Ritesh Agarwal University of Pennsylvania
Wei Lu University of Michigan
Oliver Hayden Siemens AG
Akram Boukai University of Michigan
W1: Nanowire Opening Session
Monday AM, November 29, 2010
Ballroom C, 3rd floor (Hynes)
9:30 AM - **W1.1
Semiconductor Nanowires: Building Blocks for Today and the Future.
Charles Lieber 1 Show Abstract
1 , Harvard University, Cambridge, Massachusetts, United States
Advances and breakthroughs in nanoscience depend critically on development of nanostructures whose properties are controlled during synthesis. This presentation focuses generally on this concept using nanowires as a platform material. First, the synthesis of complex modulated nanowires in which rational design can be used to precisely control composition, structure and most recently structural topology will be discussed. These unique material characteristics, which have led to the emergence of nanowires as a central material in nanoscience, will be exploited to investigate fundamental properties and performance limits of photovoltaic devices at the single nanowire level. In addition, the capabilities of nanowire will be highlighted through their unique capability to create unprecedented active interfaces cells and tissue. Recent work pushing the limits of both multiplexed extracellular recording at the single cell level and the first examples of intracellular recording will described, as well as the prospects for truly blurring the distinction between nonliving and living information processing systems.
10:00 AM - W1.2
Tuning the Color of Silicon Nanowires.
Linyou Cao 1 , Mark Brongersma 2 Show Abstract
1 , University of California, Berkeley, Berkeley, California, United States, 2 , Stanford University, Stanford, California, United States
Empowering silicon (Si) with optical functions is one of the most important problems in materials research. Nanofashioning represents a general strategy to turn Si into a useful photonic material and Si structures have been engineered to realize light emission, optical cloaks, high confinement waveguides, nonlinear optics, enhanced light absorption and Raman scattering. Here, we demonstrate that a wide spectrum of colors can be generated by harnessing the strong resonant light scattering properties of Si nanowires under white light illumination. The strong spectral dependence of the light scattering on the structure size, dielectric environment, and illumination conditions opens up entirely new applications of Si and puts this material in a new light.
10:15 AM - W1.3
Transforming Semiconductor Nanowires Into Heterostructures and Superlattices by Size-dependent Cation Exchange Reactions.
Ritesh Agarwal 1 , Bin Zhang 1 , Yeonwoong Jung 1 , Hee-Suk Chung 1 , Lambert van Vugt 1 , Ju Li 1 Show Abstract
1 Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
The unique properties of nanostructured materials enable their transformation into complex, kinetically-controlled morphologies which cannot be obtained during their growth. Solution-phase cation-exchange reactions can transform sub-10 nm nanocrystals/nanorods into varying compositions and superlattice structures; however, due to their small size, cation-exchange reaction rates are extremely fast which limits control over the transformed products and possibilities for obtaining new morphologies. Nanowires are routinely synthesized via gas-phase reactions with 5-200 nm diameters and their large aspect ratios allow them to be electrically addressed individually. To realize their full potential, it is desirable to develop techniques which can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase to be compatible with nanowire growth schemes. We report transformation of single-crystalline cadmium-sulfide nanowires into composition-controlled ZnxCd(1-x)S nanowires, core-shell heterostructures, metal-semiconductor superlattices (Zn-ZnCdS), single-crystalline ZnS nanotubes, and eventually metallic Zn nanowires by utilizing size-dependent cation-exchange reaction along with temperature and gas-phase reactant delivery control. Simulations that account for elastic interactions due to atomic size mismatch and diffusional kinetics reveal the conditions for forming these structures, mapping out critical theoretical issues like nucleation versus growth rate of domains, and the peculiarities of 1D versus 3D systems. This versatile synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits.B. Zhang, Y. Jung, H.-S. Chung, L.K. v Vugt and R. Agarwal. "Nanowire Transformation by Size-Dependent Cation Exchange Reactions," Nano Letters, v.10, 2010, p. 149.
10:30 AM - W1.4
Room Temperature Photoluminescence in Silicon Nanowires.
Vladimir Sivakov 1 , Felix Voigt 1 2 , Florian Talkenberg 1 , Bjoern Hoffmann 1 , Gerald Broenstrup 1 , Gottfried Bauer 2 , Silke Christiansen 3 1 Show Abstract
1 , Institute of Photonic Technology, Jena Germany, 2 , Carl-von-Ossietzky University, Oldenburg, Germany, 3 , Max Planck Institute for the Science of Light, Erlangen Germany
Silicon nanowire (SiNW) ensembles with different architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(111) and p-Si(100)) with an etching hard mask of silver nanoparticles that are deposited by wet electroless deposition on polystyrene pattered silicon surfaces. Two steps electroless wet chemical etching (WCE) is al method to produce silicon nanowire (SiNW) material from crystalline silicon wafers. SiNWs built by WCE were investigated by photoluminescence (PL) measurements with excitation at 488 nm and power density of about 3.2 mW per mm2. Strong visible (red-orange) room temperature photoluminescence has been observed in wet chemically etched heavily (1020 cm-3) and lowly (1015 cm-3) doped SiNWs. The as-prepared samples show strong visible PL at room temperature peaking at 1.5 eV to 1.6 eV. After treatment by hydrofluoric acid (HF) PL partly vanishes, but substantial PL remains, peaking now at 1.4 eV. In this paper the possible origins of PL of the SiNW and PL dependence on the WCE kinetic’s are investigated. We interpret the results in the framework of a two media model, assuming that one part of the PL contribution stems from quantum confined nano-crystalline states located at the SiNW sidewalls and another part of the PL arising from SiOx related states located around the SiNW surfaces and on top of the sample surface. Considering the known possibilities of PL origin for various Si based material compositions we deduce a coherent picture describing the PL origin of the SiNW based samples under investigation. Our observations strongly suggest that visible light emission at room temperature of SiNWs is a result of the rough sidewall structure that can be such that nanoscale features form that make quantum confinement most probable. Significant light absorption (over 90% in a range between 300-2000 nm) was observed in the SiNWs covered by the TCO (Al doped ZnO) thin layers performed via Atomic Layer Deposition. The strong absorption, less reflection of visible and infra-red light and room temperature photoluminenscence of the SiNW ensembles strongly suggest that such a material has a real potential to be applied in the fields of opto-electronics, photonics, sensoric and photovoltaics. The morphology, crystallographic and surface structure, and optical properties of SiNWs will be presented and discussed in details.
10:45 AM - W1.5
Thermoelectric Properties of Nanostructured Tensile Strained Silicon.
Xiao Guo 1 , Xianhe Wei 1 , Arun Kota 1 , Anish Tuteja 1 , Akram Boukai 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
The thermoelectric figure of merit is determined by ZT = S2T/ρκ. It is known that n-type, tensile strained silicon shows increased electron mobility, due to the reduced intervalley scattering and lighter electron effective mass. Thus, the power factor (S2/ρ) could be potentially increased. It is also predicted that the thermal conductivity, κ, can be significantly decreased to near the minimum theoretical value, 1W m-1K-1, by utilizing nano-scale boundary confinement. Therefore, the combination of tensile strained silicon and nano-scale features could lead to a high figure of merit.Here we report a unique method of patterning nano-scale arrays of lamellar and cylindrical domains with block copolymer nanolithography. In particular, the block copolymer is used as a template to transfer the nano-scale lamellar and cylindrical features to a thin film (15nm) silicon layer by plasma ion etching. Eventually, tensile strained silicon with these nanostructures is obtained and studied using thermal/electrical connections, which are fabricated by photolithogaphy. We will present thermoelectric measurement results of seebeck coefficient, electrical conductivity, and thermal conductivity obtained on these strained nanostructured materials. A theoretical model incorporating strain effects on thermoelectric properties will also be demonstrated. F. Schäffler. High-mobility Si and Ge structures. Semicond. Sci. Technol 12, 1515-1549 (1997) D. G. Cahill, S. K. Watson & R. O. Pohl. Lower limit to the thermal conductivity of disordered crystals. Phys. Rev. B 46, 6131-6140 (1992) R. Ruiz, et al. Density multiplication and improved lithography by directed block copolymer assembly. Science 321, 936-939 (2008)
W2: Nanowire Growth I
Monday AM, November 29, 2010
Ballroom C, 3rd floor (Hynes)
11:30 AM - **W2.1
Growth of Hybrid Group IV-group III-V Nanowires.
Frances Ross 1 Show Abstract
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Many lattice mismatched semiconductors do not grow readily as planar layers but have been combined successfully in nanowires. The efficient strain relaxation permitted by the nanowire geometry allows, for example, segments of GaP and InAs, with mismatch over 10%, to be grown together without defects. The ability to combine these III-V materials is exciting for applications such as solid state lighting and photovoltaics. Successful growth of III-V with group IV semiconductors would open an even wider range of applications, but has not been explored in as much detail. Here we discuss the growth of “hybrid” nanowires that contain segments of GaP, Si and Ge. The structures are grown in situ in a transmission electron microscope by supplying triethylgallium and phosphine, disilane, and digermane sequentially to an Au-decorated substrate. This allows us to follow details of the wire structure and catalyst phase as the different segments grow. We describe the sequence of phase changes in the catalyst and the structure of the growth interface during formation of a heterojunction, and discuss the kinetic and thermodynamic parameters that determine whether the wire grows straight or forms a kink at the interface. We consider changes in the catalyst during growth of each segment, describing synergistic effects in which the presence of one material in the catalyst affects the growth of others. This allows us to evaluate favourable conditions for growth of hybrid nanowires and discuss possible applications.
12:00 PM - W2.2
Copper as Seed Particle Material for InP Nanowires.
Karla Hillerich 1 , Maria Messing 1 , Reine Wallenberg 2 , Jonas Johansson 1 , Knut Deppert 1 , Kimberly Dick 1 2 Show Abstract
1 Solid State Physics, Lund University, Lund Sweden, 2 nCHREM/Polymer and Materials Chemistry, Lund University, Lund Sweden
Nanowires are 1D structures with diameters in the nanometer range and a high aspect ratio. Semiconducting nanowires are mostly grown with help of a metallic seed particle, where gold is the most widely used particle material. Gold, however, is known to be a deep level impurity in semiconductors. Therefore, alternative seed particle materials must be found. Metals like copper and aluminum have served as seeds for Si and Ge nanowires with good results. Epitaxial growth of vertically-aligned III-V nanowires seeded by alternative materials has not been shown previously. We report here epitaxial growth of InP nanowires seeded by copper particles in MOVPE for the first time. Copper is expected to be electronically less interfering than gold and can also directly be coupled to the copper used as interconnect material for ICs. We will present the parameter range, growth rates and activation energies for InP nanowire growth from Cu seed particles, and compare these to growth using Au particles.Copper thin films were evaporated onto InP (111)B substrates. These films split up into islands during in-situ annealing under H2 and PH3 in the MOVPE growth chamber. Growth was performed at temperatures between 290°C and 420°C. A wide range of molar fractions and V/III ratios of trimethylindium and phosphine was investigated.In the temperature range of 340°C to 370°C epitaxial nanowires grow in <111>B direction with high yield. Nanowire growth brakes down above 390°C. Below 310°C there are still structures growing, but not vertically aligned. The nanowires show no tapering even after extended growth times and have a strongly faceted particle at the tip after growth. XEDS investigations and examination of the diffraction patterns and atomic distances in the post-growth particle suggest that the nanowires grow from a solid η(Cu2In) phase particle. The growth parameters, i.e. temperature, V/III ratio and total molar fractions, are considerably lower than those commonly used for growth from Au particles.The use of copper as seed particle material opens up new parameter regimes and may help to understand the nanowire growth mechanism.
12:15 PM - W2.3
Elementary Processes in Nanowire Growth.
Jerry Tersoff 1 , Klaus Schwarz 1 Show Abstract
1 , IBM Watson Center, Yorktown Heights, New York, United States
In vapor-liquid-solid growth, uniform nanowires often grow side by side with a zoo of unintended morphologies. One common example is kinking of a wire from one growth direction into another. Another is growth of lateral wires, formed by crawling of the catalyst along the surface. Most applications require suppression of such unintended growth modes. On the other hand, by controlling kinking and crawling it is possible to grow novel device structures. For either purpose, it is important to understand the underlying mechanisms that lead to these complex growth morphologies.Here we show that the complexity of nanowire growth can be understood as arising from the interplay of just three simple elements: facet dynamics (i.e. growth of the facet by liquid-phase epitaxy from the catalyst); droplet statics (i.e. ordinary wetting); and the formation of new facets at the trijunction. We incorporate these processes in a simple model for VLS growth. Our models addresses fully faceted nanowires, such as are seen in experiment.Using this model, simulations of nanowire growth exhibit many of the same phenomena observed in experiment. We present several examples, including: (1) kinking from one direction to another under an external perturbation; (2) a competition between growth of free-standing nanowires and wires growing along the surface by catalyst crawling; and (3) the occurrence of different shapes for the wire-catalyst interface under different growth conditions.
12:30 PM - W2.4
Growing Nanowires without Catalysts: Vapor-solid Process or Vapor-liquid-solid Process?
Xudong Wang 1 , Jian Shi 1 Show Abstract
1 Materials Science and Engineering, University of Wisconsin Madison, Madison, Wisconsin, United States
Self-assembled nanowire (NW) structures are typically formed either with the assistance of metal catalyst or without. Vapor-liquid-solid (VLS) process is a widely accepted mechanism for growing NWs when foreign metal catalysts present. When no other metal catalyst is used, the formation of NWs is generally believed to result from a vapor-solid (VS) process. However, the mechanism of the one-dimensional (1D) growth directly from a vapor phase is not as clear as the VLS process. A recently proposed screw dislocation driven mechanism showed success on explaining the formation mechanism when the vapor components incorporated into the NW lattice stoichiometrically. However, when different elements deposit onto the nanowire with significantly different rates, a single-element phase would form first and catalyze the 1D growth. This is known as the self-catalyzed NW growth. Nonetheless, this mechanism is still a hypothesis. Little evidence has been shown to provide further proof and understanding of this unstable transitional growth stage. Recently, using ZnO as an example, by carefully adjusting the deposition supersaturation, we successfully manipulated the amount of Zn that was precipitated to catalyze the growth of ZnO nanowires. Different amount of Zn would lead to different nanostructure morphology. Increasing the amount of Zn on ZnO surface would eventually eliminated the distinction among the ZnO crystal surfaces, therefore smooth and curvy ZnO nanowires were received. The randomly orientated hemispherical ZnO NW tips suggested the existence of a liquid phase during the NW growth. Diffusion of Zn atoms into the lattice also rendered a strong green luminescence of the ZnO NWs. Based on the experimental observation and theoretical calculation, we suggested that for any two- or multi-element compound, even there is no foreign metal catalyst, the growth mechanism of NWs would still be a VLS process as long as the elements incorporate into the lattice non-stoichiometrically. This research brought new understandings to the growth mechanisms of NWs and would be valuable for guiding morphology and composition control of 1D nanostructures.
12:45 PM - W2.5
Anomalous Nucleation of Si Nanowires Smaller than 10nm.
Lea Marlor 1 , Bong Joong Kim 1 , Jerry Tersoff 2 , Frances Ross 2 , Eric Stach 1 Show Abstract
1 School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States, 2 IBM Research Division, T.J. Watson Research Center, Yorktown Heights, New York, United States
Nanoparticle size effects are of great importance when studying the catalyzed nucleation and growth of self-assembled nanowires. Previous studies of the nucleation of Si from Au catalysts focused on catalyst sizes greater than 10 nm in the low pressure regime, and found that the time to nucleation increases linearly with increasing diameter. This process is controlled by the rate of dissociative adsorption of disilane onto the catalyst. Here we study nucleation events in catalysts that are less than 10 nm in diameter at higher pressure to assess the kinetics over a wider range of conditions in order to understand the Gibbs-Thomson effect. We used environmental TEM at 300kV to allow real time imaging of the nucleation process at nanometer scale resolution. Au catalysts with diameters ranging from 5 –15 nm were deposited on amorphous SiN membranes. The samples were then heated to temperatures ranging from 450°C – 525°C and exposed to disilane at pressures ranging from 1 x 10-5 – 1 x 10-2 Torr. At 450°C and 1 x 10-2 Torr, nucleation took place in 10 nm diamater catalysts within a few seconds of introducing the disilane. However, catalysts with diameters between 12 and 15 nm required approximately 15 seconds of exposure to disilane before nucleation occurred. Particles under 10 nm do not nucleate: instead they either coarsen to sizes greater than 10 nm and nucleate thereafter, or they remain small and never nucleate. At lower pressures we find that the electron beam accelerates disilane decomposition, and have systematically explored the effect of electron irradiation on the nucleation kinetics. We will discuss the relationship between existing models of nucleation and our quantitative observations, and describe a model for nucleation that includes the Gibbs-Thomson effect in catalysts with a diameter below 10 nm. These experiments show that there is substantial difficulty in creating ultrasmall nanowires.
W3: Nanowire Growth Mechanisms
Monday PM, November 29, 2010
Ballroom C, 3rd floor (Hynes)
2:30 PM - **W3.1
Screw Dislocation-driven Nanomaterial Growth: Nanowire Trees, Nanotubes, and Beyond.
Song Jin 1 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
I will discuss a nanowire formation mechanism that is different from the well-known vapor-liquid-solid (VLS) growth. Axial screw dislocations provide the self-perpetuating steps to enable 1-dimensional (1D) crystal growth, unlike previously understood mechanisms that require metal catalysts. This mechanism was initially found in hierarchical nanostructures of lead sulfide (PbS) nanowires with helically rotating branches resembling “pine trees”. I will further explain how dislocations result in the spontaneous formation of nanotubes and use classical crystal growth theory to confirm that their anisotropic growth is driven by dislocations. Dislocation-driven growth should be general to many materials grown in vapor or solution phase and is underappreciated in modern nanomaterial literature. Our discoveries will create a new dimension in the rational design and synthesis of nanomaterials. It could enable the applications of novel complex hierarchical nanostructures in solar energy harvesting and our understanding will allow large scale synthesis of nanowire materials for practical applications.
3:00 PM - W3.2
Step Flow and Interfacial Dynamics During Catalytic Germanium Nanowire Growth.
Andrew Gamalski 1 , Caterina Ducati 2 , Renu Sharma 3 , Stephan Hofmann 1 Show Abstract
1 Electrical Engineering, University of Cambridge, Cambridge United Kingdom, 2 Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 3 Nanofabrication Research Group, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Implementing bottom-up grown semiconductor nanowires in future nano/opto-electronic devices will require a detailed understanding of nanowire growth mechanisms. We present environmental transmission electron microscopy video data of Au catalyzed Ge nanowire growth under digermane exposure at temperatures between 240 - 400°C . We focus on catalyst-nanowire interface dynamics during vapor-liquid-solid and vapor-solid-solid growth following the initial nucleation stage from both above and below the eutectic temperature . Kinetic and thermodynamic modeling gives insight into the importance of surface energies and catalyst-interface dynamics to nanowire growth and geometry. A. Gamalski et al, submitted (2010) A. Gamalski et al, Nano Letters, X, XXX (2010)
3:15 PM - W3.3
Growth Mechanisms of InSb Nanowires by Chemical Beam Epitaxy.
Alexander Vogel 1 , Johannes de Boor 1 , Michael Becker 1 , Samuel Mensah 1 , Joerg Wittemann 1 , Peter Werner 1 , Volker Schmidt 1 Show Abstract
1 Exp. II, Max Planck Institute of Microstructure Physics, Halle (Saale) Germany
There 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 up to 1000 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%). A nanowire grown heteroepitaxially on a lattice mismatched substrate 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. They are strongly influenced by the amount of lattice mismatch as well as by the growth parameters.We are going to present detailed growth studies 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. Compared to MBE precursor flow control and switching is much easier due to the use of electronic mass flow controllers. Trimethylindium and triethylantimony were used to grow InSb nanowires. After growth, samples were characterized using SEM, TEM, XRD and Raman spectroscopy. We identified two very different growth regimes. In the low temperature growth regime, at a growth temperature of around 350°C, nanowire growth is actually promoted by a liquid indium droplet rather than an Au-In-alloy. TEM investigations showed that wires grown at those temperatures exhibit a large number of stacking faults and twin-planes. But the stacking fault density could be heavily reduced by growing at higher temperatures.In the high temperature growth regime, around 430°C, completely defect free InSb nanowires were grown. Those nanowires had a length of up to 2,8 µm with diameters as small as 35 nm. Post-growth characterization suggested that an AuIn2 alloy promotes nanowire growth within this regime.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.
3:30 PM - W3.4
Simulation of VLS Nanowire Growth with Axial Grain Boundaries and Comparison to Experimental Structures via Cross-sectional TEM.
Edwin Schwalbach 1 , Eric Hemesath 1 , Lincoln Lauhon 1 , Peter Voorhees 1 Show Abstract
1 Dept. of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
We have developed a phase-field model for the simulation of Vapor-Liquid-Solid (VLS) growth of nanowires that includes the effects of convection via viscous fluid flow in the catalyst droplet. We apply this model to the growth of nanowires with grain boundaries oriented along the growth axis and compare simulated structures to reconstructions of experimental bi-crystal nanowires obtained from cross-sectional high resolution transmission electron microscopy and tomography. These internal planar defects act as trapping sites for catalyst atoms in the case of Au catalyzed Si nanowires, and the degree of catalyst incorporation is found to depend sensitively on the tilt of the grain boundary. We examine and compare two possible mechanisms for the genesis of such wires: coalescence of neighboring droplets on a substrate and growth from two nuclei within the same droplet. Our model highlights the importance of fluid flow in the early stages of nanowire growth and shows how adjacent droplets can coalesce. Furthermore, the model allows us to determine the relative importance of diffusion and convection within the liquid droplet. Finally, we consider the stability of adjacent grains growing within the nanowire and investigate the complex morphology of the solid-liquid interface for such wires via a simple model of interfacial anisotropy.
3:45 PM - W3.5
Controlling the Transition Region Width of VLS-Grown Axial Nanowire Heterostructures by Catalyst Alloying.
Daniel Perea 1 , S. Tom Picraux 1 Show Abstract
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
We demonstrate the liquid phase growth of Si/Ge axial heterostructures with significantly increased abruptness by in situ trimethylgallium alloying of the liquid Au catalyst with Ga. For nanowire heterostructure applications such as in tunnel field effect transistors or thermoelectric devices, the formation of a compositionally abrupt heterointerface is important for optimizing device performance. However, for group IV semiconductors, abrupt heterojunction formation has been a challenge due to the relatively high solute solubilities in the commonly used Au catalyst during vapor-liquid-solid (VLS) growth. As VLS growth of a nanowire heterostructure, composed of species A and B, is mediated through a liquid metal alloy nanoparticle, the transition region width is dictated by the depletion rate of species A from the liquid, which in turn is dictated by its relative solubility in the liquid. Recently, it was shown by others that compositionally-abrupt axial heterojunctions could be made in Si-Ge nanowires using solid Al-Au catalyst particles. For their case of growth from a solid catalyst, the solid solubility of both Si and Ge is very low, thus allowing for a compositionally abrupt interface, although at an inherently slow solid phase growth rate. In this work, we have taken a new approach of using a low-solubility liquid Ga-Au alloy catalyst to create a sharper axial heterojunction in Ge-Si, relative to that obtained using pure Au. By lowering the solubility of Ge within the liquid Ga-Au alloy catalyst, we show for the first time, the formation of a progressively sharper Ge-Si heterojunction. For example, for 60 nm diameter nanowires the Ge-Si heterojunction width decreases from 53±14 nm for growth with a pure Au catalyst, to 14±4 nm with an increasing Ga/Au catalyst ratio of up to 1±0.05. The sharper interface corresponds to a progressively lower Ge solubility in the Ga-Au alloy. Based on the only moderately lower growth rates compared to Au-catalyzed growth rates, we conclude that nanowire growth from the Ga-Au alloy proceeds via VLS growth making it practical for heterostructure device growth. SEM, TEM, and EDS analyses demonstrate 100% Ge to Si compositional change with good morphology control. This work provides motivation to further explore alternative catalyst metals and metal alloys with Au in order to tailor interfacial abruptness by manipulating the semiconductor and dopant solubility in the catalyst.
4:30 PM - W3.6
Controlled Growth of Three Dimensional Kinked-silicon Nanowire Structures.
Soonshin Kwon 2 , Ji Hun Kim 1 , Zack Chen 1 , Jie Xiang 1 2 Show Abstract
2 Materials Science and Engineering, Univ. California, San Diego, La Jolla, California, United States, 1 Electrical and Computer Engineering, Univ. California, San Diego, La Jolla, California, United States
Semiconductor nanowires have attracted significant interest because of their novel physical properties and diverse potential for electronic and optoelectronic device applications. Adding to their attractiveness is the ability to design and integrate via in-situ synthesis of core-multi-shell, axial heterojunction as well as branched heterostructures. Recently Tian et.al.  reported controllable 2D structure of multiple zigzag shape kinked nanowires in which the straight sections are separated by fixed 120 degree triangular joints. Here we report rational growth of 3D multiple kinked-nanowire structures with diameter as small as 20 nm synthesized using programmed periodic reduction of precursor partitial pressure in a low pressure CVD system. SEM and high resolution TEM clearly showed the generation and 3-fold symmetry breaking process of the kinked joints, which preserve the single crystallinity. The clean, epitaxial joint presents unique mechanical properties. Specialized device functions could be introduced into localized kinked joints in the nanowires. This new technology could open up the possibility of introducing nanoelectronics, nanophotonics, or biological sensors into complex nanoscale structures. Finally, electromechanical properties and applications of such nanojoints will also be discussed.  Bozhi Tian et al, Nature nanotechnology 4, 824 (2009)
4:45 PM - W3.7
Fabrication of Ga2O3 - SnO2 Heterostructure Nanowires by Vapor-liquid-solid Method and Atomic Layer Deposition and Their Gas Sensing Properties.
Yun-Guk Jang 1 , Won-Sik Kim 1 , Dai Hong Kim 1 , Seong-Hyeon Hong 1 Show Abstract
1 Department of Materials Science and Engineering , Seoul National University, Seoul Korea (the Republic of)
Metal oxide nanowire (NWs) semiconductor sensors are the most promising devices among the solid state chemical sensors, because they have many advantages such as a large surface to volume ratios and a Debye length comparable to their dimensions. Therefore, the synthesis of one-dimensional nanostructures has been stimulated to intense research activity about SnO2, ZnO, In2O3, Ga2O3, etc. Moreover, modifications of sensing material have been intensively studied to enhance the gas sensing performance by various routes such as doping, addition of catalyst, and formation of heterostructures. Among these methods, the design of a new sensor composition by forming a heterostructures with different sensing materials has a great potential for tuning the gas response and for accomplishing the selectivity. However, compared with a considerable development in homogeneous systems, researches on multi-compositional sensing materials are still in the early stages. Thus, the formation of heterostructures and characterization of their gas sensing properties are highly required. Ga2O3 and SnO2 are well-known n-type semiconductor gas sensor materials. Ga2O3 sensor shows the oxygen-sensing capabilities as well as reducing gases, but it has too high working temperature (over 600 oC). SnO2 is most sensitive materials toward various gases, but the selectivity towards target gases is lacking. Recently, one-dimensional Ga2O3-SnO2 heterostructures have been extensively studied as promising materials for gas sensor due to their structural defects and combination of their own sensing capabilities, but the gas sensing properties of this structure have not been studied yet.In the present study, we demonstrate an effective strategy for formation of Ga2O3-SnO2 heterostructures and their gas sensing properties. Ga2O3 nanowire (core) was synthesized by typical vapor-liquid-solid (VLS) methods and SnO2 layer (shell) was coated by atomic layer deposition. The detailed process was followed. First, Au was coated on SiO2/Si substrates with interdigitated Pt electrodes, and then nanowire were grown on Au-coated substrate by evaporating Ga powder. As-grown Ga2O3 NWs were coated with SnO¬2¬ by ALD methods to fabricate Ga2O3-SnO2 heterostructures. In our process, we chose DBTDA (dibutlytindiacetate) as Sn precursor to carry out ALD process at 100 oC with O2 plasma. The SnO2 shell thickness, from 5 to 100 nm, was controlled by number of ALD cycle. Ga2O3-SnO2 heterostructures were characterized by XRD, SEM and TEM. The gas sensing properties was measured by flow type sensing equipment. We measured sensing properties towards various gases, such as H2, CO, NH3 and NO2, and the sensing properties with shell (SnO2) thickness will be discussed in this presentation.
5:00 PM - W3.8
Atomic Layer Epitaxy on Nanowire Surfaces at Low Temperatures.
Ren Bin Yang 1 , Nikolai Zakharov 1 , Oussama Moutanabbir 1 , Kurt Scheerschmidt 1 , Li-Ming Wu 2 , Ulrich Goesele 1 , Julien Bachmann 3 , Kornelius Nielsch 3 Show Abstract
1 , Max Planck Institute , Halle Germany, 2 State Key Laboratory of Structural Chemistry, Institute of Research on the Structure of Matter, Fujian China, 3 Institute of Applied Physics, University of Hamburg, Hamburg Germany
Atomic layer deposition (ALD) is a very suitable method for the conformal deposition of semiconductor nanowires with high aspect ratios, while offering the precise tuning of the layer thickness and high uniformity. We have grown nanowires of V-VI semiconductors by cyclic vapour liquid solid growth mode. The synthesized Sb2Se3 and Sb2S3 nanowires have been applied successfully as three dimensional substrates for epitaxial atomic layer deposition. The core-shell structures that will be presented demonstrate the possibility of driving Atomic Layer Deposition at temperatures far below those used so far. Conformal atomic layer deposition of thin Sb2S3 layers takes place epitaxially at 65 °C. More elevated deposition temperatures increase the mobility of the solid and result in the diffusion of Sb2S3 along surface energy gradients. We have observed that the growth of the double-segmented structures relies on the high crystal anisotropy of a layered solid, which drives a perfectly anisotropic growth in a manner. On Sb2Se3 wires, which present the high-energy c facet at their extremity, this results in the axial elongation of the wire with a Sb2S3 segment. When Sb2S3 wires, whose c planes are exposed on the sides, are used as substrate, the homoepitaxy collects material laterally and yields nano-objects with a rectangular cross-section. This work was supported by the German Priority Program SPP 1386 on Nanostructured Thermoelectrics.Reference: R.B. Yang et al., J. Am. Chem. Soc. 132, 7592 (2010)
5:15 PM - W3.9
Tuning the Electronic Properties of Core-shell Nanowires through Control of Strain.
Melodie Fickenscher 1 , Mohammad Montazeri 1 , Howard Jackson 1 , Leigh Smith 1 , Jan Yarrison-Rice 2 , Jung Hyun Kang 3 , Qiang Gao 3 , Hoe Tan 3 , Chennupati Jagadish 3 Show Abstract
1 Department of Physics, University of Cincinnati, Cincinnati, Ohio, United States, 2 Department of Physics, Miami University, Oxford, Ohio, United States, 3 Department of Electronic Materials Engineering, Australian National University, Canberra, Australian Capital Territory, Australia
We show through detailed Raman and CW and Time-Resolved photoluminescence measurements that the band symmetry and electronic properties of strained core-shell GaAs/GaP nanowires can be controlled through variation of the core and shell relative thickness. Raman scattering from as-grown highly strained GaAs/GaP core-shell nanowires with 50 nm diameter GaAs cores and 25 nm GaP shells show that the degree of compressive hydrostatic and shear strain of the GaAs core can be separately determined. Analysis of the shift and splitting of the TO-mode Raman spectra shows that the GaAs core has a -1.2% compressive hydrostatic strain and a -0.7% shear strain. Our measurements are consistent with 8-band k.p calculations and predict a 260 meV increase of the GaAs core band gap and a ~100 meV heavy hole-light hole splitting of the valence band. Detailed low temperature photoluminescence (PL) and time-resolved PL spectra from the highly strained GaAs/GaP core-shell nanowires (NWs) confirm these results, and show, consistent with theoretical modeling, that the band structure of the NWs can be tuned by changing the ratio of the core radius to total NW radius. The ratio was changed by altering either the thickness of the GaP shell or the GaAs core radius with the other held fixed. Cross-sectional TEM is used to measure the range of core and shell radii. The PL from both methods confirms that the band gap can be shifted to dramatically higher energies from the 1.515eV GaAs free exciton peak and is consistent with the theoretical predictions as well as direct Raman measurements of the strain. These results open up new opportunities to strain engineering of the band structure by varying the nanowire core/shell ratio. We acknowledge the financial support of the National Science Foundation through grants DMR-0806700, 0806572 and ECCS-0701703, and the Australian Research Council. The Australian National Fabrication Facility is acknowledged for access to the facilities used in this research.
5:30 PM - W3.10
Self-induced and Site-selective Growth of Vertical InAs Nanowire Arrays on Si (111) by Molecular Beam Epitaxy.
Gregor Koblmuller 1 , Simon Hertenberger 1 , Kristijonas Vizbaras 1 , Max Bichler 1 , Jinping Zhang 2 , Timothy Veal 3 , Ian Maskery 3 , Gavin Bell 3 , Gerhard Abstreiter 1 Show Abstract
1 Technical University Munich, Walter Schottky Institut, Garching Germany, 2 Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou China, 3 Department of Physics, University of Warwick, Coventry United Kingdom
The desire for integration of III–V semiconductor nanowire (NW) devices on Si has fueled large materials synthesis research. For the III–arsenides several reports have shown freestanding NWs on Si, mostly grown by MOCVD with or without the use of gold (Au) catalyst. Very limited – and mainly Au–catalyzed – attempts to grow freestanding III–As NWs on Si by MBE were reported, despite enormous advantages of MBE (low impurity content, sharp composition/doping control, sophisticated core–shell heterostructures). Here, we report a detailed growth study of vertical InAs NW arrays on Si (111) by solid–source MBE. For self–induced, catalyst–free growth two strategies were employed; (i) by use of a thin SiOx mask on Si (111) for self–assembled NWs and (ii) by means of patterned Si (111) substrates (ebeam lithography) for site–selective growth of NWs and control over position, geometry and NW size. Growth of the NW arrays was recorded in situ by RHEED and QMS to provide information of the nucleation kinetics. Microstructure analysis was performed using SEM, HRXRD, Raman spectroscopy and TEM. Results on optical properties by photoluminescence (PL) and surface electronic properties by valence band spectra analysis using XPS are also reported.The InAs NWs grown by the self–assembled method exhibited vertical directionality [along (111)] with straight, hexagon–shaped, non–tapered geometries. Significant NW size variation was achieved with substrate temperature, producing maximum length/minimum diameter (~40 nm) in the 430–460 °C range. On the other hand, growth on patterned Si (111) substrates provided site-selective growth of (111)–oriented InAs NWs with preferential nucleation at the predefined holes and very high yields close to 100 percent. Size variation of the NWs depended here critically on pitch and growth time but much less on hole size.The epitaxial relationship between the InAs NWs and the Si (111) substrate was confirmed by HRXRD 2theta-omega scans. Over a wide range (0–60 deg) only the zincblende (ZB) InAs (111) peak at 25.3° and Si (111) peak at 28.3° were observed. Rocking curves of the on–axis (111) reflection yielded a full width half maximum (FWHM) of ~1° for self–assembled InAs NWs and less than ~0.5° for site-selective NWs, confirming the excellent vertical (111) directionality. The dominant ZB structure and low–defect density of the NWs was further confirmed by HRTEM and the high-intensity transversal optical E1 Raman mode. PL at 20K yielded a peak emission at 0.445 eV on representative NW arrays with a FWHM of ~33 meV, and a slight blue–shift (30 meV) with respect to the bulk InAs reference indicative of quantum confinement effects. Finally, XPS of cleaned NW samples revealed valence band spectra with shapes very different to those observed for bulk InAs; however the surface Fermi level to valence band maximum separation (0.50 eV) was identical for InAs bulk and NWs and highlighted significant surface Fermi level pinning.
5:45 PM - W3.11
Controlling the Growth Location and Length of Indium Nanowires by Introducing Patterns on Substrates.
Wardhana Sasangka 1 2 , Chee Lip Gan 1 2 , Carl Thompson 2 3 , Daquan Yu 4 Show Abstract
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 2 Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, Nanyang Technological University, Singapore Singapore, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), Singapore Singapore
We have observed formation of Indium nanowires after sputter deposition and post-annealing in vacuum. The growth locations of these nanowires were controlled by introducing trenches on the substrate. The lengths of the nanowires were also controlled by varying the size of the circular trenches. Microstructure characterizations using SEM, FIB, TEM and EDX were employed to investigate the mechanism of growth.Arrays of circular SiO2 trenches with a depth of 1 um, diameters ranging from 1-5 um and pitches of 3-20 um were fabricated on silicon wafers using standard CMOS fabrication processes. Subsequently, nominally 100 nm-thick Cr films were deposited using sputter deposition. The vacuum level in the sputtering system before and during deposition was maintained at ~10-6 torr and ~10-3 torr, respectively. Without breaking the vacuum, ~40 nm nominal thickness films of Indium were deposited at a rate of ~1.3 nm/min at a substrate temperature of 100oC. Post annealing at 200oC for 15 mins was then carried out inside the sputtering chamber.Many short nanowires were found to grow from the walls of the trenches. The diameters of these short nanowires were in the range 40–100 nm with lengths between 0.2–2 um. In addition, a single long nanowire was found in every trench. These long nanowires grew from the base of the trench. The diameters of these long nanowires were in the range 50–200 nm. The lengths varied from 1–20 um, depending on the trench size (with longer wires growing in larger diameter trenches). The composition of the nanowires was determined to be pure Indium using TEM/EDX analysis.The as-deposited Indium films had discontinuous island-like structures. Annealing, allows In diffusion on the surfaces of the substrate and ledges. Whether the wires form from pre-existing islands or are nucleated at the ledge is unclear, though the latter is suggested by their specific location at ledges. Given that longer wires grow in larger circular trenches suggests, it appears that the In that contributes to their growth is deposited within the circular recesses. The fact that all the wires are faceted suggests that surface energy or growth velocity anisotropy play an important role in causing their growth. Screw dislocations were not observed in HRTEM images. Penta-twinned growth is unlikely as most of the nanowires have hexagonally arranged side facets. Based on these observations, we believe that the high rate of axial growth is related to barriers to ledge nucleation on the side facets.
Ritesh Agarwal University of Pennsylvania
Wei Lu University of Michigan
Oliver Hayden Siemens AG
Akram Boukai University of Michigan
W4: Nanowire Characterization
Tuesday AM, November 30, 2010
Ballroom C, 3rd floor (Hynes)
9:30 AM - **W4.1
Atom-level View of Dopant and Catalyst Incorporation in Nanowires.
Lincoln Lauhon 1 Show Abstract
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Catalyst-mediated growth enables the control of nanowire size and composition in regimes where useful new properties are manifest. Our understanding of catalyst mediated doping and contamination is rather limited, however, considering the importance of impurity doping to control of semiconductor device function. The primary challenge lies in characterization; the extremely small scale of nanowires demands an atom-level view of dopant and catalyst incorporation as even a single impurity represents a significant ‘concentration’. We will describe recent progress on understanding impurity incorporation using two approaches. First, we will describe the use of atom probe tomography to understand the role of the catalyst in controlling doping rates and the abruptness of dopant homojunctions. We identify an intrinsic limitation in simultaneously achieving high doping rates and abrupt junctions with catalyst-mediated growth. Second, we apply scanning transmission electron microscopy and tomography to the analysis of gold impurity incorporation nanowires. Through sequential electron tomography and high resolution cross-sectional imaging, we correlate the locations of impurities with characteristic planar defects and relate the defect structure to the shape of the growth interface. By combining electron and atom probe tomography, we are developing a comprehensive understanding of how the chemical and physical structure of the catalyst influences impurity incorporation.
10:00 AM - W4.2
Doping of Vertical Si Nanowires and Carrier Profiling by Scanning Spreading Resistance Microscopy.
Xin Ou 1 2 , Pratyush Das Kanungo 2 , Reinhard Koegler 1 , Peter Werner 2 , Ulrich Goesele 2 , Wolfgang Skorupa 1 Show Abstract
1 , Forschungszentrum Dresden-Rossendorf, Dresden Germany, 2 , Max Planck Institute of Microstructure Physics, Halle Germany
The future application of silicon nanowires (Si NWs) in nano electronics requires their doping and the precise control of their electrical properties. However, the dopant incorporation process in Si NWs is not yet fully understood. In this study, individual vertical MBE-grown Si-NWs doped either by ion implantation or by in-situ dopant incorporation are investigated by scanning spreading resistance microscopy (SSRM). The carrier profiles across the axial cross sections of the NWs are derived from the measured spreading resistance values and calibrated by the known carrier concentrations of the connected Si substrate or epi-layer. Furthermore, three-dimensional (3D) SSRM of the NW was obtained by measuring the cross sections at different depth position of the same NW in succession. Carrier profiling reveals a multi-shell structure of the carrier distribution across the NW diameter which consists of a lower doped core region, a higher doped shell region and a carrier depleted sub-surface region.
10:15 AM - W4.3
Direct Correlation of Structural and Optical Properties in Wurtzite/Zinc-blende GaAs Nanowire Heterostructures.
Martin Heiss 1 2 , Sonia Conesa-Boj 1 3 , Emanuele Uccelli 1 2 , Francesca Peiro 3 , Joan Ramon Morante 4 , Jordi Arbiol 5 , Anna Fontcuberta i Morral 1 2 Show Abstract
1 LMSC, Institut des Matériaux , École Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 2 Walter Schottky Institute, Technical University Munich, Garching Germany, 3 Departament d’Electrònica, Universitat de Barcelona, Barcelona, CAT, Spain, 4 , Catalonia Institute for Energy Research, Barcelona, CAT, Spain, 5 ICREA and Institut de Ciencia de Materials de Barcelona, CISC, Bellaterra, CAT, Spain
A new type of heterostructure has recently attracted attention where the chemical composition of the material is constant but the crystalline structure varies along the growth axis from cubic zinc-blende to hexagonal wurtzite [1,2]. The change in crystal structure results in a different band structure, leading to new optical and electronic properties . We report direct correlation experiments combining micro photoluminescence spectroscopy and high resolution transmission electron microscopy (HRTEM) experiments on single GaAs nanowires that exhibit zinc-blende/wurtzite polytypism. By performing both characterizations on the same nanowire, we obtain a direct local correlation between structural and electronic properties. The photoluminescence measurements are performed at 4.2 K on nanowires transferred to a holey carbon film of a numbered TEM copper grid. This allows to subsequently localize the same position with TEM in order to obtain both PL and HRTEM characterizations on the same nanowire. We have applied this technique to polytypic zinc-blende/wurtzite heterostructure nanowires that are presenting a gradient of phase composition along the length. This allows distinguishing emission from nanowire sections that are predominantly composed from zinc-blende, wurztite or mixed phases. In this way, the dimensions of the quantum heterostructures are correlated with the light emission, allowing us to estimate the band gap in wurtzite GaAs to 1.50 eV and the band alignment between the two crystalline phases. Our experimental data is in agreement with recently predicted band structure for wurtzite GaAs.  Ross, F. M. Nat Nano 2009, 4, 17–18. F. M. Davidson, D. C. Lee, D. D. Fanfair, B. A. Korgel, J. Phys. Chem. C 2007, 111, 2929–2935 D. Spirkoska et al., Phys. Rev. B 80, 245325 (2009)
10:30 AM - W4.4
Crystallographic Orientational Imaging of Gallium Nitride Nanowires via Confocal Raman Imaging.
Adam Schwartzberg 1 , Jeffrey Urban 1 Show Abstract
1 The Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, California, United States
Gallium Nitride (GaN) has become a ubiquitous material in modern technology with wide usage in light emitting diodes and optoelectronic devices. Thin films of GaN are common, but increasingly nanowires have become a favored material due to their potential in nanoscale lasing applications and directed charge transport. The imaging of such structures and determination of crystallographic orientation has been limited to transmission electron microscopy due to their small size and homogeneous triangular cross section. The identification of orientation is critical in both device development and basic research applications, and an inexpensive, effective method to determine this is critical. In the present work, we present a method by which crystallographic orientation with respect to a surface can be determined by simple confocal Raman mapping. By monitoring the A1(TO) and E2 Raman peaks we have been able to observe crystallographic contrast in wires as thin as 250 nm, a counterintuitive result as this is both near the diffraction limit at our pump wavelength, and the crystallographic orientation of the crystal does not change with respect to the scanning focal volume. We believe the observed contrast is due to significant waveguiding within the high index of refraction (~2.4) wires, directing Raman scattered photons generated within the wire away from the focal volume, while surface generated photons will be collected at high efficiency. The result is a surface dominated Raman signal that can differentiate between the two possible surface crystal planes. While this work has focused on GaN structures, this type of measurement is possible on any reasonably high index nanostructure and should be viewed as a general technique with significant potential.
10:45 AM - W4.5
Atom-probe Tomographic Analyses of Al-catalyst Grown Si Nanowires.
Oussama Moutanabbir 2 , Dieter Isheim 1 3 , Horst Blumtritt 2 , Ulrich Goesele 2 , David Seidman 1 3 Show Abstract
2 , Max Planck Institute of Microstructure Physics, Halle Germany, 1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 3 Northwestern University Center for Atom-Probe Tomography, Northwestern University, Evanston, Illinois, United States
Synthesis and properties of silicon nanowires (NWs) are of significant scientific and technological interest since they can be used for components in microelectronic devices and their basic compatibility with many existing semiconductor technologies. Si NWs grown with an Al catalyst are particularly interesting because Al avoids the electronic degradation and CMOS-processing compatibility problems of Au catalyst, which is currently used for producing most Si NWs. Catalyst-grown NWs incorporate trace levels of catalyst atoms effectively doping the NW. A quantitative knowledge of local concentrations and dopant atom distributions is crucial for understanding and designing a NW’s physical properties. We utilize laser-assisted local-electrode atom-probe (LEAP) tomography to characterize Si NWs grown with an Al-catalyst via a vapor-solid-solid (VSS) process. Dual-beam focused-ion beam milling in conjunction with micromanipulation is employed to prepare NWs for LEAP tomography. Pulsed evaporation of individual atoms is achieved employing picosecond ultraviolet (355 nm wavelength) laser pulses. LEAP tomography generates a three-dimensional atom-by-atom reconstruction encompassing the entire cross-section of a NW, including the Al-catalyst particle at the end of the NW. Al concentrations up to 0.33 at.% are measured in the interior of the NWs, which is significantly greater than the equilibrium solubility of Al in bulk Si at the growth temperature. Potential reasons for the increased solubility and implications for the electronic properties of Si NWs are discussed. OM, HB, and UG were financially supported through the Max-Planck Institute for Microstructure Physics, Germany. DI and DNS acknowledge the US-Israel Binational Science Foundation for financial support. APT measurements were performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph was purchased and upgraded with funding from NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781) grants.
11:30 AM - W4.6
Direct Measurement of Strain in Germanium-silicon Core-shell Nanowires.
Aditya Mohite 1 2 , Shadi Dayeh 1 , Wei Tang 3 , Gregory Swadener 4 , S. Picraux 1 , Han Htoon 1 2 Show Abstract
1 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Chemistry and Applied Spectroscopy, Los Alamos National Lab, Los Alamos, New Mexico, United States, 3 Material Science and Engineering, Universityof California Los Aangeles, Los Angeles, California, United States, 4 Mechanical Engineering, Aston University, Birmingham, Birmingham, United Kingdom
Semiconductor nanowire growth techniques like the Vapor Liquid Solid (VLS) and Vapor Solid (VS) methods provide significant flexibility in creating both axial and radial heterostructures for- a wide variety of applications like photodiodes, photovoltaic’s, high performance FETs etc. An important consequence of such nanowire heterostructures is that materials with different lattice constants such as silicon (Si) and germanium (Ge) can be combined to tailor the strain distributions and thus engineer the band structure of the strained nanowire core-shell heterostructure devices. Measuring the degree of strain is critical in tailoring the heterostructure’s interface in order to obtain optimal electronic and optical properties. Here we have used confocal Raman imaging on individual Ge-Si core-shell nanowires to directly probe the strain in each layer for variable core-fixed shell and variable shell - fixed core diameter structures. For nanowires with a Ge core of 10 nm and Si shell of 4 nm, we observe a clear splitting and a red shift in the Ge Raman peak in 90% of the nanowires. For larger core diameters nanowires, the percentage of nanowires showing splitting in the Ge peak decreases to 15% for 50 nm cores and 2% for 100 nm cores at fixed Si shell thicknesses of 4 nm. The splitting is as large as 15-20 cm-1and is attributed to the hydrostatic and shear strain on the Ge core which results in lifting of degeneracy of the LO and TO phonon modes. The magnitude of splitting does not change as we scan along the length of the nanowires, consistent with core diameter and shell thickness being uniform along the nanowire. To directly correlate the strain- induced changes with the observed Raman signals we have combined Raman and HRTEM measurements on the same nanowires which are deposited on thin silicon nitride membranes. For a fixed core diameter of ~ 20 nm and a 10 nm thick Si shell, Ge Raman peak degeneracy disappears, and a single peak at the Ge bulk value evolves, which is indicative of crossing the coherency limit and relaxed Si shell on Ge. The experimental results are compared with predicted strains and strain stability limits - using our recently developed  molecular dynamics simulations1 of the strain distributions between the Ge core and Si shell. xxxxxxxxxxxxxxxx Strain distribution and electronic property modifications in Si/Ge axial nanowire heterostructures, J. G. Swadener & S. T. Picraux J. Appl. Phys 105, 044310 (2009).
11:45 AM - W4.7
Tuning the Electronic Properties of Ultra-strained Silicon Nanowires.
Alois Lugstein 1 , Mathias Steinmair 1 , Andreas Steiger 1 , Hans Kosina 1 , Emmerich Bertagnolli 1 Show Abstract
1 , Technical university of vienna, Vienna Austria
We demonstrate that under ultra high strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a micro-electro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the(111)growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires.For low strain levels (nanowire elongation less than 0.8%) our measurements revealed the expected positive piezoresistance effect, whereas for ultra high strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes no fatigue failures are observed for several hundred loading cycles. Our simulations clearly show that at room temperature even a considerably reduced bandgap does not give any relevant contribution of the minority carriers to the total current. The same holds, if carrier lifetimes are varied by a few orders of magnitude. This means, current remains unipolar and the reduction in resistivity can only be attributed to a mobility increase, and not to the onset of a bipolar conduction mechanism. The simulations performed indicate that the resistivity characteristics mainly reflect the strain-dependent hol