodegradation, which can be explained by the difference of work function among the deposited metals. Among the three metals, Pt has the largest work function [2], which means that the difference between the conduction band of TiO2 and the Fermi level of metal is the largest in Pt-decorated case. It has been pointed out that the driving force of electron transfer from semiconductor to metal is proportional to such energetic difference [3]. In the current case, Pt-decorated TiO2 NWs may possess higher driving force for charge separation and consequently showed better photocatalytic efficiency as compared to Ag- and Au-decorated samples. It should be noted that when compared with the commercial product like P-25 TiO2 powder, Pt-decorated TiO2 NWs showed better photocatalytic performance, demonstrating their potential as an efficient photocatalyst in relevant redox reactions. In addition, no obvious decay of photocatalytic efficiency was observed in Pt-decorated NWs after repeatedly used in MB photodegradation for three times. This result reveals the promising potential in the long-term course of photocatalysis for the present Pt-decorated TiO2 NWs.References[1] Yun, H. J.; Lee, H.; Joo, J. B.; Kim, W.; Yi, J. J. Phys. Chem. C 2009, 113, 3050.[2] Arabatzis, I. M.; Stergiopoulos, T.; Andreeva, D.; Kitova, S.; Neophytides, S. G.; Falaras P. J. Catal. 2003, 220, 127.[3] Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943.
9:00 PM - EE9.19
Designing the Electric Transport Characteristics of ZnO Micro/Nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects.
Youfan Hu 1 , Yanling Chang 1 , Peng Fei 1 , Robert Snyder 1 , Zhong Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractSemiconductor nanowires are the fundamental materials for fabricating a wide range of nanodevices. In the oxide family, ZnO nanowires and nanobelts have been widely studied as a key 1D oxide nanomaterial for numerous applications. Due to the unique piezoelectric and semiconducting coupled properties, a range of novel nanodevices of ZnO have been developed, such as nanogenerators, piezoelectric field effect transistors, piezoelectric diode, and strain sensors. A direct band gap of 3.4 eV and a large exciton binding energy (60 meV) at room temperature make ZnO a prominent candidate in optical applications, such as an ultraviolet detector, optical pumped laser, and light emitting diodes. Although a lot of progress has been made in each research field as stated above, very limited research has been conducted on the localized and quantitatively controlled coupling of the piezoelectric effect and photoexcitation on a ZnO nanowire nanodevice.In this report [1], by introducing a controllable stepping strain and using a focused laser beam, we have investigated the localized coupling between the piezoelectric effect and photoexcitation of ZnO microwire devices that exhibit various controlled electrical transport characteristics. By using microwire devices with Schottky-Ohmic, Ohmic-Ohmic, and Schottky-Schottky contacts, we have demonstrated the finetuning of contact characteristics, such as from Schottky to Ohmic or from Ohmic to Schottky, by varying the individual contributions made by piezoelectricity and photoexcitation at local contacts. On the basis of this concept, a design of electric transport properties is demonstrated. This study reveals a new principle for controlling the coupling among mechanical, electrical, and optical properties, which can be the basis for fabricating piezo-photonic-electronic nanodevices, which is referred to as piezophototronics.[1]"Designing the Electric Transport Characteristics of ZnO Micro/Nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects" ACS Nano,4,1234.
9:00 PM - EE9.2
Controllable Transport Properties of ZnO-based Polytypoid Nanowires.
Sean Andrews 1 2 , Melissa Fardy 1 2 , Michael Moore 1 2 , Shaul Aloni 2 , Velimir Radmilovic 2 3 , Peidong Yang 1 2
1 Department of Chemistry, University of California- Berkeley, Berkeley, California, United States, 2 Material Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractDue to their unique properties, nanostructured materials have shown great promise in a myriad of applications, including sensing, photonic manipulation, and energy conversion. The practical utilization of these materials is heavily dependant on the amount of control one has over the properties of interest toward a specific application. While nanowires are geometrically ideal for integration into such practical devices, careful control over their transport properties is required to fully exploit their intrinsic advantages. To this end, we employ a simple diffusion scheme to create M2O3(ZnO)n (M = In, Ga) nanowires with controllable polytypoid structures, whereby adjusting the amount of metal precursor and annealing conditions tune the nanostructured features. After conversion into the polytypoid structure, thermal and electrical measurements on single In2-xGaxO3(ZnO)n nanowires reveal modified transport properties from both their ZnO nanowire starting point and analogous bulk counterparts. Using aberration-corrected transmission electron microscopy and spatially resolved electron dispersive X-ray spectroscopy, we were able to fully analyze the M2O3(ZnO)n structure, as well as the ZnO-M2O3 interface, providing insight into the effect of the structural features on both electronic and phononic transport. Because of the unique relation between the structural features and transport properties, this nanowire system may be suitable for practical device implementation.
9:00 PM - EE9.20
CdTe-ZnO Nanocone Photovoltaic Solar Cells.
Lee Sang Hyun 1 , D. Barton Smith 1 , Xiaoguang Zhang 1 , Jun Xu 1
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractOne of the most important issues in photovoltaic (PV) technology is how to improve PV conversion efficiency. Two scientific approaches include (1) improving charge collection efficiency by minimizing trapping and recombination centers and (2) obtaining higher light absorption by reducing incident light reflection. Quasi one-dimensional semiconducting nanostructures are potentially beneficial for both approaches due to the large probability that minority carriers will cross the nanojunction, the preferable electric field distribution in the nanostructure, and increased light absorption. We will present recent results about ZnO-CdTe nanocone photovoltaic devices, where vertically aligned n-type ZnO nanocones were grown on ITO-coated glass substrate and a p-type CdTe matrix was grown on the ZnO nanocones using thermal evaporation of CdTe powder to form the p-n junction. Scanning electron microscopy (SEM) revealed that the CdTe matrix does not completely cover the surface of ZnO nanocones, suggesting that the CdTe vapor did not diffuse past the nanocone tips to fill the space between the nanocones. However, with only partial contact at the interface of the matrix and nanocone tips, the nanocone-based p-n junction showed a photovoltaic conversion efficiency that is larger than that of a comparable planar CdTe-ZnO junction. The enhancement may be attributable to efficient charge collection in nanocone structure through the electric potential difference. Additional improvements of structural and electric properties of CdTe on ZnO nanocone were accomplished by CdCl2 post-treatments. The contact area of CdTe with ZnO nanocones was increased by CdTe recrystallization during the CdCl2 treatment, and as a result the photoconductivity of CdTe was increased by about 3 orders of magnitude over that of as-grown CdTe films.
9:00 PM - EE9.21
Electrical Contact Fabrication on Vertically- aligned ZnO Nanowires Investigated by Current Sensing AFM.
Vaibhav Jain 1 , Gary Kushto 1 , Antti Maekinen 1
1 Optical Sciences, Naval Research Lab, Washington DC , District of Columbia, United States
Show AbstractWe present a facile method for generating arrays of vertically-aligned ZnO nanowire (NW) arrays with tailored electrical properties, suitable for photodetector and photovoltaic applications. ZnO NWs were synthesized on Si substrate in house using a hydro-thermal method, producing dense and highly organized vertical ZnO NW (~100 nm) arrays as revealed by scanning electron microscopy (SEM). The vertical geometry of the NW arrays facilitated further processing of the NW structures and streamlined in-situ characterization of individual NWs as well as entire NW ensembles. The I-V characteristics of the ZnO NWs were measured using current sensing atomic force microscopy (CS-AFM) where a Pt-coated AFM tip made contact with individual metalized ZnO NWs while the growth substrate provided the back contact. In order to eliminate current leakage between NWs an insulating polymer film (Poly (methyl methacrylate) (PMMA) or Silsesquioxanes based material, SOG - 400F) was spin-coated onto NW arrays which were then etched back to reveal bare NW ends for metallization. The top contact metallization comprised depositing a thin film of low work function metals such as aluminum (5 nm) followed by a thin film of gold (5 nm). Compared with arrays of as-grown ZnO NWs the metalized top contacts of NWs resulted in improved I-V characteristics with good rectifying behavior, attributed to Schottky barrier formation at the metal-semiconductor contact. The formation of a Schottky barrier is thought to be critical for high photosensitivity and short response time in ZnO NW –based sensor structures.
9:00 PM - EE9.22
Enhanced Pure Ultraviolet Emission of n-ZnO Nanowire/i-MgO/p+-Si Heterojunction LEDs Fabricated by Metal-organic Chemical Vapor Deposition.
Byung oh Jung 1 , Dong Chan Kim 1 , Ju Ho Lee 2 , Hyung koun Cho 1 , Jeong Yong Lee 2
1 Advanced Materials Science and Engineering, Sungkyunkwan Univ., Suwon Korea (the Republic of), 2 Advanced Materials Science and Engineering, KAIST, Daejon Korea (the Republic of)
Show AbstractRecently, ultraviolet (UV) LEDs are gaining a lot of attention in the medical science fields. Most of the UV lightening used fluorescent lamps based on mercury, lead and cadmium. But its noxious nature, inefficiency, and short span of illumination need the replacement of a semiconductor light source. Although many studies on the development of UV LEDs using the ZnO with UV bandgap in progress, they focused on the use of sapphire substrates and the efficiency is still lower than GaN due to high dislocation density. In addition, the cost effective ZnO/Si heterojnuction LEDs are well known to have low efficiency due to polycrystalline film. On the other hand, the studies regarding ZnO nanowires on Si substrates provide valuable solutions, which contain high quality single crystal nanowires without defects and the enhancement in the emissive area. In particular, the nanowires as an active layer allow the use of cost-effective Si substrates.It is anticipated that the use of appropriate dielectric layers such as SiO2, Al2O3, and MgO is very profitable in the LED fabrication using the n-type ZnO nanowires/Si substrate heterojunctions for charge carrier confinement. Among them, the MgO/ZnO junction is favorable for hole movement due to small valence band offset. In this research, the ZnO nanowires/MgO/Si heterostructure was fabricated by MOCVD and the density of nanowires was controlled by growth temperature. We investigated the effect of the introduction of MgO dielectric layers on the luminescence properties and found that the use of MgO layers provides bright UV emission.
9:00 PM - EE9.23
Influence of Stoichiometry on the Metal-insulator Transition in Suspended VO2 Nanobeams.
In Soo Kim 1 , Shixiong Zhang 1 , Rongrong Cheacharoen 1 , Lincoln Lauhon 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractSingle-crystal VO2 nanobeams exhibit single domains across the nanobeam width, and therefore provide a model system to study the characteristics of the metal-insulator transition with improved control over the boundary conditions that influence structural domain formation. Wu et al. have shown that the interfacial strain between substrate and nanobeam leads to the self organization of alternating metal-insulator domain patterns along the nanobeam axis1, and interfacial strain has been hypothesized to control domain formation and the prevalence of the metastable M2 phase2. The effect of stoichiometry on the metal-insulator transition in nanobeams, however, has not been discussed. We have investigated the influence of stoichiometry on the metal-insulator transition in free-standing single crystalline VO2 nanobeams using Raman spectroscopy and electron diffraction to characterize crystal structure. VO2 nanobeams were synthesized using physical vapor deposition on Si (100) substrates pre-treated with VO2 seed particles. In singly clamped nanobeams, variations in stochiometry induced by thermal annealing produce domain structure similar to that induced by interfacial strain, with alternating domains of rutile and monoclinic phases observed at room temperature. Through small variations of growth and annealing conditions, we have established a pseudo-phase diagram with dimensions of temperature and relative stoichiometry. Reducing conditions lower the metal-insulator transition temperature, as has been observed in thin films, whereas oxidizing conditions favor the formation of metastable M2 and M3 phases. The transition in the treated beams is not abrupt, and the domain structure suggests variations in induced defect concentration. Given the absence of interfacial strain in these nanobeams, it is clear that stoichiometry must also be considered in the interpretation of switching for nanobeams on substrates. Such effects are important to consider in processing of VO2 devices with high surface-area-to-volume ratios, and could also provide new functionality.References1. J. Wu, Q. Gu, B. S. Guiton, N. P. de Leon, L. Ouyang and H. Park, Nano Lett. 6 (10), 2313-2317 (2006).2. S. Zhang, J. Y. Chou and L. J. Lauhon, Nano Lett. 9 (12), 4527-4532 (2009).
9:00 PM - EE9.24
Fabrication of Transition Metal Oxide Nanofiber-based Thin Film Transistors.
Jeonghyun Kim 1 , Junghyun Choi 2 , Sangkyu Lee 1 , Hyunjung Park 2 , Joo Hyun Kim 2 , John Rogers 3 , Ungyu Paik 1 2
1 Department of Materials Science & Engineering, Hanyang University, Seoul Korea (the Republic of), 2 WCU Department of Energy Engineering, Hanyang University, Seoul Korea (the Republic of), 3 Department of Materials Science and Engineering, Beckman Institute for Advanced Science and Technology, and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractTransition metal oxide (TMO) has been considered to be an alternative material for conventional amorphous silicon-based thin film transistors (TFTs) due to high mobility (~1-100 cm2 V-1 s-1). For the fabrication of the TMO-based TFTs, active semiconductor materials have been formed by either gas-phase deposition or solution processes. In this work, we demonstrate the performence of electro-spinning (e-spinning) derived TMO nanofibers as active materials in TFTs. The resultant TMO nanofibers with one-dimensional nanostructure was anticipated to provide better device performance in TFT architecture unlike using conventional metal oxide frame-work. TMO semiconductor sol was prepared by dissolving metal nitrates and poly(vinyl pyrrolidone) in de-ionized water. The solution was electro-spun on Si wafers with a thermally grown oxide. After thermal annealing process (300-500°C), source and drain electrodes were defined on the e-spun TMO semiconductors for the fabrication of TFTs. The electrical performance of e-spun nanofiber-based TFT was characterized using a semiconductor parameter analyzer, which shows a typical n-type behavior. And we also analyzed transparency of e-spun TMO semiconductor using an UV-Vis spectrophotometer. Our approach could be applied to fabricate transparent and high performance device due to the possibility of controlling the dimension of e-spun TMO nanofiber.
9:00 PM - EE9.25
Low-temperature Synthesis of Density-controlled ZnO Nanorod Arrays from an Aqueous Solution for Nanorod-based Solar Cell Applications.
Yinghuan Kuang 1 , Karine H. van der Werf 1 , Z.Silvester Houweling 1 , Ruud E. Schropp 1
1 , Utrecht University, Utrecht Netherlands
Show AbstractWe propose a third generation solar cell concept in which an array of zinc oxide nanorod electrodes is coated with an a-Si:H n-i-p junction to form a novel nanorod/a-Si:H solar cell. For this concept, the fabrication of vertically aligned nanorods with a desired aspect ratio and site-density on a substrate of a large area size is essential. In this study, density-controlled ZnO nanorod (ZNR) arrays were synthesized on large area pre-treated Corning glass substrates by a simple aqueous solution growth technique using different growth conditions. Equal molar ratio mixtures of zinc acetate dihydrate and hexamethylenetetramine dissolved in de-ionized water were used as the growth solution. The effects of the reactant concentration, the growth temperature and the time on the morphology and crystallinity of the ZNRs were systematically investigated by scanning electron microscopy and X-ray diffraction. Optical properties of the prepared ZNRs were also studied. It was demonstrated that vertically aligned single-crystal ZNRs can be prepared at a low temperature (below 90°C). The as-synthesized ZNRs posses a wurtzite crystal structure with a preferential growth orientation along the [002] direction. The concentration of reactants has a strong impact on the density and the vertical alignment of the ZNRs. The desired site-density of the ZNRs can be easily controlled by adjusting the solution concentration and the desired nanorod aspect ratio by the growth time. Large-scale, density-controlled and low-cost ZNR arrays prepared by this technique were applied to the ZNR-based solar cells. The photovoltaic performance of the novel three-dimensional nanorod solar cells will also be reported in this study.
9:00 PM - EE9.26
Indium-tin-oxide Nanowire-based Photoelectrodes for Photoelectrochemical System.
Sangwook Lee 1 , Sangbaek Park 1 , Hyun Soo Han 1 , Kug Sun Hong 1 , Jun Hong Noh 2 , Hyun Suk Jung 1
1 , Seoul National Univ., Seoul Korea (the Republic of), 2 , Kookmin University, Seoul Korea (the Republic of)
Show AbstractTin-doped indium oxide (Sn:In2O3, ITO) nanowires were grown on ITO thin film/glass substrate by vapor transport method. For vapor-liquid-solid (VLS) reaction, Sn and In metal powders were used as source, and Au seeds were used as a catalyst. The ITO nanowires were grown vertically on the substrate with over 10 μm of length and 100 nm of diameter. The length of the wires varied from 10 μm to 50 μm with increasing the reaction time. The growth direction of the nanowire is [100] crystallographic direction of the cubic system. In order to apply the ITO nanowire-based electrodes to photoelectrolysis system, TiO2 nanoshell was deposited on the surface of the aligned ITO nanowires via atomic layer deposition (ALD) method. Compared to TiO2 nanoparticle/ITO film-based photoelectrode, the TiO2/ITO nanowire structure-based photoelectrode was favorable for absorption of photons and for collecting photo-generated charges, due to the one-dimensional nanostructure and transparent/conductive nature of the ITO nanowires. Under UV light illumination, photoelectrochemical characteristics and photoelectrolytic hydrogen evolution property of the TiO2/ITO nanowire-based photoelectrodes were investigated.
9:00 PM - EE9.27
Synthesis of Gallium Doped ZnO Nanorods and Their Application as a CO Gas Sensor.
sang Kyoo Lim 1 , Sung-Ho Hwang 1 , Soonhyun Kim 1 , Daeic Chang 1 , Seong Hui Hong 1
1 DiVision of Nano-Bio Technology, DGIST, Daegu Korea (the Republic of)
Show AbstractControlled syntheses of one-dimensional nanostructured materials such as nanotubes or nanorods have been sucessfully carried out and have received much attention due to their potential applications by many researchers. ZnO is an n-type semiconductor with a wide bandgap and it is one of the most promising materials due to its use in a wide range applications in various fields, including short wavelength light-emitting diode and room temperature ultraviolet(UV) lasing diode, solar cell, UV-absorber, transparent conductor, gas sensor, etc. In this study, Gallium doped ZnO(GZO) nanorods were synthesized by microemulsion method with different types of surfactants. The phase and the morphology of the above nanorods were investigated by using SEM and XRD. Scanning electron microscopy observations show that the ZnO nanorods have diameters around about 70~200 nm and lengths up to several micrometers. The room temperature photoluminescence (PL) spectrum of GZO nanorods exhibited a sharp and strong ultraviolet bandgap at 383 nm and a relatively weaker emission associated with the defect level. The gas sensing properties of the Gallium doped ZnO nanorods based sensors to carbon monoxide(CO) in air were analyzed.
9:00 PM - EE9.28
Single Crystal Rutile TiO2 Nanorods on Fluorine-doped Tin Dioxide Substrates for Dye-sensitized Solar Cells.
Bin Liu 1 , Emil Enache-Pommer 1 , Eray Aydil 1
1 , University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractDye-sensitized solar cells (DSSCs) made from oriented, one-dimensional (1D) semiconductor nanostructures such as nanorods, nanowires, and nanotubes are receiving wide attention because photogenerated charge carriers can be easily transported to the collection electrodes along such 1-D nanostructures. Fast charge transport could allow one to increase the optical density of the photoanode without sacrificing collection efficiency. Specifically, oriented single-crystalline TiO2 nanorods or nanowires on a transparent conductive substrate would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this presentation, we describe a facile hydrothermal method to grow oriented, single-crystalline rutile TiO2 nanorod films on transparent conductive fluorine-doped tin dioxide (FTO) substrates. The diameter, length, and density of the nanorods could be varied by changing the growth parameters such as growth time, growth temperature, initial reactant concentration, acidity and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a small lattice mismatch plays a key role in driving the nucleation and the subsequent growth of the rutile TiO2 nanorods. DSSCs assembled using TiO2 nanorods grown on FTO as the photoanode exhibited light-to-electricity conversion efficiency of 3% even with only 4 μm-long TiO2 nanorods. Surprisingly, comparison of electron transport and recombination rates in these single-crystal rutile TiO2 nanorods with TiO2 nanoparticle films with similar thickness show that the electron transport time constant in single-crystal rutile TiO2 nanorods is approximately the same and even slightly slower than in TiO2 nanoparticle films. This suggests that electron diffusion rate is still determined by the residence time in surface traps even in single-crystal TiO2 nanorods.
9:00 PM - EE9.29
Synthesis and Optical Properties of Cl-doped ZnS Nanoribbons.
Yankuan Liu 1 , Linbao Luo 1 , Xing Xie 1 , Yucheng Dong 1 , Chunyan Luan 1 , Juan Antonio Zapien 1
1 Department of Physics and Materials Science, City university of Hong Kong, Hong Kong, Hong Kong, China
Show AbstractZnS is an important semiconductor for a number of applications. It is a promising triboluminescence material, especially for UV light-emitting diodes (LEDs) and laser diodes (LDs), because of the direct and wide bandgap of 3.77 eV, and a relatively large exciton binding energy (~40meV) for the hexagonal wurtzite phase at room temperature. The ion doping can cause intense fluorescence and persistent phosphorescence in ZnS nanostructures. Cl-doped wurtzite ZnS nanoribbons on Au-coated Si wafers have been successfully synthesized through a chemical vapor deposition based on the vapor-liquid-solid (VLS) mechanism. Co-dopant Cl-1 ions can serve as donors, producing a donor-acceptor pair transition with the ZnS valence band level. Photoluminescence and cathodoluminescence were used to study the optical properties of Cl-doped ZnS nanoribbons. Compared with the undoped counterpart, the blue emission peak at 2.78 eV was observed due to the co-dopant Cl-1 luminescence centers, which indicates its potential applications in visible light-emitting diodes, photodetectors and also some applications in transport properties.
9:00 PM - EE9.3
N-type ZnO Nanowire Based Opto-electronic Devices.
Min Young Bae 1 , Kyung Whon Min 1 , Jeong Sook Ha 1
1 Department of Chemical and Biological Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractZnO nanowires (NWs) have attracted a great attention as active channel materials for various devices including light emitting diodes and sensors owing to their excellent electronic and optoelectronic properties. However, the alignment of randomly grown ZnO NWs onto the desired position of the devices is one of the major challenges in fabricating the array of opto-electronic devices reproducibly. In this work, we report on the fabrication and characterization of ZnO NW- based opto-electronic devices. ZnO NWs grown by chemical vapor deposition (CVD) were transferred onto the desired position of the device substrate by a sliding transfer method, resulting in uniformly aligned ZnO NW channels. Photoluminescence of both the CVD grown pristine and the transferred ZnO NWs showed a sharp peak at 385 nm. ZnO NWs FET array exhibited excellent n-type electron transfer properties with a high electron mobility of 370 cm2/Vs and on/off current ratio of 106. In particular, ultraviolet (λ = 254 nm) sensitivity of the n-ZnO NW FET device was ~107 in off-state, implying the high potential application to UV FET sensor array. PN hetero-junction devices consisting of patterned p+-Si substrate and aligned n-ZnO NWs were also fabricated and they showed the rectifying behavior with a high rectification ratio of ~103 at ± 1 V and an ideality factor of 1.7. The long-term stability of the pn-junction device was obtained by passivation with PMMA coating. Strong electroluminescence at ~ 380 nm was observed, indicating the potential application of such-formed hetero-junctions to practical electro-optic devices.
9:00 PM - EE9.30
Optical Studies of ZnO Nanowires.
Martha Coakley 1 , Athavan Nadarajah 1 , Rolf Koenenkamp 1
1 Physics, Portland State University, Portland, Oregon, United States
Show AbstractWe report a detailed optical characterization of doped ZnO nanowire films grown at temperatures below 100oC in aqueous solutions. The incorporation of dopants, and the activation of these dopants upon low temperature thermal annealing and laser annealing are observed. We characterize the optical properties in terms of the photo- and electroluminescence behavior. Our results indicate that Al-doping generates strong near-gap luminescence and 16% internal quantum efficiency from photoluminescence measurement. We also characterized the optical surface scattering from ZnO nanowire films and found that the scattering behavior strongly depends on the wire morphology and orientation. We show that nanowires grown in electroless solution growth show promise as antireflection coatings on Si and other semiconductor materials. The reflectance in the visible region is below 10%.Doping of the ZnO nanostructures was achieved electrochemically using solvable chloride compounds as precursors. SIMS measurements and TEM-EDX line profiles indicate impurity incorporation and doping in the bulk of the nanowires. Thermal and laser annealing took place in air at a temperature of 380oC and a frequency-tripled Nd-YAG laser with pulse energies in the range of 10 to 40mJ at a wavelength of 355nm respectively. The photoluminescence of as-grown and Al doped ZnO nanowires before and after annealing were studied. This indicated that the concentrations of impurities can often be changed by annealing and Al doped annealed ZnO nanowires exhibit improved UV luminescence peak at 393nm. The internal quantum efficiency for PL was measured to be as high as 16 percent for Al-doped samples annealed at 380oC. The PL measurements also show that the excitonic luminescence is preferentially guided while the defect emission is more isotropically emitted.In addition, we performed the room-temperature electroluminescence from these annealed and as-grown ZnO nanowires in a hybrid p-n junction arrangement. It consists of a hole-conducting polymer and n-ZnO nanowires grown on FTO. After annealing at 380oC the defect related electroluminescence peak exhibits a shift from 620nm to higher wavelengths depending on the dopant. Aluminum incorporation leads to the emergence of a narrow excitonic luminescence peak close to the bandgap of ZnO around 393nm. The antireflection properties of ZnO nanowires for solar cells were also examined using wires grown by electrochemical deposition on ITO coated silicon and by electroless chemical bath on a planar ZnO layer on silicon. The unevenness in the length of the wires creates a graded index of refraction, which acts as a broadband antireflection coating. Vertically aligned ZnO wires grown by chemical bath reduce incident reflection of light on silicon better than randomly aligned wires grown by electrodeposition. Different theoretical models are examined and compared with experimental results.
9:00 PM - EE9.33
Novel Fabrication Method of Morphology and Composition-controlled 1D Pt(Pd)/ZnO Hybrid Nanostructures and Sensor Applications.
Mi Ae Lim 1 2 , Inkyu Park 1 2
1 Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejon Korea (the Republic of), 2 KI for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejon Korea (the Republic of)
Show AbstractOne dimensional (1D) ZnO nanostructures have recently received much attention due to their useful functionalities such as piezoelectricity, chemical sensitivity, wide-bandgap semiconductivity, etc. These unique properties could be further improved for specialized applications such as chemical sensors and photocatalysts by surface modification with metal nanoparticles (NPs). In the present work, we have developed an extremely simple method to the fabrication of one dimensional (1D) hybrid nanostructures of ZnO and metal (Pt, Pd) with diverse morphologies and chemical compositions, ranging from Pt(Pd) NP-coated ZnO nanowires, Pt(Pd) / ZnO hybrid nanotubes, and Pt(Pd) nanotubes. This method is based on the low temperature reaction between ZnO nanowires hydrothermally grown on the substrate and aqueous solution of Pt(Pd) precursors. Tuning of morphology and composition of 1D Pt(Pd) / ZnO hybrid nanostructures from Pt(Pd) NP-coated ZnO nanowires to Pt(Pd) / ZnO hybrid nanotubes, and to Pt(Pd) nanotubes was achieved simply by varying synthesis parameters such as Pt(Pd) salt concentration, pH in aqueous solution of Pt(Pd) salt, and citrate amount used as reducing agent. The formation of nanotubes is attributed to the anisotropic etching effect of ZnO nanowires in the acidic condition and acceleration of ZnO dissolution by the decrease of pH in the solution via the reduction of metal salt (PtCl42- or PdCl42-). Furthermore, this process is also compatible with flexible polymer substrate due to the low-temperature and mild chemical conditions. Pt nanotube film was fabricated on polyimide film by this novel method for its application as flexible strain sensor. The normalized change in electrical resistance of Pt nanotube film increased in linear proportion to applied strain with considerably higher sensitivity (gauge factor=15) compared to conventional metal foil strain sensor. In addition, the application of Pd/ZnO hybrid nanostructures as chemical sensors with high sensitivity and fast response time is presented.
9:00 PM - EE9.35
Combined Effects of SPR and FRET on Enhanced Light Emission of ZnO Nanowires.
Jeeyoung Jang 1 , Yun-Mo Sung 1
1 Materials Sci. & Eng., Korea University, Seoul Korea (the Republic of)
Show AbstractHigh-quality and high-density ZnO nanowires (NWs) were successfully grown on indium tin oxide (ITO) glass substrates using a mild wet chemical approach. Zn-acetate layer coated at the surface of ITO substrates was heat treated and the ZnO nanoparticle (NP) layer was formed as a seed layer for the ZnO NW growth. Ag NPs and CdSe/ZnS quantum dots (QDs) were synthesized using salt reduction and inverse micelle templates. First, the ZnO NW array was dipped into a CdSe/ZnS QD-containing solution and subsequently it was dipped into the Ag NP-containing solution to form ZnO NW//CdSe/ZnS QD//Ag NP heterostructures.The ZnO NWs showed relatively weak radiative recombination emission at ~380 nm, while they showed strong defect emission at ~550 nm. The ZnO NW//CdSe/ZnS QD//Ag NP heterostructures showed apparently enhanced radiative recombination radiation as well as the suppressed defect emission. The defect level energy in ZnO NWs transferred to CdSe/ZnS QDs through fluorescence resonance energy transfer (FRET), since the wavelength of defect emission overlaps with the light absorption of CdSe/ZnS QDs. The excited free electron in the QDs can transfer to ZnO NWs due to the difference in the conduction band energy level. Surface Plasmon resonance (SPR) of Au NPs can excite more number of balance band electrons in ZnO NWs. Thus, ZnO NWs could have increased band-edge transition and in turn show enhanced radiative recombination emission at ~380 nm. Interestingly, we could not observe the FRET in the ZnO NWs//CdSe/ZnS QD hetereostructures, which implying that the Au NPs not only brought SPR but also assisted the FRET between ZnO and CdSe.
9:00 PM - EE9.36
Electrical Transport in Individual ZnO Nanorods Studied by Photo-conductive Atomic-force Microscopy.
Christian Teichert 1 , Igor Beinik 1 , Markus Kratzer 1 , Gerhard Brauer 2 , Xinyi Chen 3 , Yuk Hsu 3 , Aleksandra Djurisic 3
1 Institute of Physics, University of Leoben, Leoben Austria, 2 Institut für Strahlenphysik , Forschungszentrum Dresden-Rossendorf, Dresden Germany, 3 Department of Physics, University of Hong Kong, Hong Kong China
Show AbstractOne-dimensional ZnO nanostructures exhibit technological potential for many device applications, like efficient low-cost ZnO nanorod-polymer solar cells [1]. Conductive atomic-force microscopy (AFM) is a valuable tool for nanometer-scale electrical characterization of such nanorods [2]. Here, we present a complementary study of electrical transport in individual upright standing ZnO nanorods (NRs) grown by thermal evaporation [4] using conductive AFM (C-AFM) and photoconductive AFM (PC-AFM) [5]. Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps measured at different bias voltages applied to the sample contact mode. Further, C-AFM was utilized to determine the local current-to-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs.Further, we pioneered the application of PC-AFM to resolve the photoconductivity spectra measured from a single as-grown ZnO NR. PC-AFM is similar in concept to C-AFM: the sample surface is biased and additionally irradiated from a Xe light source connected to a monochromator. The current through the AFM tip is measured as a function of illumination intensity and/or wavelength. PC-AFM investigations reveal that I-V curves taken from a single upright standing NR under illumination appear more degraded with respect to the non-illuminated state. Analyzing the photoconductivity spectra it has been found that the band gap of ZnO NR is reduced by about 220 meV with respect to the known value of 3.37 eV at room temperature. Using PC-AFM, we also observed persistent photoconductivity from a single ZnO NR. We believe that both phenomena can be attributed to the processes of oxygen desorption/re-adsorption from the ZnO NR surface. Moreover, these observations are in good agreement with theoretical predictions of the influence of oxygen vacancies on the electronic structure of ZnO [6].Supported by Austrian Science Fund FWF under project # P19636.[1] E. Greene, et al., Nano Lett. 5, 1231 (2005).[2] G. Brauer, et al., Phys. Status Solidi C 6, 2556 (2009).[3] C. Teichert and I. Beinik, in “Scanning Probe Microscopy in Nanoscience and Nanotechnology”, Vol. 2, Edited by B. Bhushan, (Springer, Heidelberg, 2011).[4] Y. F. Hsu, Y. Y. Xi, A. B. Djurišić, W. K. Chan, Appl. Phys. Lett. 92, 133507 (2008)[5] H. Sakaguchi, et al., Jpn. J. Appl. Phys. 38, 3908 (1999).[6] S. Lany and A. Zunger, Physical Review B 72, 035215 (2005).
9:00 PM - EE9.38
Ordered Nanowire Array Blue/near-UV Light Emitting Diodes.
Sheng Xu 1 , Chen Xu 1 , Ying Liu 1 , Youfan Hu 1 , Rusen Yang 1 , Qing Yang 1 , Jae-Hyun Ryou 1 , Hee Jin Kim 1 , Zachary Lochner 1 , Suk Choi 1 , Russell Dupuis 1 , Zhong Lin Wang 1
1 School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractZnO-based light emitting diodes (LEDs) have been considered as a potential candidate for the next generation of blue/near-UV light sources. While the pursuit of stable and reproducible p-ZnO is still undergoing, heterojunctions of n-ZnO and p-GaN are employed as an alternative approach in this regard by considering the similar crystallographic and electronic properties of ZnO and GaN. Compared with the thin film/thin film LEDs, which may suffer from the total internal reflection, n-ZnO nanowire/p-GaN thin film heterostructures are utilized in order to increase the extraction efficiency of the LEDs by virtue of the waveguiding properties of the nanowires. But in all of these cases, the n-ZnO nanowires are randomly distributed on the substrate, which largely limits their applications in high performance optoelectronic devices. Here in this work, we demonstrate the capability of controlling the spatial distribution of the blue/near-UV LEDs composed of position controlled arrays of n-ZnO nanowires on a p-GaN thin film substrate [1]. The device was fabricated by a conjunction of low temperature wet chemical methods and electron beam lithography (EBL) [2]. The EBL could be replaced by other more convenient patterning techniques, such as photolithography and nanosphere lithography, rendering our technique low cost and capable of scaling up easily. Under forward bias, each single nanowire is a light emitter. By Gaussian deconvolution of the emission spectrum, the origins of the blue/near- UV emission are assigned particularly to three distinct electron hole recombination processes. By virtue of the nanowire/thin film heterostructures, these LEDs give an external quantum efficiency of 2.5%. This approach has great potential applications in high resolution electronic display, optical interconnect, and high density data storage [3][4].[1] Sheng Xu , Chen Xu , Ying Liu , Youfan Hu , Rusen Yang , Qing Yang , Jae-Hyun Ryou, Hee Jin Kim , Zachary Lochner , Suk Choi , Russell Dupuis and Zhong Lin Wang, “Ordered nanowire array blue/near-UV light emitting diodes”, Adv. Mater., 2010, doi:10.1002/adma.201002134.[2] Sheng Xu, Yaguang Wei, Melanie, Kirkham, Jin Liu, Wenjie Mai, Dragomir Davidovic, Robert L. Snyder and Zhong Lin Wang, “Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low temperature without Catalyst”, J. Am. Chem. Soc., 2008, 130, 14958-14959.[3] Research supported by DARPA, DOE and NSE.[4] For more information: http://www.nanoscience.gatech.edu/zlwang/
9:00 PM - EE9.39
Tailored Synthesis of Crystalline WO3 Nanowires and their Application in Novel NOx Sensor Prototypes.
Dachi Yang 1 , Luis Valentin 1 , Luis Fonseca 1
1 Department of Physics and Institute for Functional Nano materials, University of Puerto Rico Rio Piedras, San Juan United States
Show AbstractTungsten trioxide nanowires (WO3 NWs) have been synthesized inside the nanochannels of porous Anodic Aluminum Oxide (AAO) template by first sputtering an Au layer on its planar surface side and subsequent electrodeposition. The method allows the crystallization of single- and poly-crystalline NWs with tailored length and diameter as required. The approaches on manipulation of single and multiple WO3 NWs between two different electrodes have been developed. NOx sensor prototypes made of single and multiple WO3 NWs are constructed, showing that both are sensitive to NOx gas, and the single NW prototype shows much shorter response. Our research may provide experimental base for the production of sensitive NOx sensors and future nanotechnology.
9:00 PM - EE9.4
Metal-oxide-nanowire-based Electronic Nose Using Heterogeneous Catalysis as a Functionalization Strategy.
Jeong Min Baik 1 3 , Myung Hwa Kim 2 3 , Alec M. Wodtke 3 , Martin Moskovits 3
1 , Ulsan National Institute of Science and Technology, Ulsan Korea (the Republic of), 3 , University of California, Santa Barbara, Santa Barbara, California, United States, 2 , Ewha Womans University, Seoul Korea (the Republic of)
Show AbstractOne-dimensional (1-D) nanostructures of metals and metal-oxides are currently the subject of intense research both in order to discover fundamental science at the nanoscale as well as for their potential as sensing, catalytic, and other electronic applications. Furthermore, the development of MEMS microfabrication techniques and nanotechnology has significantly generated cost savings in materials used or savings in the time it takes to perform a process. Here, I will show you some sensor performances obtained by using 1-D metal-oxide nanostructures. An electronic nose (e-nose) strategy is described based on SnO2 nanowire arrays whose sensing properties are modified by changing their operating temperatures and by decorating some of the nanowires with metallic nanoparticles. Since the catalytic processes occurring on the metal nanoparticles depend on the identity of the metal, decorating the semiconducting nanowires with various metal nanoparticles is akin to functionalizing them with chemically specific moieties. Other than the synthesis of the nanowires, all other steps in the fabrication of the e-nose sensors were carried out using top-down microfabrication processes, paving the way to a useful strategy for making low cost, nanowire-based e-nose chips. The sensors were tested for their ability to distinguish three reducing gases (hydrogen, carbon monoxide, and ethylene), which they were able to do unequivocally when the data was classified using linear discriminant analysis. The discriminating ability of this e-nose design was not impacted by the lengths or diameters of the nanowires used. With the devices characteristics, the synthesis and properties of 1-D novel metal-oxides such as VO2, RuO2, Fe3O4 will be also shown here.
9:00 PM - EE9.40
Surface Elasticity Effect on a Bent Piezoelectric Nanowire Based on Core-shell Model.
Haiyan Yao 1 , Guohong Yun 1
1 College of Physical Science and Technology, Inner Mongolia University, Hohhot China
Show AbstractNanowires have attracted tremendous promise as nanoscale devices and nanoelectromechanical systems (NEMS). Recently, a wurtzite crystal, zinc oxide (ZnO) is fabricated to nanowires/nanobelts and applied as power generators[1], sensors[2], Schottky diodes and switches[3] , and field-effect transistor (FET)[4]. Owing to it’s well coupled piezoelectric and semiconducting properties, ZnO nanowires/nanobelts become important candidates of NEMS.The theoretical method of nanogenerator and nanopiezotronics is reported by Yifan Gao and Zhong Lin Wang[5], under applying the perturbation theory and continuum model . Though the results show accuracy and validity, they ignore ZnO NWs surface elasticity importance. Chen et al[6] report the Young’s modulus of ZnO NWs is significantly higher than that of bulk ZnO with diameter decreasing, especially less than 120nm.This paper is based on Core-Shell model, considering surface elasticity effect, calculates charge density and potential distribution in a bent ZnO nanowire. The conclusion shows that the piezoelectric potential distribution with radius changing across the middle of NWs cross section ( and ) is smaller than the results reported by Yifan Gao et al[5] because of high surface Young’s modulus influence. The piezoelectric polarization charge density is also independent of coordinate . In addition we mainly discuss the potential distribution with nN and nm to 50nm , which are the experiment parameters[1]. Our calculated theory value is about 9 to 10.7mV, which is much smaller than 17.8mV using the model of Yifan Gao et al[5]. Our results agree well with the experiment measurement of the output voltage ~10mV in the literature of Science (2006) [1].References1. Z. L. Wang and J. H. Song, Science 312, 242 (2006).2. J. Zhou, Y. D. Gu, P. Fei, W. J. Mai, Y. F. Gao, R. S. Yang, G. Bao, and Z.L. Wang, Nano Lett. 8, 3035 (2008).3. J. Zhou, P. Fei, Y. D. Gu, W. J. Mai, Y. F. Gao, R. S. Yang, G. Bao, and Z.L. Wang, Nano Lett. 8, 3973 (2008).4. X. D. Wang, J. Zhou, J. H. Song, J. Liu, N. S. Xu, and Z. L. Wang, Nano Lett. 6, 2768 (2006).5. Yifan, Gao; Z. L. Wang; Nano Lett. 7, 2499-2505 (2007).6. C.Q. Chen, Y. Shi, J. Zhu, Y. J. Yan, Phys.Rev. Lett. 96, 075505 (2006).
9:00 PM - EE9.42
Hollow ZnO Nano Cone Synthesis via Catalyst Free MOCVD.
Wei Zhang 1 , Karheinz Strobl 1
1 , CVD Equipment Corp., Ronkonkoma, New York, United States
Show AbstractZinc oxide is a wide band gap material with semiconducting, photonic and piezoelectric properties. In the past ten years zinc oxide nano-structures such as nanowires and nanorods have received great interest due to their unique dimensional and material properties in the area ranging from photonics, electronics, mechanics, energy recovery, etc. In this paper we report the manufacturing process of a new shape, i.e. hollow hexagonal ZnO nano cones. We grew them on Si wafers using a low pressure, catalyst free, metal organic chemical vapor deposition (MOCVD) process on a FirstNano EasyTube 3000™ MOCVD system. Nitrogen was used as carrier gas to bring the reactants, DEZ and H2O, to the Si wafer surface. At the right balance of process temperature and carrier/precursor gas flow rate the ZnO nano structure transitioned into a hollow hexagonal cones growth mode. The both two and three dimensional aspects of these catalyst free hollow hexagonal ZnO nano cones, which are novel to the best of our knowledge, could lead to new applications in photonics, near field probing, chemical sensors, quantum confinement, electronic, etc.
9:00 PM - EE9.44
Effects of Piezopotential Spatial Distribution on Local Contact Dictated Transport Property of ZnO Micro/Nanowires.
Yan Zhang 1 2
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Physics, Lanzhou University, Lanzhou, Gansu, China
Show AbstractWe present a study on the effect of deformation induced piezoelectric potential piezopotential in a ZnO micro/nanowire on its electrical transport properties by choosing different contacting locations on the wire to form a metal-ZnO contact. When a ZnO wire is under nonuniform deformation, the local piezopotential distribution at different positions shows significant effect on and distinct trend of variation in the charge carrier transport characteristic. This study has a broad impact on understanding the characteristics of piezotronic devices based on nanowires of wurtzite materials by controlling local contact position and contact size.
9:00 PM - EE9.5
ZnO-GaN(p) Heterostructures for Optoelectronics and Chemical Sensing.
Camilla Baratto 1 , Silvia Todros 1 , Alessio Cherubini 1 , Elisabetta Comini 1 , Guido Faglia 1 , Giorgio Sberveglieri 1 , Andreas Weimar 2
1 Chemistry and Physics, University of Brescia and CNR-IDASC, Brescia Italy, 2 , OSRAM Opto Semiconductors GmbH, Regensburg Germany
Show AbstractZnO is a material of great interest for optoelectronic applications due to a direct wide bandgap energy of 3.37 eV, a large exciton binding energy of 60 meV at room temperature: ZnO single crystalline nanowires based light emitting diodes (LEDs) have been considered as a potential candidate for the next generation of blue/ near-UV light sources. Since stable p-ZnO was not obtained yet, heterojunctions of n-ZnO and p-GaN are employed as an alternative approach. Compared with the thin film/thin film LEDs, which may suffer from the total internal reflection, n-ZnO nanowire/p-GaN thin film heterostructures are utilized in order to increase the extraction efficiency of the LEDs by virtue of the waveguiding properties of the nanowires. Moreover, ZnO performances for conductometric chemical sensing have been known for forty years, while the properties as optical chemical sensor were recently discovered. An overview of the most recent results we have obtained in the study of optical properties of ZnO nanowires will be presented, focusing mainly on reversible photoluminescence (PL) quenching produced by gaseous NO2 traces on nanowires. Hybrid heterostructures were realized growing ZnO nanowires by VLS and VSS growth mechanisms on p-GaN thin film on sapphire substrate. The study of experimental parameters, required to achieve the super-saturation conditions to produce nanowires aligned perpendicular to the surface, will be thoroughly described. Basic investigations techniques were used to study the materials properties of ZnO and GaN and of the junctions, like PL at room temperature and morphological characterization by Field Emission Scanning Electron Microscope (FESEM). PL spectrum of ZnO nanowires at room temperature is composed of band edge emission at 380 nm and defect emission in the visible region (large band centered at 500 nm). For LED device realization PMMA was deposited as insulating layer and plasma etching was used to remove PMMA from tip of nanowires. The effect of plasma etching on PL properties of nanowires was studied in details. After that contacts were deposited by sputtering and followed by IV heterojunction characterization. At the end electroluminescence in the UV and visible region was proved from heterojunction. We acknowledge for funding ORAMA project FP-NMP-2009-LARGE-3 NMP-2009-2.2-1, Grant Agreement 246334: Oxide materials for electronics applications.
9:00 PM - EE9.6
ZnO Nanowire Gas Sensing Device: Electrical and Optical Characterization.
Andre Luis Cauduro 1 2 , Giovani Pesenti 1 2 , Henri Boudinov 1 2 , Joao Oliveira 1 2 , Fernando Zawislak 1 , Daniel Baptista 1 2
1 Physics, Federal University of Brazil, Porto Alegre, RS, Brazil, 2 Microelectronics Program (PGMicro), Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, RS, Brazil
Show AbstractSingle crystalline zinc oxide nanowires (NWs) have attracted a lot of interest in the last years due to the possibility of fabricating special nanoelectronic and nanophotonic devices. ZnO is a II-VI direct wide band gap (3.37 eV) semiconductor with large exciton energy [1] (60 meV, more than two times the exciton binding energy of gallium nitrate which is the most used material in optoelectronics devices). In addition, ZnO may present n-type behavior due to oxygen vacancies (VO) and zinc interstitials (ZnI) [2], having potential applications on gas and chemical sensors [3].ZnO NW’s semiconductors have enormous potential in high-sensitive, fast and selective sensing applications due to its high surface-to-volume ratio and Debye length comparable to the nanowire radius over a wide range of temperature and doping. ZnO nanowires can improve a nano-sensor gas device which quickly detects small quantities (p.p.m. and p.p.b.) of a certain substance, such as CO, CO2, O2, NH3, H2, NO, etc., becoming a powerful device to control an environment from the presence of harmful substances, for instance. In the present work, we report the development of crucial steps in the micro/nanofabrication of a ZnO nanowire gas sensor using e-beam lithography technique. Ohmic and schotkky contacts were performed in order to measure the electronic transport responsible for the nanowire sensing mechanism. The described processes involve the synthesis as well as electrical and optical characterization of ZnO nanowires grown on sapphire and silicon substrates by the vapor phase method [4]. The growth mechanism is still controversial until today and it is also a crucial and primary research step for the development of nanowire-based sensors. Electrical measurements on the ZnO nanowires in controlled gaseous environment show a change in conductivity with respect to different adsorbed species on the nanowire surface. The surface defects play a fundamental role in the gas sensing mechanism. Then, we present a low temperature photoluminescence measurement in order to investigate the energy levels of those point defects that are responsible by changing the potential surface and, consequently, cause an energy band bending on the semiconductor.Work supported by CAPES Brazilian Federal Agency and Microelectronics Program (PGMicro). [1] S. Y. Kim. Chem. Phys. Lett. 462, 100 (2008).[2] A. F. Kohan. Phys. Rev. B 61, 15019 (2000).[3] J. HAHM, Nano Lett. 4, 51 (2004).[4] Y. W. HEO, Mat. Sci. and Eng. R. 47, 1 (2004).
9:00 PM - EE9.7
Highly Sensitive ZnO Nanowire UV Photodetectors with Dielectric MgO Layers.
Dong Chan Kim 1 , Byung Oh Jung 1 , Hyung Koun Cho 1 , Ju Ho Lee 2 , Jeong Yong Lee 2
1 School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do, Korea (the Republic of), 2 Department of Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractRecently, ultraviolet (UV) photodetectors have drawn a great deal of attention due to a wide range of civil and military applications. The most UV detectors were mainly Si-based photodetectors. However, they have limitations which need a filter to eliminate low energy photons of visible and IR light. To avoid these disadvantages, UV photodetectors based on wide band-gap semiconductors have been studied due to their intrinsic visible-blindness. Among them, the ZnO is currently considered to be a promising material for UV detectors due to its direct and wide energy band-gap (3.37 eV), and makes it possible to prepare UV detectors with different cut-off wavelengths. The ZnO thin films based detectors mainly possess the ZnO/Si heterojunction and use photoelectrons generated from the depletion layer with anisotropic band offset. However, the UV selectivity with respect to background signal was low, because of the contribution of the photoelectrons generated from the Si side by visible light. As an alternative method, the inserting of insulating dielectric layers (MgO & SiO2) at the ZnO/Si interface significantly enhanced the selectivity and the responsivity of UV wavelength. On the other hand, the sensing performance of nanowire based UV detectors is strongly dependent on the quantity of oxygen molecular absorbed on surface of nanowires. However, the most nanowire detectors used the TFT based photodiode structure using single nanowire, while studies concerning the UV detectors using vertical arrayed nanowires have not actively been performed. In particular, the nanowire detectors grown by CVD have fast recovery time and low responsivity by increasing background current level due to conducting property of nanowires. In this study, it is demonstrated that the responsivity, recovery time, and selectivity were significantly improved by inserting an ultrathin dielectric MgO layer in n-ZnO nanowires/n-Si substrates. All growth process was performed by MOCVD. Based on the spectral response of the UV detectors, we pointed out that the inserted MgO layers only contribute to the decrease of the background current by blocking photo-generated electrons, and the improved UV sensing property was due to tunneling current induced by two-dimensional electron gas (2DEG) under UV illumination.
9:00 PM - EE9.9
Nanowire-metal Hybrid Structures: The Influence of the Deposition Technique on the Optical Properties.
Apurba Dev 1 , Raphael Niepelt 2 , Jan Richters 1 , Carsten Ronning 2 , Tobias Voss 1
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 Institute of Solid State Physics, University of Jena, Jena Germany
Show AbstractSemiconductor nanowires have drawn widespread attention for their potential use in many optical devices. Despite of that, the low quantum efficiency of spontaneous emission from these nanowires has so far hindered any commercial production of such devices. With decreasing nanowire diameter, the influence of the surface states becomes dominant and strongly reduces the luminescence intensity by introducing additional non-radiative recombination channels for the excited carriers. Among various means to mitigate this problem, the use of surface plasmon resonances of metal nanoparticles (NPs) has been found to be most effective as the excited carriers in this case can relax very fast by transferring the energy to the surface plasmons. However, the energy of the plasmon resonance and hence the efficiency of such a process strongly depend on the size, kind and distribution of the metal NPs. For nanowires it is very difficult to get a homogeneous coverage of suitable metal NPs due to the non-flat nature of the surface. Although sputtering in argon plasma provides a simple and effective means to deposit metal NPs homogeneously on nanowires, we have observed that such deposition technique itself induces some additional effects.We investigated the time-integrated and time-resolved photoluminescence (PL) properties of ZnO nanowires coated with Au, Ag and Pt nanoparticles which were deposited by DC sputtering in Ar plasma. To separate the influence of the Ar plasma from that of the metal particles on the PL properties of the ZnO nanowires, we performed a mild Ar plasma treatment using a sputter-coating system while shielding the nanowires to avoid metal deposition. A strong enhancement of the near-band-edge emission and a quenching of the deep-level emission were observed in all cases. Time-resolved spectroscopy performed on both plasma-treated and metal-nanoparticle-coated samples showed a reduction of the radiative lifetime in all samples irrespectively of size and kind of metal particles. The enhancement and quenching at room temperature were measured quantitatively using an integrating sphere. Photoluminescence studies at 4 K revealed a strong hydrogen donor-bound-exciton line indicating unintentional incorporation of hydrogen during the plasma treatment. To confirm the results, hydrogen was implanted into ZnO nanowires with a low ion energy of 600 eV and different fluences. The results can be explained by considering the passivation of deep centers by hydrogen and the introduction of a large amount of hydrogen donors to which excitons can bind.
Symposium Organizers
Volker Schmidt Max Planck Institute of Microstructure Physics
LincolnJ. Lauhon Northwestern University
Takashi Fukui Hokkaido University
GeorgeT. Wang Sandia National Laboratories
Mikael Bjoerk IBM Research GmbH
Symposium Support
AIXTRON SE
FEI Company
IBM Research Zurich
JEOL USA
Veeco
EE11: Biological Applications and II-VI Nanowires
Session Chairs
Friday PM, April 29, 2011
Room 3001 (Moscone West)
2:30 PM - EE11.1
Array of Silicon Field Effect Transistors to Detect Charges Propagation in Neuron Circuit.
Cecile Delacour 1 , Guillaume Bres 1 , ghislain Bugnicourt 1 , Thierry Crozes 1 , Thierry Fournier 1 , Catherine Villard 1
1 , Institut Néel-CNRS, Grenoble France
Show AbstractOne of the most existing new tasks of nanoelectronics is to measure neuronal activity. And one of the most promising tools for that is silicon nanowires as they act as ultra-sensitive transistors and are CMOS compatible allowing large scale integration. They have already proved their ability to measure single charge events with a field effect detection[1]. For our specific purpose, nerve cell should act as a top gate electrode and shift the conductance of the nanowire when a potential crosses it, ie when cell is carrying an information.Few years ago, a short voltage pulse related to the propagation of charges in a nerve has been detected with silicon nanowires [2]. More recently the coupling between a neuron and an array of nano-FETs has been obtained [3]. Our goal is to measure real propagation of neuronal signal through a single neuron and eventually through a defined path formed by coupled neurons. This would help to go deeper in our understanding of information proceesing in the nerve system and in a distant future would contribute to the study of abnormal neural activity involved in some medical diseases like Parkinson. In this work, an organized growth of neuronal cells with axonal differentiation along chosen partterns has been obtained. This allows us to control the propagation path of neuronal signal. In addition, we show the realization of silicon nanowires (100nm) and their characterization to study effects of doping, annealing and schottky barriers on their electronic properties. We investigate the sensitivity of these devices in physiological liquid to detect weak changes of charge and their coupling to complementary patterns of neurons. An innovative nano-fabrication technic of lithography with an atomic force microscope [5] is also used to reduce the section of the silicon wire and investigate the effect on their detection efficiency. To record neural propagation, we have developed a specific electronic card to implement a synchronous detection of nanowires in the matrix. Nanowires multiplexing is obtained up to Khz-rate (keeping capacitive effects low and detection times short). At the present time, this card appears as one of the best and will be a promising tool to study charges transport through single and coupled nerve cells. [1] A. Fujiwara et al. Appl. Phys. Lett. 88, 053121 (2006). [2] R. Alexander Kaul, Naweed I. Syed, and Peter Fromherz, PRL 92, 038102-1 (2006).[3] F. Patolsky et al. Science 313, 1100-04 (2006) .[4] C.Delacour et al. Appl. Phys. Lett. 90, 191116 (2007)
2:45 PM - EE11.2
Nanowire-nanopore Transistor Sensor for DNA Detection.
Ping Xie 1 , Qihua Xiong 2 3 , Ying Fang 4 , Quan Qing 1 , Charles Lieber 1 5
1 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Singapore, 3 School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore Singapore, 4 , National Center for Nanoscience and Technology, Beijing China, 5 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractNanopore sequencing, as a promising low cost, high throughput sequencing technique, has been proposed more than a decade ago. Due to the incompatibility between small ionic current signal and fast translocation speed and the technical difficulties on large scale integration of nanopore for direct ionic current sequencing, alternative methods rely on integrated DNA sensors have been proposed, such as using capacitive coupling or tunnelling current etc. But none of them have been experimentally demonstrated yet. Here we show that for the first time an amplified sensor signal has been experimentally recorded from a nanowire-nanopore field effect transistor sensor during DNA translocation. Independent multi-channel recording was also demonstrated for the first time. Our results suggest that the signal is from highly localized potential change caused by DNA translocation in none-balanced buffer condition. Given this method may produce larger signal for smaller nanopores, we hope our experiment can be a starting point for a new generation of nanopore sequencing devices with larger signal, higher bandwidth and large-scale multiplexing capability and finally realize the ultimate goal of low cost high throughput sequencing.
3:00 PM - EE11.3
Nanowire-based Single Cell Endoscopy.
Ruoxue Yan 1 2 , Ji-Ho Park 1 2 , Yeonho Choi 4 , Chuljoon Heo 3 5 , Seung-Man Yang 5 , Luke Lee 3 , Peidong Yang 1 2
1 Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Biomedical Engineering, Korea University, Seoul Korea (the Republic of), 3 Bioengineering, University of California, Berkeley, Berkeley, California, United States, 5 Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractNanostructured smart probes that can be safely internalized into biological cells hold enormous potential for selective targeting, efficient gene or drug delivery, intracellular imaging and sensing of molecular dynamical processes, all of which are in critical demand for understanding complex biological events and theranostics. However, our efforts to use them for optical communication cross the cellular membrane at the subwavelength level remains very limited. Here, we show that an optical nanoprobe, composed of a nanowire-based waveguide with diameter of 100~250 nm and length of up to 100 µm that is optically coupled to a tapered conventional optical fiber, can guide and confine visible light into intracellular regions selectively. The optical nanoprobe can deliver payloads into the cell with a high spatiotemporal precision, illuminate intracellular compartments and detect optical signals from subcellular regions with high spatial resolution. The intracellular insertion and illumination of the optical nanoprobe did not induce any significant cytotoxicity. Like the endoscope used in medicine that can directly image and examine the interior of the human body, this minimally invasive optical nanoprobe could become a powerful tool for nanoscale payload delivery, imaging, and probing in single living cells.
3:15 PM - EE11.4
Quantitative Characterization of ZnO Nanowire Arrays by Quantum Efficiency.
Daniel Gargas 1 2 , Hanwei Gao 1 2 , Hungta Wang 1 , Peidong Yang 1 2
1 Chemistry, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractDue to their high crystallinity and unique material properties, semiconductor nanowires have garnered considerable interest in photonic and opto-electronic applications. Further understanding of the relation between electronic and optical properties with respect to material quality remains crucial for the realization of nanowire devices. While the electronic properties of semiconductor nanowires have been thoroughly investigated, their direct correlation with intrinsic optical properties, such as photoluminescence, remains relatively unknown. External Quantum Efficiency (EQE), which measures the absolute loss within a material beginning from exciton creation to radiative recombination, is inherently tied to the photovoltaic and electronic properties and, therefore, can be utilized to optimize materials for solar and LED applications. Here we present the optimization of room-temperature EQE of vertical ZnO nanowire arrays grown by chemical vapor transport. Additionally, we show a dependence of EQE on the oxygen percent and cooling-rate during gas-phase synthesis, and report the corresponding electronic properties, such as carrier concentration and electron mobility, of individual ZnO nanowires.
3:30 PM - EE11.6
Functionalization and Environmental Stabilization of ZnO Nanobridge Sensors Fabricated using Carbonized Photoresist.
Ashley Mason 1 , Chien-Chih Huang 1 , Saki Kondo 2 , Myra Koesdjojo 2 , Nate Stephon 1 , Vincent Remcho 2 , John Conley 1
1 EECS, Oregon State University, Corvallis, Oregon, United States, 2 Chemistry, Oregon State University, Corvallis, Oregon, United States
Show AbstractTwo of the major challenges to the application of nanowires as bio-sensors are integration of nanowires into electrically accessible device and sensor selectivity. A novel method for the selective growth and directed integration of ZnO NWs was achieved using a lithographically patterned carbonized photoresist layer (C-PR). This C-PR method avoids the use of metal catalysts, seed layers, and additional patterning processes. Growth and electrical connection of NWs take place simultaneously for many devices. Electrical measurements on three-terminal C-PR/ZnO nanobridge/C-PR devices show field effect modulation of the conductivity of the ZnO channel via a back gate. Nanobridge devices were found to perform well as gas (O2 and H2O) sensors and show excellent sensitivity to UV exposure (> 10^4 increase in current). In addition, the response of the three-terminal nanobridge sensors to UV and oxygen was enhanced by application of a negative bottom gate voltage. To demonstrate selective sensing, ZnO nanobridge devices were functionalized via the adsorption of biotin on the surface. However, biotinylated ZnO NWs were found to rapidly dissolve in an aqueous environment. Parylene-A was investigated as a moisture barrier and potential functionalization platform. A CVD process for parylene-A was developed and it was demonstrated that parylene-A coated ZnO NWs do not show any signs of dissolution after 24 hours in an aqueous solution. The impact of the parylene-A coating on the electrical performance and sensitivity of ZnO nanobridge devices was investigated. A comparison of UV and O2 sensitivity for uncoated and coated devices revealed that a thin coating of parylene-A attenuates both the UV and O2 response, suggesting the ability to modulate environmental sensitivity using this non-covalently bound polymer passivation layer. It has been demonstrated by others that the available amine groups on parylene-A can bind to the carboxylate groups on BSA by utilizing a crosslinker, such as glutaraldehyde to form a covalent bond. Thus, in addition to providing protection of the ZnO NWs for liquid and vapor-phase sensing, parylene-A may also serve as an activation layer for further functionalization. In conclusion, this work demonstrates the novel use of a parylene-A coating as a potential starting platform for the general functionalization of ZnO NW devices for selective sensing in the liquid or vapor phase.
3:45 PM - EE11: Bio App
BREAK
4:30 PM - EE11.7
Electronic Properties of Sulfurized Antimony Selenide Nanocrystals and Assemblies.
Rutvik Mehta 1 , C. Karthik 1 , Wei Jiang 1 , Binay Singh 1 , Yunfeng Shi 1 , Richard Siegel 1 , Theo Borca-Tasciuc 2 , Ganpati Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractAntimony selenide is a pnictogen chalcogenide semiconductor with ~1 eV band-gap and a fast amorphous-crystalline transition making it attractive for photovoltaics and phase-change memory applications. It is also a promising thermoelectric with a high Seebeck coefficient (α), but its figure of merit is limited by its low electrical conductivity (σ). Here, we report a rapid and scalable (gram-a-minute) microwave synthesis of one-dimensional nanocrystals of sulfurized antimony selenide that exhibit ~104-1010 times higher σ than non-nanostructured bulk or thin film forms of this material, while retaining a high α >-500 μV/K and a diminished thermal conductivity (κ) <1 W/mK, resulting in factorial gains in the thermoelectric figure of merit over bulk forms. Nanocrystal assemblies also show high σ, making the nanocrystals attractive building blocks to realize nanostructured thin film and bulk forms for thermoelectric device applications. Sulfurized antimony selenide nanowires were synthesized by exposing a mixture of a polyol with antimony and selenium precursors and a mercaptan-terminated surfactant to 2.4 GHz microwave radiation for 60-120s. The surfactant performs roles of shape-direction, oxide-suppression and dopant-delivery. We tune nanowire diameter and length between 30 nm to 400 nm, and 100 nm to 2 μm, by adjusting the microwave dose and control the sulfur content through pre-cursor concentration. As the nanocrystal diameter increases >100 nm, the nanowires transform into nanotubes through void formation and coalescence driven by axial rejection of sulfur incorporated into the nanowires from the surfactant; The dynamic morphological nanowire-nanotube transformation was captured using molecular dynamic simulations and we show that our approach permits independent tuning of morphology by the microwave dose and of substitutional sulfur content through chemical control.We show the presence of sulfur surface states and metastable sulfur gradients result in the transformed electronic behavior including up to ten-billion fold higher σ than bulk, ambient dependent σ, a negative α and a unique reversible electronic threshold switching behavior. Individual nanowires and nanotubes exhibit a charge carrier transport activation-energy of <60 meV arising from surface sulfur donor states, while the sulfurization induced degenerate carrier concentrations ~1018-1019 cm-3 provide sufficiently high density of shallow levels underlying the switching phenomena. Thermoelectric measurements on individual nanocrystals reveal large α with n-type behavior opposed to p-type bulk on account of the sulfur-mediated carrier reversal, and a size-scattering induced κ diminution. Finally we demonstrate that the enhanced electrical behavior in the one-dimensional nanocrystals can be transferred to bulk forms as thin films and bulk nanostructured solids, making sulfurized antimony selenide nanocrystals appealing for photovoltaic, phase-change memory and thermoelectric devices.
4:45 PM - EE11.8
Ferroelectric Arrays of Sb2S3 Nanowires.
Justin Varghese 1 2 3 , Sven Barth 1 2 3 , Lynette Keeney 2 , Roger Whatmore 2 , Justin Holmes 1 2 3
1 Chemistry, University College Cork, Cork Ireland, 2 , Tyndall National Institute, Cork Ireland, 3 , Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Dublin Ireland
Show AbstractAntimony sulfide or stibnite (Sb2S3) is semiconducting as well as a weak ferroelectric material, which exhibits phase transitions at 292 K and 420 K [1]. The interest in Sb2S3 arises from its strong anisotropic properties, including optical, ferroelectric, and piezoelectric which could be exploited at the nanoscale [2]. Grigas and Karpus revealed highly anisotropic dielectric permittivity behaviour, along the c-axis rather than in the (a-b) plane in Sb2S3 [1]. Therefore, if the polar c-axis of Sb2S3 nanowires is aligned parallel to the nanowire axis, its anisotropic ferroelectric and piezoelectric properties could be exploited. Thus, making vertically aligned, c-axis oriented, nanowire arrays of single crystal Sb2S3 is a potential way of harvesting these properties. To address the functionality of individual Sb2S3 nanowires, we have investigated the piezo and ferroelectric responses using piezo response force microscopy (PFM).Vertically aligned Sb2S3 nanowires were synthesized inside the pores of anodic alumina template (AAO) by a solvent-less method using a single-source precursor. High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) showed single crystalline Sb2S3 nanowires with a predominant <001> growth direction. The presence of ferroelectricity in Sb2S3 nanowires was established using switching spectroscopy piezoresponse force microscopy (SS-PFM). In addition, SS-PFM results show that Sb2S3 nanowires show electrostriction and weak piezoelectric behaviour. The observed ferro/piezoelectric behaviour at room temperature can be attributed to the polarisation results from small structural changes associated with the Sb and S atoms within the (Sb4S6)n chain along the c-axis [1,3]. References[1]. Grigas, J. Ferroelectrics 1978, 20, 173.[2]. Grigas, J. Microwave dielectric spectroscopy of ferroelectrics and related materials Gordon and Breach Publishers: Amsterdam B.V., 1996; Vol. 9, p. 124-143.[3]. Grigas, J.; Talik, E.; Lazauskas, V. Phase Transitions: A Multinational Journal 2002, 75, 323.
5:00 PM - EE11.9
Carrier Recombination Dynamics in Individual CdSe Nanowires.
Felix Vietmeyer 1 , Pavel Frantsuzov 2 , Boldizsar Janko 2 , Masaru Kuno 1
1 Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana, United States, 2 Department of Physics, University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractSemiconductor nanowires (NWs) are an emerging class of materials that offer unique optical and electrical properties. Applications include polarization sensitive photodetectors, lasers, light emitting diodes, logic gates and solar cells. These uses illustrate the versatility of NW optical/electrical properties for developing next generation technologies.In all cases, the ability to control NW carrier dynamics as well as recombination processes is key to realizing these applications. However, despite the fact that NWs have been readily employed in numerous proof-of-concept applications, associated carrier recombination mechanisms remain to be more thoroughly explored. Among important properties not yet fully characterized are emission/carrier lifetimes, quantum yields, the effects of varying carrier densities and disorder along the NW length.The present study therefore focuses on better elucidating room temperature carrier recombination mechanisms in solution-grown CdSe nanowires. In particular, carrier dynamics in single CdSe nanowires (NWs) have been studied using various approaches. They include measurements of single wire emission intensities as a function of pump fluence, excitation intensity-dependent emission quantum yields and excited state lifetimes. Ensemble transient differential absorption studies of induced bleach dynamics have also been conducted.Results of these studies show super linear growth of the emission intensity as a function of excitation intensity. This is corroborated by single nanowire emission quantum yields that vary as a function of excitation fluence and range from 0.1% to values over 10%. At the same time, measured emission lifetimes are short (<100 ps). By contrast, the nanowire band edge bleach persists for over a nanosecond. To explain all of the abovementioned results, a kinetic model that accounts for not only the nature of photogenerated carriers within the wires but also their subsequent recombination dynamics has been developed. Quantitative agreement with experiments using CW and pulsed excitation at various excitation fluences is achieved. These insights add to our basic understanding of semiconductor nanowire photophysics and may ultimately aid their future use in NW-based devices.
5:15 PM - EE11.10
Controlling Polarization through Aspect Ratio in Nano ``Rods in Rods” Heterostructures.
Amit Sitt 1 , Asaf Salant 1 , Uri Banin 1
1 The Institute of Chemistry and the Center of Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem Israel
Show AbstractStudying the behavior of nanostructures as they develop from the zero-dimensional to the one-dimensional regime is significant for unveiling the modifications that occur in the electronic structure of the particles as the length to width aspect ratio is increased. Such understanding can lead to better design and control of the particles properties, making them attractive for a wide spectrum of technological applications, such as lasers and polarized single photon emitters.In this work, we present the effect of the 0D to 1D transition in core/shell structures on their optical properties and electronic structure. We compare the optical properties of CdSe/CdS core/shell nano heterostructures of different morphologies including “sphere in a sphere”, “sphere in a rod” and “rod in a rod” with various core and shell aspect ratios. The novel “rods in rods” heterostructures are prepared using colloidal seeded growth synthesis and exhibit attractive properties such as highly polarized emission with improved quantum efficiencies. We discuss the changes which occur as the dimensionality of the particles changes, and the ability to control the optical properties, and in particular the polarization, by carefully tuning the dimensions and aspect ratios of the particles.
5:30 PM - EE11.11
Onset of Stimulated Emission in Lead Sulfide Nanowires.
Minghua Sun 1 , Patricia Nichols 1 , Debin Li 1 , Leijun Yin 1 , Zhicheng Liu 1 , Cunzheng Ning 1
1 Electrical Engineering, Arizona State University, Tempe, Arizona, United States
Show AbstractOptoelectronic devices operating in the mid-infrared (MIR) wavelength (2-5 µm) are very important for a wide range of applications. But it has been traditionally very challenging to realize these devices, due to strong non-radiative process in the narrow gap materials. Lead sulfide (PbS), one of the oldest known lead salt semiconductors, has many unusual and interesting optical properties and important applications in MIR optoelectronic devices including lasers, light emitting diodes, detectors and sensors. PbS has a direct bandgap of less than 0.5 eV at the L point of the Brillouin zone. The edges of conduction and valence bands of PbS have mirrorlike structure with almost the same effective masses. Such band structures enhance photon emission and at the same time reduce the Auger recombination. It is well established that very low carrier density is required for the onset of stimulated emission for lead salts because of their high interband matrix elements and low joint density of states. Therefore, PbS nanowires (NWs) provide one of the most favored conditions for achieving NW lasing in the MIR frequency range. However, due to various technical challenges of performing optical experiments in this wavelength range, there has been no report of photoluminescence (PL) study of these distinctive optical properties of PbS NWs. Here we present the results of our systematic and comprehensive temperature-dependent and excitation-intensity dependent PL characterizations of PbS NWs. The PbS NWs were grown on Si substrates using a Chemical Vapor Deposition (CVD) system and the length of these NWs is more than 10 µm with diameter as large as 600 nm. The NWs are high quality single crystals with the same rock-salt cubic structure as the bulk PbS. The PL peaks of PbS NWs showed the abnormal blue shift with increasing temperature. The consistency of peak positions between measured and calculated temperature-dependent PL spectra further confirmed the band-edge origin of these emissions. Strong band-edge emission can still be observed even at temperature as high as 450 K indicating high crystal quality and high luminescence efficiency of these PbS NWs. Excitation-dependent PL measurements were performed at different temperature and the obtained PL spectra were compared with the calculated spontaneous PL spectra. Most interestingly, we observed three different types of linewidth dependence on pumping power. At room temperature, the measured PL width increases with pumping power, consistent with spontaneous emission spectra; at temperature of 77 K, the linewidth first increases and then decreases with pumping, showing the onset of stimulated emission. At 10 K, linewidth shows a monotonous decrease with pumping starting from very low level, indicating the dominance of stimulated emission. In conclusion, we observed clear onset of stimulated emission in PbS NWs, a critical step in achieve lasing in this important and challenging wavelength range.
5:45 PM - EE11.12
On-nanowire Heteroepitaxial Integration of Diode and PCRAM.
Inchan Hwang 1 , Yong-Jun Cho 1 , Moon-Ho Jo 1 2
1 Department of Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of), 2 Division of Advanced Materials Science, POSTECH, Pohang Korea (the Republic of)
Show AbstractPhase change memory (PCM) has been the subject of considerable recent interest as next-generation nonvolatile data storage, offering good cycling endurance, extended scalability, and reduced switching times. PCM programs the resistance of a cell by changing the phase of phase-changing material such as Ge2Sb2Te5 (GST) using Joule heating from the electrical current. A linear integration of a phase change memory (PCM) cell and a p-n diode can be an interesting architecture, since it can further increase the area density of PCM cells by minimizing single memory-cell sizes. For that, axially heteroepitaxial integration of a PCM cell and a diode can be a candidate for this unidirectional programming and reading of PCM using diode as a driving device. Here, we report on-nanowire (NW) integration of Si and Ge p-n diodes and GeTe in a heteroepitaxial manner. Axial NW heteroepitaxy has been successfully achieved by catalytic chemical vapor syntheses vapor-liquid-solid (VLS) mechanism, in which Si and Ge p-n diode NWs are synthesized by complementary doping, followed by axial elongation of GeTe PCM NW cells. We also discuss a basic circuit characteristics around the roles of the unique one-dimensional geometry of the cells in comparison to the bulk counterpart.