D1: Nano and Molecular Contacts
Chair: A. Alec Talin
Chair: Richard Martel
- Tuesday AM, April 10, 2012
- Moscone West, Level 2, Room 2002
8:30 AM - *D1.1
Electrical Contacts to Nanostructures: Lessons, Opportunities, and Challenges
, Sandia National Laboratories, Livermore, California, USA.Show Abstract
Nanostructures such as carbon nanotubes, nanowires and graphene are being intensively explored for future electronic, photonic, and energy applications. In order for these nanosystems to progress from the research laboratory to technology, it is critical to precisely understand and control charge injection at the electrical contacts. While the scientific community and the semiconductor industry have invested significant resources to develop and control metal contacts to bulk semiconductor materials, charge injection at metal/nanostructure interfaces has received much less attention despite the obvious technological importance. Because nanostructures possess unique properties that differ significantly from bulk semiconductors, existing models of electrical contacts in bulk devices are often inapplicable at the nanoscale. In this presentation, experimental and theoretical works that have highlighted the much different physics and materials science of electrical contacts to nanostructures will be discussed, and key research and development challenges that must be addressed to understand and control nanocontacts will be addressed.
9:00 AM - *D1.2
Scanning Probe Microscopy of In-based Nanocontacts and Nanorings on CdZnTe(110)
Faculty of Engineering, Tel Aviv University, Ramat Aviv, Israel.Show Abstract
Unlike in Si or even GaAs technology, understanding complex relations between the macroscopic device performance and the metallurgy of contact formation on the atomic level in cadmium zinc telluride (CdZnTe) radiation detectors, remains a formidable challenge. To bridge that macro-nano knowledge gap, we conducted a series of controlled experiments aimed at correlating electrical properties of the In contact to n-type CdZnTe(110) surface with a step-by-step contact formation process, which could only be achieved by using high spatial resolution techniques, capable of conducting highly localized measurements on the nano- and subnano-scale. More specifically, scanning tunneling microscopy was used in-situ to monitor the behavior of various In atom coverages on atomically flat and ordered CdZnTe surface under well-controlled molecular beam epitaxial conditions in ultra-high vacuum, whereas electrical derivatives of atomic force microscopy, such as contact potential difference and spreading resistance in torsion resonance tunneling mode, were used ex situ to measure the electrical contact properties. It was concluded, that In atoms preferentially reacted with Te atomic-rows already at room temperature, forming nanometric patches of indium-telluride Schottky-type contacts, in particular at very low and moderate coverages. At a relatively high In coverage, the dominant morphology consisted of large three-dimensional hillocks, that transformed into quantum rings and "camel humps" upon annealing. The transformation model was proposed. The methods developed in this study, in terms of both nano-contact fabrication and characterization, should be applicable and beneficial in basic and applied research of any metal-semiconductor system.
9:30 AM - D1.3
Correlation between Quantum Conductance and Atomic Arrangement of Silver Atomic-Size Nanocontacts
Lagos1 2, Daniel
Applied Physics, State University of Campinas, Campinas-SP, Sao Paulo, Brazil; 2,
, LNLS, Campinas, Sao Paulo, Brazil.Show Abstract
The intense work of the nanotechnology community has increased the capabilities of researchers to produce new materials at the nanometric scale. As a result, novel physical and chemical behaviors are frequently reported opening opportunities for creating new kind of devices. These new devices will require a precise knowledge of the physical properties of atomic-size contacts and nanowires (NW)/interconnects. The generation of these atomic-size metal wires by the mechanical stretching has allowed the study of a wide range of metals at nanoscale. Due to the dominant role of surface energy in this size regime, several anomalous wire structures have already been reported to form during the stretching of very tiny wires, as hollow tubular metals and the size-limit to the existence of defects in NWs [1-3]. In this work we have studied the relevance of thermal effects on the structural and transport response of Ag atomic-size nanowires generated by mechanical elongation. Our study involve time-resolved atomic resolution transmission electron microscopy imaging and quantum conductance measurement using a ultra-high-vacuum mechanically he controllable break junction in association with quantum transport calculations. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements and theoretical modeling we have been able to establish a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along  direction.  M. J. Lagos, F. Sato, J. Bettini, V. Rodrigues, D. S. Galvao and D. Ugarte, Nature Nanotechnology v4, 149 (2009)  P. A. S. Autreto, M. J. Lagos, F. Sato, J. Bettini, V. Rodrigues, D. Ugarte, and D. S. Galvao, Phys. Rev. Lett. v106, 065501 (2011).  M. J. Lagos, F. Sato, D. S. GalvÃ£o, and D. Ugarte, Phys. Rev. Lett. v106, 055501 (2011).
9:45 AM - D1.4
Conductance Statistics from a Large Array of sub-10 nm Single Grain Au Nanodot Electrodes
, IEMN - CNRS, Villeneuve d'Ascq, France; 2,
, LPN, Marcoussis, France.Show Abstract
Devices made of few molecules constitute the miniaturization limit that both inorganic and organic-based electronics aspire to reach. However, integration of millions of molecular junctions with less than 100 molecules each has been a long technological challenge requiring well controlled nanometric electrodes. Here we report molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodots electrodes, a new approach that allows us to measure the conductance of up to a million of junctions in a single conducting Atomic Force Microscope (C-AFM) image. We focus on alkylthiol junctions (with 8, 12 and 18 carbon atoms) as an archetype and for the sake of comparison with an abundant litterature for this molecule. We show that the number and respective amplitudes of the conductance peaks vary, depending on the molecular organisation in the junctions and the atomic structure of the electrodes (i.e. single crystal, polycrystal, amorphous). Since nanodot dimensions are much smaller than that of AFM tip, at constant force, the force per surface unit is increased compared to molecular junctions with substrate electrodes. As a consequence, we observe a lower decay factor than usually measured for molecular junctions with such molecules. By reducing the force to few nN, decay factor increases significantly. We also investigate, using the transition voltage spectroscopy (TVS) method, the electronic structure of junctions belonging to each of the observed conductance population, and we demonstrate how the energy position of the molecular orbitals (with respect to the electrode Fermi energy) depends on chain length, molecular organisation, electrode structure and C-AFM applied force. N. Clement, G. Patriarche, K. Smaali, F. Vaurette, K. Nishiguchi, D. Troadec, A. Fujiwara and D. Vuillaume, Small 7, 2541 (2011) K. Smaali, N. Clement, G. Patriarche and D. Vuillaume, submitted to ACS Nano
10:00 AM -
10:30 AM - D1.5
Room-temperature Spin Injection into Si in a Metal-oxide-semiconductor Field Effect Transistor Structure with a High-quality Schottky-tunnel Contact
Ando1 2, Kohei
Department of Electronics, Kyushu University, Fukuoka, Japan; 2,
INAMORI Frontier Research Center, Kyushu University, Fukuoka, Japan; 3,
Research Center for Silicon Nano-Science, Tokyo City University, Tokyo, Japan; 4,
PRESTO, Japan Science and Technology Agency, Tokyo, Japan.Show Abstract
For realizing silicon-based spintronic devices with low power consumption, spin injection and detection technologies for silicon (Si) channels without an insulating tunnel barrier have been explored.[1,2] In this study, we demonstrate atomically controlled epitaxial growth of CoFe layers directly on Si by means of molecular beam epitaxy, and realize room-temperature spin injection into a Si channel in a metal-oxide-semiconductor field effect transistor (MOSFET) structure with the fabricated CoFe/n+-Si Schottky-tunnel-barrier contact. By applying electric fields to the Si channel, the magnitude of the spin-accumulation signals can be manipulated at room temperature.
To demonstrate spin injection into Si using Schottky-tunnel-barrier contacts, the 10-nm-thick CoFe layer was grown directly on 75-nm-thick (111)-oriented silicon on insulator (SOI) (N=~4.5Ã—1015 cm-3) at 60 oC. An atomically flat interface was demonstrated without the formation of any silicide materials. An n+-Si layer (Sb: 1Ã—1019 cm-3) was inserted between CoFe and SOI. Conventional processes with electron-beam lithography and Ar+ ion milling were used to fabricate three-terminal lateral devices with a backside gate electrode. With a constant source-drain bias voltage, the source-drain current I gradually increased by applying the gate voltage (VG). This means that the conduction channel was formed from the vicinity of the interface between SOI and BOX, indicating that this device can operate as a MOSFET.
The three-terminal Hanle-effect measurements were performed at room temperature by a dc method. For VG = 8.0 V at I = -1.0 ÂµA, where the electrons were injected from the CoFe electrode into the Si channel, a clear voltage change (Î”V) caused by the spin accumulation and its depolarization in the Si channel was observed even at room temperature. The magnitude of Î”V, |Î”V|, was ~11.5 ÂµV. When the gate voltage was further applied up to V G = 54 V, |Î”V| was decreased to ~7.8 ÂµV surprisingly despite the same value of the injection current. A lower limit of spin lifetime for V G = 8.0 and 54 V can be estimated to be ~ 1.30 and ~ 1.27 nsec, respectively. According to the simple spin-diffusion model, the reduction in the |Î”V| can be explained by the change in the channel resistance by applying V G . Therefore, this feature indicates reliable evidence for the spin injection into an intrinsic Si channel.
This work was partly supported by PRESTO-JST and STARC.
 Y. Ando et al., Appl. Phys. Lett. 94, 182105 (2009).  Y. Ando et al., Appl. Phys. Lett. 99, 012113 (2011).  Y. Maeda et al., Appl. Phys. Lett. 97, 192501 (2010).  Y. Ando et al., Appl. Phys. Lett. 99, 132511 (2011).
10:45 AM - D1.6
High Performance Switchable Thermal Diode Based on Metal-insulator Interface
, University of California, Berkeley, San Jose, California, USA.Show Abstract
Thermal Diode (rectifier), in which thermal conductance depends on the sign of the thermal gradient, representing an advanced thermal management, is critical for thermal energy conversion and the future electronic cooling. Despite the fact that multiple mechanisms have been proposed for thermal rectification, high performance thermal rectification has rarely been achieved. Here for the first time, thermal rectification (20%) is observed in metal-insulator interfaces. More strikingly, the thermal rectification can be turned on/off by a third terminal. Therefore it represents a significant advancement towards a three-terminal thermal transistor. With the high performance, nanoscale size and unique property, this type of thermal rectifier can serve as a novel building block for the future development of thermal circuit.
11:00 AM - D1.7
Molecule/Electrode Interface Energetics in Nanocontact Molecular Junction: A ``Transition Voltage Spectroscopy'' Study
IEMN, CNRS, Villeneuve d'ascq, France.Show Abstract
In this work, we investigated the electronic structure of various molecule/electrode interfaces by Transition Voltage Spectroscopy (TVS). We fabricated molecular junctions with various electrodes: semiconductor (silicon with and without native oxide), oxide-free metal (gold) and oxidized metals (aluminum, mercury and eutectic GaIn). Self Assembled Monolayers (SAM) formed the organic materials in the junctions: alkyl-thiol molecules (CH3-(CH2)n-SH) grafted on gold substrate, alkyl-alcene molecules (CH=CH2-(CH2)n-CH3) on hydrogenated silicon and alkyl-trichlorosilane (CH3-(CH2)n-SiCl3) on naturally oxidized silicon. Various approaches were used to electrically characterize the SAM: from nanocontact (Conducting-AFM with gold tip) to macroscopic contact (micro-pore junction, aluminum patterns, eGaIn and mercury drop). The TVS was introduced to measure the energy offset Î” (i.e. the position of one of the molecular orbitals with respect to the electrode Fermi energy) at the electrode/molecule interface in a molecular junction [1;2]. The voltage Vmin obtained at the minimum of the Fowler-Nordheim graph (i.e. a plot of ln(I/V^2) against 1/V) scales linearly with the energy offset (Î”=Î±.Vmin with 1<Î±<2) [3;4]. Albeit, the exact value of Î± and the physical origin of Vmin are still under debate [3-5] TVS has became an increasingly popular tool in molecular electronics. Our results show that Vmin measured for the thiol junctions by C-AFM (without oxide at both interfaces), is 1.27Â±0.07V, in agreement with Beebe et al. [1;2]. In the same way, for the Si/alcene junctions measured by C-AFM, the value of Vmin of 0.96Â±0.15V is close to the LUMO-Si CB offset of 1.5eV as measured by transport experiments and IPES  (regardless the Î± factor). These two families of â€œcleanâ€ junctions (without oxide at the interfaces) present higher values for Vmin than junctions with oxide. Indeed, by using an oxidized electrode (Al, Hg or eGaIn) on alcene SAM, Vmin decreases to 0.2-0.4V. Similarly, for trichlorosilane junctions on slightly oxidized Si substrate, Vmin values are also low ca. 0.25V. These results are understood if Vmin is ascribed to the situation when the tail of a density-of-states (DoS) enters the energy window defined by the applied bias , this DoS being related to the molecular orbitals (HOMO or LUMO) in the case of "clean" junctions (with Vmin>1V), and due to some oxide states in the other cases (Vmin<0.4eV). To conclude, even if the precise determination of the molecular orbital energy by TVS is still under debate, this method seems a good approach for a quick assessment of the quality of the molecule/electrode interfaces in molecular junctions.  J.M. Beebe et al., Phys. Rev. Lett., 2006, 67, 026801  J.M. Beebe et al., ACS Nano, 2008, 2, 827  E.H. Huisman et al., Nano Letters, 2009, 9, 3909  J. Chen et al., Phys. Rev. B, 2010, 82, 121412  M. Araidi et al., Phys. Rev. B, 2010, 235114  A. Salomon et al., Phys. Rev. Lett., 2005, 95, 266807
11:15 AM - D1.8
Nanocontact Reliability for Nanomechanical Logic Switches: Lifetime Performance of Platinum and Conducting Diamond Contacts under Extreme Conditions
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, USA; 2,
Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania, USA; 3,
Center for Nanoscale Materials, Argonne National Laboratories, Argonne, Illinois, USA; 4,
Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, USA.Show Abstract
Nanomechanical logic based on nanoelectromechanical systems (NEMS) switches promises a significant reduction in total switching energy over conventional, solid-state logic. This has merited its inclusion in the International Technology Roadmap for Semiconductors as an Emerging Research Device. However, the reliability of the contact interface of these ohmic switches is a crucial barrier to their commercialization. The adhesiveness and reactivity of conventional contact materials (i.e. metals) results in permanent adhesion, or the buildup of insulating tribofilms, at the contact. In the present study, atomic force microscopy (AFM) was used to efficiently evaluate 1) the lifetime response of conductivity and adhesion for conventional, metallic nanocontacts (platinum) and 2) the robustness of a possible next-generation contact material (conductive diamond). The lifetime response of conductivity and adhesion of platinum-platinum single asperity contacts subjected to forces relevant to NEMS logic operation were investigated using amplitude modulation AFM. Applied electrical potentials at the contact interface and relative humidity (RH) during testing were chosen to mimic anticipated (<1 V and near 0% RH) and extreme (5 V and 50% RH) variants of the operational environment of NEMS logic. We simultaneously measured the electrical and adhesive performance of the nanocontacts for up to 200 million contact cycles. Results show significant fluctuations in contact resistance and adhesive force when tested in high RH with high (5.0 V) electrical potentials. This behavior illustrates the need to consider alternate materials for NEMS contacts. Ultrananocrystalline diamond (UNCD) is a promising candidate for a next-generation NEMS logic contact material; it has chemical inertness, tunable electrical conductivity via nitrogen incorporation (N-UNCD), and ultralow adhesion and wear. Here we assess the robustness of N-UNCD under applied contact stresses and electrical potentials. A platinum-coated AFM tip was raster-scanned over N-UNCD surfaces with MPa-to-GPa applied contact stresses, a variable bias (0.0-10.0 V), and variable relative humidity (<5 - 45%). Changes to carbon and oxygen bonding in the scanned surface regions were then interrogated using photoelectron emission microscopy. N-UNCD showed no observable chemical modification or buildup of insulating tribofilms in the absence of wear-through of the platinum coating on the AFM tip. These findings show that N-UNCD is tribologically robust under extreme contact stresses and electric fields, which are relevant to NEMS switch environments. Together, these results demonstrate the critical advantage of using AFM to study contact reliability. Specifically, AFM enables quantitative, fundamental studies at single asperity contacts which can be evaluated both in situ and ex situ to elucidate, for the first time, the physical processes that dominate nanoscale switch contact performance.
D2: Contacts to Nanotubes, Graphene and Beyond
Chair: M. Saif Islam
Chair: Yu Huang
- Tuesday PM, April 10, 2012
- Moscone West, Level 2, Room 2002
1:30 PM - *D2.1
Carbon Nanotube Array Electrodes for Carrier Injection
Chemistry, Universite de Montreal, Montreal, Quebec, Canada.Show Abstract
We have investigated the charge injection efficiency of carbon nanotube (CNT) electrodes on organic semiconductors and compared their performance to that of traditional metal electrodes. Our study was conducted on three different organic semiconductors: pentacene, phenyl-C61-butyric acid methyl ester (PCBM) and copper phthalocyanine (CuPc) in a three-terminal FET configuration. The carbon nanotube electrodes are arrays made of individual single-walled carbon nanotubes bonded at one end to a Ti metal pad and embedded at the other end in the semiconducting layers. Compared to conventional Au electrodes, these CNT array electrodes provided better injection efficiency at low bias, improved switching behavior, higher electron mobility, and lower contact resistance. Despite large offsets between the CNT work function and the energy levels of the organic layers, the injection characteristics were Ohmic irrespective of the type of carrier being injected. These good performance characteristics appear general and indicate that carbon nanotubes should be considered as better alternatives to the traditional metal electrodes for next-generations of field-effect transistors. The mechanism of injection will also be discussed on the basis of tunneling across the Schottky barrier, a process that appears to be enhanced by the strong electrostatic effects at the CNT/organic interfaces. Work done in collaboration with F. Cicoira, C. M. Aguirre and P. Desjardins
2:00 PM - D2.2
Charge Injection at Carbon Nanotube-metal Contacts
, Sandia National Laboratories, Livermore, California, USA.Show Abstract
Due to their unique structural and electrical properties, carbon nanotubes (CNTs) are promising candidates for next-generation electronic and optoelectronic devices, and the CNT-metal contact plays a crucial role in their performance. In this talk, we discuss our recent theoretical and modeling work to examine the nature of charge injection at metal contacts to individual CNTs and CNT arrays. The simulations reveal a variety of phenomena that govern the efficiency of charge injection in CNT devices. Electrostatic modeling shows that competing length scales can result in a potential barrier that prevents charges from crossing the channel-contact interface, which can significantly increase the contact resistance. We also show that the contact resistance is affected by the length of the contact, as indicated by recent experiments, and we provide an atomistic calculation of the relevant charge injection length, and discuss the factors that govern it. A charge transport model based on non-equilibrium Greenâ€™s functions allows us to examine the importance of tunneling in CNT Schottky contacts. Our results will be discussed in the context of a variety of devices, including single-tube field-effect transistors, Schottky diodes, and CNT arrays.
2:15 PM - D2.3
Negative Contact Resistances Apparently-appeared at Graphene/Metal Contacts
WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan; 2,
Department of Physics, Tohoku University, Sendai, Japan.Show Abstract
Charge carriers in graphene show intrinsically ultrahigh mobility, and thus graphene is now recognized as a promising material for future electronic devices. The carrier transport properties should be measured using metallic electrodes. However, metal-graphene contacts introduce an additional resistance, known as contact resistance, RC. This resistance is a limiting factor for the performance of electronic devices. The relative contribution of RC to the total device resistance becomes larger in devices with shorter inter-electrode spacings, i.e., in shorter channel devices. Therefore, RC becomes a predominant factor to consider when attempting to achieve miniaturization and integration of graphene devices. We have studied the effect of metal contacts to transfer characteristics (the gate-voltage dependence of the drain current) of graphene field-effect transistors [1-4]. The metal contacts have been reported to affect the electronic property through charge transfer (CT) from the metals to graphene . In this presentation, we show that the CT is accountable for apparently-appeared "negative" RC extracted by the transmission line model (TLM) or four-terminal measurement, which should include a contribution from an additional resistance due to the metal-contact doping RCD, in addition to the actual tunnel resistance precisely at the metal-graphene contacts, RCI. The apparently negative RC is considered to be a characteristic feature of Dirac-cone systems such as graphene and topological insulators.  R. Nouchi, M. Shiraishi and Y. Suzuki, Appl. Phys. Lett. 93, 152104 (2008).  R. Nouchi and K. Tanigaki, Appl. Phys. Lett. 96, 253503 (2010).  R. Nouchi, T. Saito and K. Tanigaki, Appl. Phys. Express 4, 035101 (2011).  R. Nouchi and K. Tanigaki, Jpn. J. Appl. Phys. 50 (2011) 070109.  G. Giovannetti et al., Phys. Rev. Lett. 101, 026803 (2008).
2:30 PM - D2.4
Graphene for Metal-semiconductor Ohmic Contact
Heo1, Hyun Jae
Samsung Advanced Institute of Technology, Samsung Electronics, Yongin-Si, Gyeonggi-do, Republic of Korea.Show Abstract
Metal-Semiconductor (MS) junction is one of the most important interfaces for many technologies including Si technology. However, for the most MS junctions, Fermi-level of metals at the interface is pinned at a certain level regardless of the metals, and reducing the Fermi-level pinning effect has been a critical technical challenge. 
In our recent results, we discovered that inserting graphene between MS junction reduces the Fermi-level pinning effect, selectively decreasing the Schottky barrier height of the MS junction. Graphene is chemically inert and does not form silicide with Si unlike transition metals when it is transferred onto Si surface. Instead, graphene keeps the surface termination of Si surface as it is prepared. Thus, when graphene is transferred onto a completely saturated Si surface, the stable Si termination atoms/molecules remain and create van der Walls interaction between graphene and Si.
Based on the above idea, we demonstrated a method, inserting graphene at the MS junction, to reduce or even potentially eliminate the Schottky barrier at the junction of MS junction. We especially focused on n-type Si and Ni junction, since n-type Si forms higher Schottky barrier height with any metals including Ni than p-type Si. With the new contact structure and the knowledge that Ni reduces the work function of graphene , we were able to diminish the Schottky barrier height substantially and to demonstrate near Ohmic contact between Ni and Si. We also investigated the effect of other metals. We found that the metal/graphene interaction exists and the work function of graphene on metal was not a simple function of the work function of pure metal as it was predicted.  In addition, we tried various interfaces between graphene and Si with several functionals. We believe that this research opens an innovative way to control MS junction for new technology.
 F. J. Himpsel, G. Hollinger, R. A. Pollak, Phys Rev B 1983, 28, 7014
 H. Yang, J. Heo, S. Park, H. J. Song, D. H. Seo, K.-E. Byun, P. Kim, I. Yoo, H.-J. Chung, and K. Kim, Submitted.
 G. Giovannetti, PA Khomyakov, G Brocks, VM Karpan, J van den Brink and PJ Kelly, Phys Rev Lett 2008, 101, 026803
2:45 PM -
3:15 PM - *D2.5
Mechanical Annealing and Robust Conductance of Metallic and Semimetallic Nanocontacts
Física de la Materia Condensada, Universidad Autónoma de Madrid, Madrid, Spain; 2,
Física Aplicada, Universidad de Alicante, Alicante, Spain.Show Abstract
Conductance histograms have been routinely used to characterise metallic nanocontacts. These histograms typically present a broad dispersion with a lack of characteristic conductance values as a result of the lack of reproducibility of the crystalline structure at the nanoscale. I will present experimental evidence of how to obtain, in a reproducible and reversible mechanical manner, crystalline structures and conductance values ranging from a few atom section and a few quanta down to a single atom and one conductance quantum. I will also present evidence of the possible formation of a two-dimensional topological insulator in Bismuth nanocontacts with its concomitant robust conductance quantization.
3:45 PM - D2.6
Nanostructure Characterization of Carbon Nanotube/Metal Interfaces
, Santa Clara University, Santa Clara, California, USA; 2,
, Applied Material, Santa Clara, California, USA.Show Abstract
Carbon nanotubes (CNTs) synthesized by plasma-enhanced chemical vapor deposition (PECVD) offer a potentially suitable material for vias in next-generation integrated circuits. One advantage of PECVD over other techniques is the vertically-aligned geometry that is critical to the via manufacturing process. One important consideration in the via formation is the contact resistance between CNT and the underlayer metal at the as-grown interface, which tends to dominate the overall resistance of the interconnect via . A clear understanding of the nature of that interface and the resulting contact resistance is essential for assessing the potential of CNT for via applications. Recently our group presented evidence for aluminum oxide formation when aluminum is used as a metal underlayer . Further, chromium and titanium were found to be partially oxidized , which is believed to be a cause for the high contact resistance between the metal and CNT. An earlier study reported larger lattice spacing in titanium at the interface of PECVD-grown CNTs, which was attributed to the formation of titanium carbide . In the PECVD process, ammonia is used as a reducing agent during the catalyst particle formation process. Our current results, obtained using high-resolution transmission electron microscopy (TEM) with energy dispersive x-ray spectroscopy (EDX), reveal a large amount of nitrogen incorporated into the underlayer metal, thus changing its surface morphology and interface electrical properties. In this study, we focus on the nanostructure-electrical property relationship at the as-grown contact between the CNT and underlayer metal, using electrical measurements as well as TEM/EDX, as part of our objective to optimize the PECVD growth process for CNT vias.  Q. Ngo, T. Yamada, M. Suzuki, Y. Ominami, A. M. Cassell, J. Li, M. Meyyappan, and C.Y. Yang, IEEE Transactions on Nanotechnology 6, 688-695 (2007).  X. Sun, K. Li, R. Wu, P. Wilhite, T. Saito, J. Gao, and C.Y. Yang, Nanotechnology 21, 045201 (6pp) (2010).  Y. Ominami, Q. Ngo, M. Suzuki, A.J. Austin, C.Y. Yang, A.M. Cassell, and J. Li, Applied Physics Letters 89, 263114-1-3 (2006).
4:00 PM - D2.7
Quantum Confinement Effects on Charge Injection and Transport in InAs Membranes
Fang1, S. Bala
Electrical Engineering and Computer Sciences, University of California Berkeley, Berkeley, California, USA; 2,
Electrical Engineering and Computer Sciences, University of Florida, Gainsville, Gainsville, Florida, USA; 3,
Center for High Technology Materials, University of New Mexico, Albuquerque, Albuquerque, New Mexico, USA; 4,
Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan.Show Abstract
significantly modify the fundamental properties of materials. Specifically, the charge injection and transport properties of semiconductors can be drastically altered. Using the model system of ultra-thin InAs membranes, we investigate the dominant role of quantum confinement on the field-effect device properties. By varying the thickness of free-standing InAs membranes between 5-50 nm, the effect of quantization is systematically studied. The thickness directly affects the subband spacing, as shown by absorption studies of the InAs membranes. These sub-bands determine the contact resistance of the system, with the experimental values consistent with the expected number of quantum transport modes available for a given thickness. Additionally, the effective electron mobility of InAs membranes is shown to exhibit anomalous field and thickness dependencies that are in distinct contrast to the conventional MOSFET models, arising from the strong quantum confinement of carriers. The results provide an important advance toward establishing the fundamental device physics of two-dimensional semiconductors. References:  K. Takei, H. Fang, S. B. Kumar, R. Kapadia, Q. Gao, M. Madsen, H. S. Kim, C.-H. Liu, Y.-L. Chueh, E. Plis, S. Krishna, H. A. Bechtel, J. Guo, A. Javey. "Quantum Confinement Effects in Nanoscale-Thickness InAs Membranes", Nano Letters, 2011, ASAP
4:15 PM - D2.8
Theory of the Schottky Barrier at the Metal/AlN Interface
Department of Physics, The University of Texas, Austin, Texas, USA.Show Abstract
Group III-nitride semiconductors play an important role in high speed electronic and optoelectronic devices. The band gaps reach up to 6.3 eV for wurtzite AlN, making nitrides ideal for high power applications. In this work we focus on the zinc blende variant of AlN. Using density functional theory we study the band alignment at metal/AlN interfaces as a first step towards a theoretical description of a metal contact to AlN. We deduce the Schottky barriers of Al, Cs, Au, W, Hf and Pt to AlN. Comparing results of first principles calculations to those predicted by the metal induced gap states (MIGS) theory we find good agreement with the Schottky limit for Al, Au, Pt, Hf and W while Cs shows significant deviation. Our results can be explained when allowing for an effective change in AlNâ€™s electron affinity when in contact with a metal. Simulating a monolayer of Cs on AlN we find a shift in electron affinity from 1.9 eV for the bare AlN surface to -1.2 eV for the â€œcesiatedâ€ surface.
D3: Poster Session: Nanocontacts - Emerging Materials and Processing for Ohmicity and Rectification
Chair: A. Alec Talin
Chair: Christian Lavoie
Chair: King-Ning Tu
Chair: M. Saif Islam
- Tuesday PM, April 10, 2012
- Moscone West, Level 1, Exhibit Hall
5:00 PM - D3.2
A Numerical Simulation Study of Inverse Doped Surface Layer in Schottky Barrier Modification
Physics, National Institute of Technology, Hamirpur, HP, India.Show Abstract
The nonlinear Poissonâ€™s equation along with the electron and hole continuity equations are simultaneously solved by fixed point iterative method in one dimension to obtain the potential inside the semiconductor bulk near the metal semiconductor interface. After potential calculation the electron and hole current densities are calculated to obtain total current at various forward bias. Simulations were performed for different inverse layer thickness and doping concentrations at different temperatures. The barrier height (BH) and ideality factor obtained by fitting of simulated current voltage (I-V) data into thermionic emission diffusion (TED) current equation shows that the BH increases initially with the increase in inverse layer thickness and then attains an almost constant value beyond certain thickness. On increasing inverse layer doing concentration the BH starts increasing at low thickness and becomes constant thereafter. Thus the BH attains maximum saturation value at relatively lower thickness for highly doped surface layer than for the less doped surface layer. However, the maximum saturation value of BH with large inverse surface layer is same for all doping concentrations. The variation of derived ideality factor of the diode with inverse surface layer thickness was also studied. The ideality factor start increasing with increase in inverse layer thickness and attains a maximum value at a particular thickness and thereafter it starts decreasing and finally approach almost unity for larger inverse layer thicknesses at which the barrier height attains maximum saturation value. The maximum ideality factor occurs at thickness for which the barrier height values starts rising. It is observed from the inverse layer thickness and doping concentration dependence of BH and ideality factor that for large inverse layer thickness the BH attains a maximum value with unity ideality factor. Thus depending on the layer thickness and its doping concentration there are two regimes, namely, the non-ideal regime corresponding to lesser inverse layer thickness and/or its doping concentration and the ideal regime corresponding to large inverse layer thickness and/or its doping concentration. The temperature dependence of the BH and ideality factor of the Schottky diodes with inverse doped surface layer were also studied by numerical simulation at low temperatures. It is observed that for diode in non-ideal regime the BH found to decrease and ideality factor increases with decrease in temperature. However, in ideal regime the Schottky diode BH increases linearly with decrease in temperature with unity ideality factor at all temperatures. The overall barrier enhancement as a function of inverse layer thickness was found to occur from 0.71V to 0.92V. Further barrier height enhancement as a function of temperature is observed for 120nm thickness for which it is found to increase from 0.92V at 300K to 1.18V at 40K with decrease in temperature.
5:00 PM - D3.3
Low-frequency Noise in Schottky Barriers Based Nanoscale Field-effect Transistors
, IEMN - CNRS, Villeneuve d'Ascq, France; 2,
, LAAS, Toulouse, France.Show Abstract
Investigation of low-frequency noise in Schottky barriers based nanoscale Field-effect Transistors (SB-FETs) is of prime importance due to its large amplitude in emerging bottom-up devices. In addition, noise can give additional information on the charge transport mechanisms. Here, we study the 1/f noise in nanoscale silicon-on-insulator SB-FETs which we consider as a test bed because of an excellent process control of the gate oxide interface and flexible engineering to control SB height Î¦ through dopant segregation. First, we propose an analytic model for 1/f noise in SB-diodes and SB-FETs. We consider that noise is related to a voltage fluctuation across the SB, which can be obtained analytically from Richardson expression for thermoionic emission. The contribution of two fluctuators in series (SB and a serial resistance or FET channel) is then considered to get an equation valid for low SB heights. Second, SB diodes and SB-FETs were studied through DC current, 1/f noise and shot noise measurements. Barrier heights as low as 80 and 50 meV (for N- and P- type respectively and estimated independently through temperature dependence measurements) are achieved using dopant segregation. We show that even for very small Î¦<0.1 eV, the contribution from the SB to the noise is not negligible even if the current is dominated by the channel resistance. In addition, there is an exponential decay of normalized noise with Vd, which is in perfect agreement with the proposed model. Transistor channel length Lg has been varied from 30 nm to 1 Âµm. Noise dependence with Lg at Vd = 0.2 V is very different for N- and P- type segregated SB-FETs (independent on Lg and following 1/Lg, respectively). 50 meV can be considered as a maximum barrier height for low impact of SB on the noise when Vd >> 0 V. Tuning Î¦ by technological means such as dopant segregation tends to reduce the noise amplitude, although additional generation-recombination and trapping noise is noticed at small Vd for P-type devices. In agreement with SOI-SBFETs experimental results, the proposed analytic model can be considered for any diffusive SB FET including nanowires, graphene sheets and carbon nanotubes devices. It could be used either for noise amplitude comparison, interfacial quality and SB height evaluation. N.Clement, G. Larrieu and E. Dubois, Â« Low-Frequency noise in Schottky barriers based Nanoscale Field-Effect Transistors Â» IEEE Trans.El.Dev., in press. TED-.2011.2169676
5:00 PM - D3.4
Nano Devices for Spintronics with Organic Materials
Tatay Aguilar1, Clement
, Unité Mixte de Physique CNRS/Thales, Paris, Paris, France.Show Abstract
Molecular spintronics, the combination of chemistry potential to the spin degree of freedom provided by spintronics, is considered to be more than an alternative to conventional spintronics with inorganic materials. Unconventional properties and strong potentialities offered by the flexibility, chemical engineering and low production costs of molecules, add to the opportunity that spin lifetime could be enhanced by several orders of magnitude compared with inorganic materials. Very recently it has been highlighted that the metal/molecule hybridization could strongly influence interfacial spin properties going from spin polarization enhancement to its sign control in spintronics devices . A very promising direction is going towards new spintronics devices based on tailored thin organic layers such as SAMs, ultrathin polymerand organic semiconductor films. However, one of the major obstacles to be faced is the possible presence of topographic heterogeneities and electrical defects in the organic barrier. This limits the reliability and reproducibility of the devices and results in a lack of control of the metal-molecule interfaces eventually hampering the proper study of transport in wide-area devices. In this contest nanocontacts become indispensable to allow a deeper investigation on the metal-molecule interface properties. We developed the idea of nanoindentation lithography as a highly versatile and easily adaptable technique allowing the realization of nanojunctions based on the different organic barriers. During nanoindentation a conducting AFM tip is used to notch a nanohole into a previously deposited mask resist. The originality of the process relies on the real-time monitoring of the indentation process through a tipâ€“sample resistance measurement which allows controlling the indentation depth and the contact area with sub-nm precision . Different types of Ferromagnet/Organic/Ferromagnet spintronics nanodevices can be realized: ones where the nanocontact is first elaborated, enlarged by the exposition of the sample to an additional plasma treatment and then filled with the desired organic layer; others where the nanohole is directly dug through the masking resist and into the organics leaving just a thin layer. The top electrode is finally deposited onto the organic layer to complete the device. The nanometric size of the contact (Ã¸=10nm) reduces heterogeneities and allows us to explore the local properties of the junction. We will demonstrate the versatility and potential of our technique showing transport characterization for three types of organic tunnel junctions: thin polymer films, aliphathic SAMs and small molecules organic semiconductors. We will especially focus on SAMs based devices for which a significant TMR effect is measured and found to be exceptionally robust with bias voltage up to 2V. M.G. thanks EU-FP7 HINTS project.  C. Barraud et al.Nature Phys.2010,6,615  K. Bouzehouane et al.Nano Lett.2003,3,1599
5:00 PM - D3.5
Electronic Properties of Sandwiched Metal-Graphene-Metal Structures: An Experimental and Theoretical Study
Shan2 1, Robert
Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas, USA; 2,
School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China.Show Abstract
Two types of interfaces can be formed between metals and graphene depending on the strength of the metal-graphene interaction: weak (metal physisorption) and strong (metal chemisorption) interfaces.1,2 â€œPhysisorptionâ€ interfaces (e.g., with Al, Ag, Cu, Ir, Pt and Au) are characterized by a larger metal-carbon distance (>3 Ã…) with some charge transfer between metal and graphene (i.e. doping of graphene) that maintains its overall Ï€-band dispersion. â€œChemisorptionâ€ interfaces (e.g. with Ni, Co, Pd, and Ti) are characterized by a smaller metal-carbon distance (<2.5 Ã…) and strong orbital hybridization between metal-d and carbon-pz orbitals, resulting in the destruction of the grapheneâ€™s Ï€-band dispersion around the Dirac point. Consequently, metals like Ni, Co, Pd, and Ti are commonly used as electrode materials because they form stable contacts due to their strong chemical interaction with graphene, although the graphene electronic properties are essentially destroyed. The issue therefore is to keep grapheneâ€™s intrinsic Ï€ bandstructure by using weakly interacting metals while enhancing the interface stability. In this work, we propose to use sandwiched metal/graphene/metal structures, using weakly interacting metals. The structures thus fabricated are characterized mainly by Raman and x-ray photoelectron spectroscopy (XPS). Large graphene sheets are grown by chemical vapor deposition (CVD) of gaseous methane on copper foils.3 Both Raman and XPS are used to identify the doping level and interface binding energy change before and after the formation of the sandwich structures. These structures make it possible not only to adjust the doping levels in graphene by selecting a combination of appropriate metals, but also to form stable contacts with graphene with metal films on top and bottom. Density functional theory (DFT) calculations are carried out to provide an atomic level understanding of the electronic properties of sandwiched metal-graphene-metal structures. We find that the increased interface binding strength of sandwiched structures arises from an increased interface electron repulsion effect by metal contacts at both sides of graphene. This work suggests that sandwiched structures provide a means to optimize the metal-graphene contact for device applications.  Giovannetti et al. Phys. Rev. Lett. 101, 026803 (2008).  Gong et al. J. Appl. Phys. 108, 123711 (2010).  Li, et al. Science 324, 1312-1314 (2009).
5:00 PM - D3.6
Ultra-thin and Discontinuous Metal Films for Solar Cell Electric Nanocontacts
Photovoltaic Nanotechnology and Nanosensors Laboratory, Shaw University, Raleigh, North Carolina, USA; 2,
Department of Computer Science, University of Manouba, ISCAE, Manouba, Tunisia.Show Abstract
For modern third generation solar cells and optoelectronic devices, conventional contacts made of metallic and alloys can no longer satisfy device requirements in the nanoscale range. As the thickness is decreased, ultra-thin metal films become discontinuous, thus their optical and electrical properties drastically change and so does their functionality. The coverage reduction leads to island formation and local microstructure variations. Therefore, the understanding of optical and electrical properties of metallic nano-layers thinner than a critical thickness is extremely important. We are reporting on layers of high purity Al and Ag sputter-deposited in high vacuum chamber on prime mirror polished silicon wafers. Emphasis are put on the properties of sputtered Al and Ag films with thickness in the coalescence range. Sputtered Al, with aimed uniform film thinner than 10 nm, showed by FE-SEM scattered fragments on the silicon surface. The formation of nano-sized Al islands starts at film thicknesses above 10 nm, which defines a critical thickness for coalescence of sputtered nano-particles. Depth profile of a 38nm thick Al layer exhibits roughness higher than that of the initial silicon wafer, which suggests a continuous involvement of Al nanoparticles during the film growth, even after full coalescence. Al film reflectance appeared to decreases with the thickness to reach its minimum at 50nm, then increases and peaks at 80 nm, and ultimately decays linearly as the film thickness increases. We believe that reduction of the reflectance at early stages of film formation is due to an increased scattering of the incident light by small islands formed at the initial sputtering stage. As the deposition proceeds, the reflectance increases and reaches its maximum when the inter-island regions are totally filled. With further deposition, the reflectance slowly decreases. A strong dependence of ultra-thin Al film electrical resistivity on thickness was found. The resistivity dramatically increases as the thickness (<50nm) decreases due to the discontinuity of the grown Al islands/grains. Non-conventional conduction mode occurs between those metallic nano-particles to enable current transmission from one islands to its neighbors. Beyond 50 nm film thickness, the resistivity becomes almost stable with a value of 6.1Ã—10-8 Î©-m, which is still higher than the resistivity of the bulk Al (i.e., 2.8Ã—10-8 Î©-m). The change in the conduction mode is suggested to be a determining factor for the observed conductivity transition. We found that discontinuous Ag layer produces strong fluorescence due to an oxidized phase. AFM imaging of annealed Ag ultra-thin layers showed islands formed on the silicon surface, these appeared well separated from each other. Such nanosized features offer the potential of absorption enhancement via surface plasmon modes. Raman spectroscopy has shown Ag related lines much higher than that of the Si line.
5:00 PM - D3.7
Nanometer Thickness Planar Schottky Contacts for Ultra-fast Sensing and Energy Conversion Applications
, University of Ilinois at Chicago, Chicago, Illinois, USA.Show Abstract
Recent observations of chemically induced hot electron flow over Schottky barriers in planar metal-semiconductor nanostructures provide interesting possibilities for electrolyte-free conversion of chemical energy into electricity and ultra-fast biomimetic sensing applications. Meanwhile, the short-lived nature of the hot electrons in metals imposes special requirements for the Schottky contact morphology, material selection, physical and chemical properties explained in this presentation. Thermal and chemical resilience of the Schottky contact properties and electrical continuity of the wide-area nanometer thickness cathodes play an important role for the potential practical applications. An efficient hot electron harvester must utilize a wide bandgap semiconductor anode, and therefore a fabrication procedure is required to produce both a sharp barrier layer and a diffusive Ohmic contact layer on the same semiconducting wafer; that alone imposes a remarkable challenge. The presentation involves discussion of the relevant theoretical guidelines, practical methods and techniques leading for high quality Pt/GaP/Sn, Pt/SiC/Ag, Rh/SiC/Ag and other Schottky nanodiodes, and prospects of their practical utilization.
5:00 PM - D3.8
Marked Suppression of the Fermi-level Pinning at Metal/Ge(111) Junctions with Atomically Matched Interfaces
Department of Electronics, Kyushu University, Fukuoka, Japan; 2,
PRESTO, Japan Science and Technology Agency, Tokyo, Japan.Show Abstract
Germanium (Ge) is one of possible candidates to replace silicon (Si) as the channel material in complementally metal-oxide-semiconductor (CMOS) technology due to its high electron and hole mobility. However, Fermi-level pining (FLP) effect at metal/Ge interfaces is a critical issue, disturbing the development of high-performance CMOS devices with metallic source and drain contacts. In order to solve this issue, it is important to understand the origin of FLP at metal/Ge junctions. Although it has so far been considered that the origin of FLP is metal induced gap states (MIGSs), we have obtained some results which can not be explained only by MIGSs.
In this paper, we fabricated Fe3Si/Ge(111) Schottky diodes with atomically matched interfaces with a size (S) of ~106 and ~1 Î¼m2, and measured their electrical properties at low temperatures. We found that the diodes with S = ~1 Î¼m2 show the two different current-voltage (I-V) characteristics at 100 K, i.e., Ohmic behavior and rectifying behavior. On the other hand, almost all diodes with S = ~106 Î¼m2 showed nearly Ohmic behavior at 100 K. This S dependence cannot be explained by MIGSs. Assuming Poisson distribution, we can regard the S dependence as a result of the influence of the number of defects at the interface between Fe3Si and Ge(111). In short, we infer that the number of the interfacial defects depends on S. When S is ~106 Î¼m2, there are some interfacial defects at the Fe3Si/Ge(111) interface. When S is reduced to ~1 Î¼m2, we can encounter two different interfaces, the strong contribution with defects and almost no contribution with ones. From these considerations, we can also infer that the Ohmic and rectifying behavior are arising from the defective and high-quality interfaces, respectively. Therefore, FLP at metal/Ge interfaces can be explained by the extrinsic mechanism. For discussing the formation of the Schottky barrier at metal/Ge interfaces, one should distinguish between intrinsic and extrinsic mechanisms.
K.K and S.Y. acknowledge JSPS Research Fellowships for Young Scientists. This work was partly supported by Industrial Technology Research Grant Program from NEDO and Grant-in-Aid for Young Scientists (A) from JSPS.  C. H. Lee et al., IEDM Tech. Dig., 416 (2010).  A. Dimoulas, A. Toriumi, and S. E. Mohney, MRS bulletin 34, 522 (2009).  T. Nishimura et al., Appl. Phys. Exp. 1, 051406 (2008).  K. Yamane et al., Appl. Phys. Lett. 96, 162104 (2010).  K. Kasahara et al., Phys. Rev. B (in press).
5:00 PM - D3.9
Laser Zone Annealing of Electrodeposited Gold Nanowires to Bamboo Grain Structures
Kim1 2, Mercedes
Materials Science and Engineering, UC Irvine, Irvine, California, USA; 2,
Chemistry, UC Irvine, Irvine, California, USA.Show Abstract
The four-point resistance, R, of electrodeposited gold nanowires was measured in situ through a laser zone annealing process. A stage and sample attached to a piezoelectric motor translated the nanowire over a 532nm, confocal laser spot at rates as low as 7nm/s. These nanowires were synthesized at height of 20nm and at varying widths (from 76nm to 274nm) using an electrodeposition method known as Lithographically Patterned Nanowire Electrodeposition (LPNE). The resulting resistivity of one such wire measured a reduction of 60% from its initial resistance of 2.5x10-7Î©m to 1.1x10-7Î©m. Transmission electron microscopy (TEM) was used to analyze the change in the internal grain structure and measure the large increase in the in-plane grain diameters (from 27Â±14nm as-grown state to 90Â±31nm post annealed condition). The average grain growth diameter was limited to the wire width as the wire neared a complete bamboo structure. The sharp heating profile of the concentrated laser spot could be observed in TEM micrographs as a distinct boundary in the grain structure between annealed and as-grown sections. Theoretical values of resistivities were calculated with grain diameters using analytical models primarily considering grain boundary and surface scattering. The agreement between experimental results and these calculations illustrated that significant contribution to the enhanced conductivity was primarily the reduction of grain boundary density. The improved performance was obtained without suffering instability along the wire, namely grain boundary grooving or wire discontinuity observed in bulk thermal annealing. This zone annealing technique is of interest in improving the microstructure and conductivity of interconnects without the introduction of high thermal energy.
5:00 PM - D3.10
Novel Method for Robust Bonding and Alignment of Nanowires on Electrodes
Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea; 2,
Nano Manufacturing Technology, Korea Institute of Machinery and Materials, Daejeon, Republic of Korea.Show Abstract
ZnO nanowires (NWs) synthesized by bottom-up method can be used for electronic devices such as field effect transistors, light emitting diodes, and sensors. Conventional methods for the alignment and integration of NWs have been dielectrophoresis, an external magnetic-field and so on. However, when they were aligned onto the electrodes, mechanical and electrical contacts between NWs and electrodes were not stable due to incomplete bonding and weak adhesion based on Van der Waals force. Therefore, post-processes such as focused ion beam deposition or selective electrodeposition were needed to enhance the mechanical and electrical contacts, which increases the complexity and cost of manufacturing process. In this paper, we present a novel process based on two-step transfer printing of nanowires on the metal electrodes with low melting temperatures, which allows robust bonding of nanowires onto metal electrodes without the need of additional bonding processes. The procedure of thermocompression bonding is as follows: In the first transfer step, ZnO NWs randomly grown on Si substrate by CVD method were transferred to intermediate Si/SiO2 substrate. In this process, we used contact printing method where normal and shear forces were applied to the intermediate substrate to form parallelly aligned ZnO NWs. In the second transfer step, parallelly aligned ZnO NWs on intermediate substrate was pressed against target substrate with metal electrodes by thermocompression at P=5 bar and T=100Â°C for 5 minutes. After intermediate substrate was removed from the target substrate, we could observe that ZnO NWs were successfully aligned and formed a robust bonding on metal electrodes. We used double layer metal (Au/In and Cu/In) having low melting temperature because they are softened at elevated temperature (~100Â°C). ZnO NWs can be easily embedded into metal electrodes with low pressure (~5 bar) since the metal electrodes are mechanically soft. Consequently, ZnO NWs and metal electrodes are bonded with high mechanical strength. Mechanical strength of ZnO NW â€“ metal electrode bonding was measured by applying lateral force microscopy (LFM) at center of ZnO NWs. It could be observed that ZnO NWs were broken before the failure of NW-electrode joints, proving the mechanical robustness of bonding. We could also find that the bonding between metal electrodes and ZnO NWs form a stable Ohmic contact property from electrical measurement. In summary, we demonstrated a novel method for the alignment and robust bonding of NWs onto metal electrodes by using two-step transfer printing technique. Mechanically robust bonding and stable ohmic contact between NWs and metal electrodes could be achieved by this high-speed and low-temperature process. We believe that this method will be very useful for the large-area fabrication of NW-based electrical devices such as field effect transistors, light emitting diodes, and sensors.
5:00 PM - D3.11
The Role of the Organic Layer Functionalization in the Formation of Silicon/Organic Layer/Metal Junctions with Coinage Metals
Soria1, Maria Fernanda
Fisicoquímica, Facultad de Ciencias Químicas. Univ. Nacional de Córdoba, Córdoba, CÃ³rdoba, Argentina; 2,
Matemática y Física, Facultad de Ciencias Químicas. Univ. Nacional de Córdoba, Córdoba, CÃ³rdoba, Argentina.Show Abstract
Introducing organic molecules in electronics is limited by the ability to connect them electrically to the outside world. This involves the formation of top metal overlayers on the organic surfaces. This is not a trivial task because most known methods to make such contacts are likely to damage the molecules . Understanding the atomic and molecular level interaction of metal atoms with organic surfaces is becoming increasingly important as the number of applications involving metalâˆ’organic interfaces grows. The design of silicon/alkyl layer/metal junctions for the formation of optimal top metal contacts requires the knowledge of the mechanistic and energetic aspects of the interactions of metal atoms with the modified surface. This involves a) the interaction of the metal with the terminal groups of the organic layer, b) the diffusion of metal atoms through the organic layer and c) the reactions of metal atoms with the silicon surface atoms. The diffusion through the monolayer and the metal catalyzed breakage of Siâˆ’C bonds must be avoided to obtain high quality junctions. In this work we developed a complete mechanistic and energetic picture of all the processes involved in the formation of silicon/alkyl layer/metal junctions. Our results are in agreement with the available experimental observations and provide detailed information on the reaction pathways and energy barriers involved in the formation of the junction. We performed a comprehensive density functional theory investigation to identity the reaction pathways of all the processes involved . The diffusion of gold atoms through the alkyl layer was compared for two systems: compact alkyl monolayers on Si(111) and compact alkanethiol monolayers on Au(111). In the absence of a reactive terminal group, gold atoms penetrate through the monolayers with small energy barriers. However, the presence of thiol terminal groups introduces a high energy barrier which blocks the diffusion of metals into the monolayer. On the alkylated Si(111) surface, the diffusion barriers increase in the order Ag < Au < Cu and correlate with the stability of metalâˆ’thiolate complexes whereas the barriers for the formation of metal silicides increase in the order Cu < Au < Ag in correlation with the increasing metallic radii. The reactivity of gold clusters with functionalized Si(111) surfaces was also investigated. Metal silicide formation can only be avoided by a compact monolayer terminated by a reactive functional group. The mechanistic and energetic picture obtained in this work contributes to understand the factors that influence the quality of top metal contacts during the formation of silicon/organic layer/metal junctions.  H. Haick and D. Cahen, Prog. Surf. Sci., 2008, 83, 217-261  M. F. Juarez, F. A. Soria, E. M. Patrito, P. Paredes-Olivera. Phys. Chem. Chem. Phys. 2011, DOI: 10.1039/c1cp22360g
5:00 PM - D3.12
Nanoscale Schottky Contact and Light-induced Charge Separation on 1D TiO2 Nanostructures
School of Advanced Materials Engineering, Kookmin University, Seoul, Republic of Korea.Show Abstract
TiO2 is the most intensively investigated class of materials for the applications of photocatalysts' support and photoanodes of solar cells and water splitting. Among them, nanotubular structures of TiO2 are even more interesting due to their 1 dimensional morphology as a direct pathway of the photo-induced charge carriers. For the applications, local characterizations of the nanometer scale contacts and charge separation and transport behavior through 1D nanomaterials are of paramount importance. Here, we present nanoscale electrical as well as photoelectric properties of TiO2 nanotubes (TNTs) by the modified scanning probe microscopy (SPM). SPM is an ideal tool for the local electric characterization. It could be addressed the phenomena of the trapped and/or injected charges on the nanoscale functional materials. In this presentation, two examples are to be discussed: 1) Light-induced charge separation on nanostructures of Au/TiO2 nanotubes(TNTs) characterized by modified electrostatic force microscopy (EFM), and 2) Nanometer scale Schottky contact at anatase TiO2 nanotubes/Pt interfaces. Schottky interfaces at both distal ends of the nanotube, the ideality factor and Schottky barrier height are obtained based on thermionic emission theory, revealing the enhanced tunneling. The reduced effective Schottky barrier heights are in the range of 0.28 to 0.43 eV.
5:00 PM - D3.13
Development and Characterization of Platinum Silicide as a Tunable Contact Material for NEMS Switches
Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, USA; 2,
Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, USA; 3,
Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, USA.Show Abstract
Nanoelectromechanical systems (NEMS) switches have been identified by the International Technology Roadmap for Semiconductors as a possible "beyond CMOS" technology. However, the reliability of the contact interface is currently a key challenge for the commercialization of these devices. The adhesiveness of conventional metallic contact materials in combination with the nanoscale dimensions of NEMS switches results in failure through permanent adhesion of the contact interface. In addition, many non-adhesive contact materials are reactive and can form insulating tribolayers after many switching cycles. This motivates the need for contact materials that are highly conductive, chemically inert, minimally adhesive, and amenable to CMOS fabrication processes. Platinum silicide is a promising candidate material that may satisfy these complex demands. In this work, we show for the first time that the surface chemical composition of platinum silicide (PtxSi) formed through amorphous silicon (a-Si) and platinum (Pt) diffusion may be tuned by changing the amount of a-Si available during silicidation. This gives direct control over the selection of highly conductive and chemically inert Pt-rich variants, or low-adhesion Si-rich forms. PtxSi is a promising next-generation switch contact material because it possesses metallic conductivity, is compatible with current CMOS fabrication, is chemically inert, and can be used for the etchant-free in-situ release of freestanding NEMS switches. However, successful integration of platinum silicide contacts into many NEMS switch architectures requires the use of a-Si rather than the more heavily studied single-crystalline silicon-based (sc-Si) PtxSi. Little is known about the growth sequence, growth kinetics, growth rates, and resulting chemical composition of the PtxSi formed using a-Si. In the present study, PtxSi was synthesized by annealing a 40-nm a-Si layer on a 70-nm Pt layer and a 100-nm Pt layer on a sc-Si wafer using both rapid thermal (RTA) and high vacuum (HV) annealing. Diffusion of Pt from the a-Si/Pt bilayer into the sc-Si carrier wafer was prevented through the use of an aluminum nitride diffusion barrier - a topology similar to that employed in some current micro/nano-scale switches. Differences in the formation behavior of the resulting silicides were then characterized using in-situ and ex-situ X-ray photoelectron spectroscopy. a-Si was shown to form a Pt-rich silicide (Pt3Si) in comparison to the PtSi and Pt2Si stoichiometries usually found in sc-Si/Pt silicidation. This Pt-rich stoichiometry is believed to be the result of using a thin a-Si layer - essentially robbing the silicidation process of sufficient Si for full conversion to Pt2Si or PtSi. This suggests that the thickness of thin a-Si on Pt has a direct impact on the stoichiometry and may be tuned to confer properties associated with either Pt-rich (high conductivity and inertness) or Si-rich (low adhesion) compositions.
5:00 PM - D3.14
Compositional Analysis of E-beam-induced Deposited Tungsten Contacts for Nanocarbon Interconnects
Center for Nanostructures, Santa Clara University, Santa Clara, California, USA; 2,
, Hitachi High-Technologies, Hitachinaka, Ibaraki, Japan.Show Abstract
Electron-beam-induced deposition (EBID) of tungsten (W) has been demonstrated to yield substantial improvement in the contact resistance between carbon nanomaterials and their metal electrodes . EBID could provide a lower-cost alternative to Ion-Beam Induced Deposition (IBID) schemes such as focus-ion beam (FIB), and has the ability to produce structures with high positional accuracy. Additionally, FIB has the risk of damaging the on-chip devices because of its high beam energy, resulting in the implantation of gallium from the ion beam which could lead to alteration of device properties. Therefore, we have developed an EBID technique for W deposition using a variable-pressure scanning electron microscope (VP-SEM). In this technique, the source gas (WF6) is delivered via a specially designed gas injection system (GIS)  and guided by the focused electron beam to yield deposition on a selected target at a lower energy than FIB. We have performed surface analysis of the deposited areas using energy-dispersive x-ray spectroscopy (EDS), and compared them with depositions performed by FIB using W(CO)6 on similar structures. Additionally, in order to assess the viability of this technique, we have performed a series of experiments to determine the localization of W in the area surrounding the deposits. We find that for EBID, the W content in the deposited area is generally higher than that for IBID. A detailed analysis of W composition between the electrode contacts reveals that the EBID depositions are fairly localized at the electrodes, though there is evidence that trace amounts of W are present along the length of the nanocarbon interconnect. This small lateral spread of W in the EBID is probably a result of secondary electron induced deposition, or through other migration paths such as atomic W diffusion subsequent to dissociation from the WF6 molecule. The extent of the impact of this lateral spread of W deposits on the electrical properties of the nanocarbon interconnects will be discussed.  Shusaku Maeda, Patrick Wilhite, Nobuhiko Kanzaki, Toshishige Yamada, and C. Y. Yang, AIP Advances 1, 022102 (2011).  D. C. Joy and P. D. Rack, Microscopy and Microanalysis 11, 816 (2005).
5:00 PM - D3.15
Magnetoresistance of Ni/Benzenedithiol/Ni Single Molecular Junctions at Room Temperature
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan.Show Abstract
Rapid progress has recently been made in the quantitative analysis of electrical conductance of metal/molecule/metal junctions through the development of a mechanically controllable break junction (MCBJ) method. However, experimental studies on the single molecular junctions have been focused on a charge transport and have not made use of spin degree of freedom. Theoretical studies have predicted that single molecule is useful for the transport and generation of spin-polarized electrical current . Recently Schmaus et al. have reported that the magnetoresistance (MR) of single molecular junctions reaches several tens of % using scanning tunneling microscope . We also have succeeded in measuring the MR of the single molecular junctions by using the MCBJ method . In this study we found that molecular junctions showed anomalous giant MR (GMR) at room temperature. We used A thin Si plate as a substrate. Au/Cr electrodes were prepared by conventional photolithography. Au and Ni layers were prepared by electrochemical deposition on Au/Cr electrodes to form a contact of Ni electrodes. A droplet of mesitylene containing 1 mM benzenedithiol (BDT) was then placed on the Ni electrodes to modify the electrodes. The Ni contact is broken by bending the substrate. When the molecular junction is formed, plateau is observed in the current transient. We conducted MR measurement of single molecular junctions by applying external magnetic field in Ar atmosphere at room temperature. When the magnetic field was changed from H = -2000 Oe to H = 2000 Oe, the resistance reached maximum around H = 300 Oe. MR of Ni/BDT/Ni junctions. The typical MR ratio was tens of %. Since the tunnel junction without the molecule showed only a few % of negative MR, we can attribute the origin of MR to the Ni/BDT/Ni junctions.  L. Bogani and W. Wernsdorfer, Nature Mater. 7, 179 (2008).  S. Schmaus et al., Nat. Nanotechnol. 6, 185 (2011).  R. Yamada et al., Appl. Phys. Lett. 98, 053110 (2011).
5:00 PM - D3.16
Formation of Cobalt Disilicide on 3D Structures from Highly Conformal Cobalt Nitride Thin Films by Low-temperature Chemical Vapor Deposition from a Liquid Cobalt Amidinate Precursor
Gordon1, Qing Min
Engineering and Applied Sciences, Harvard, Cambridge, Massachusetts, USA; 2,
, Dow Chemical, Midland, Michigan, USA.Show Abstract
Silicides have been widely studied and used for the low-resistance contacts, gate electrodes and local interconnections in metalâ€“oxideâ€“semiconductor field effect transistors (MOSFET) for decades. Although nickel silicide (NiSi) offers lower resistivity, the greater thermodynamic stability of cobalt disilicide (CoSi2) makes it more suitable for structures which high processing temperatures are needed. Traditionally, CoSi2 has been prepared by annealing of sputtered or evaporated cobalt films on silicon substrates. Industry is moving towards 3D transistors to continue the pace of technology advancement, however, cobalt films made by physical vapor deposition methods (sputtering or evaporation) are non-conformal over the complex 3D architectures and thus fail to meet the challenge. In this presentation, we will demonstrate the formation of CoSi2 by in-situ annealing of highly-conformal cobalt nitride films inside holes with aspect ratios over 30 to 1. The cobalt nitride films are prepared by chemical vapor deposition (CVD) using a cobalt amidinate precursor and a reactant mixture of NH3 and H2 at low substrate temperatures. We studied the reaction of cobalt nitride films with silicon under different annealing conditions. Morphological stability and interfacial smoothness are crucial for application of cobalt disilicide as a material for contacts, gates or local interconnects. We developed a method to measure the roughness of CoSi2/Si interfaces by selective backside etching followed by atomic force microscopy. Using this method, we optimized the deposition and processing conditions to make smooth interfaces between CoSi2 and various crystalline orientations of silicon.
5:00 PM - D3.18
Systematic Study of the Contact Resistance of InAs Nanowires between Wet Etching Process and In situ Argon Milling
London Centre for Nanotechnology, University College London, London, United Kingdom.Show Abstract
Nanowires are attracting a growing interest in the semiconductor industry due to their numerous potential applications including field-effect transistors, elementary logic gates and light-emitting diodes. It has also been demonstrated that nanowires could be used as Josephson elements in superconducting devices. Indium arsenide (InAs) nanowires are of special interest as InAs can form Schottky-barrier-free contacts with metals. However a native oxide layer is known to form easily on InAs nanowires and thus must be removed prior to metallization to achieve highly transparent contacts. While most of the studies involving InAs nanowires report the use of a wet etching process to treat the nanowires before deposition of the contacts, there have been few reports of the sputter-cleaning process and no quantitative comparison between the two techniques. Here we present a systematic comparative study of the contact resistance between InAs nanowires and metals following the use of (a) a wet etching process or (b) a sputter cleaning process. The InAs nanowires are grown via molecular beam epitaxy on Si (111) substrates without the use of metal catalysts. They have a diameter between 50 and 100nm and an average length of 3Âµm. HRTEM measurements show that the structure of the nanowires is a mixture of hexagonal and cubic phases. The metallic contacts are attached to the nanowires by using an electron-beam lithography process. From field-effect measurements, we have established that the InAs nanowires are n-type and that their mobility at room temperature lies between 100cm2 V-1s-1 and 600cm2 V-1s-1. Before the contact metallization, the InAs nanowires are treated either by wet etching or by argon milling. In the wet etching process, the contact area of the InAs nanowires is etched and passivated by an ammonium polysulfide solution. Different dilution levels of the solution and exposure times are investigated. In the sputter cleaning process, the InAs nanowires are milled by argon-ion sputtering with a voltage of 200V. Nanowires treated by either of these two processes exhibit lower contact resistances by up to three orders of magnitude compared with untreated nanowires . While both processes exhibit similar values of optimized contact resistance, the use of argon milling tends to give more systematic results with a smaller sample-to-sample variance. Its use is also simpler as the metallic deposition can be done in-situ following the milling. We have also examined the influence of the material used for the drain and source electrodes (Cr/Au, Ni/Au, Ti/Au or Nb/Au) and the stability of the contacts as a function of storage time in vacuum or air.
5:00 PM - D3.19
Ohmic Contacts to n-type Germanium Using a Thin ZnO Interfacial Layer
Center of Excellence in Nanoelectronics, Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India.Show Abstract
Non-Ohmic contacts to Ge pose a significant challenge to future high performance CMOS and low thermal budget selector devices for cross-point memories. Low resistance n-type Ohmic contacts are more difficult to achieve due to large Schottky barrier heights (0.49-0.64 eV)  resulting from Fermi level (EF) pinning near the valence band and low dopant activation levels.[1,2] A thin, insulating dielectric between the metal and Ge layers unpins EF but the high tunneling resistance of the dielectric layer limits the contact resistance. Recently, a high-doped Si interfacial layer having good conduction band alignment with Ge has been used to realize low resistance Ohmic contacts to n-Ge. In this work we demonstrate n-type ZnO as another suitable interfacial semiconductor layer that can form Ohmic contacts on n-Ge due to a negative (-0.1 eV) conduction band offset at the ZnO/Ge interface, low EF pinning for metal contacts on ZnO, and high doping levels that can be realized at metal/ZnO interfaces. Thin ZnO layers (0.7-2.1 nm) were deposited on HF:DI cleaned n-Ge (1e17 cm-3) substrates using RF sputtering. X-ray diffraction on thick lpayers confirms the deposition of crystalline, c-axis oriented ZnO (0002). Hall and sheet resistance measurements show that the ZnO films are low-doped (1e17 cm-3). Metal contacts were formed using Ti/Au e-beam evaporation and backside metallization was done using Al. I-V characteristics of as-deposited contacts show a strong dependence on ZnO thickness. Thin ZnO (0.7nm) gives a significant increase in forward (10x) and reverse-bias (100x) currents due to the low (0.3 eV) barrier of the unpinned Ti/ZnO interface  as compared to the pinned 0.54 eV barrier of the control Au/Ti/n-Ge sample. However, the current decreases with increasing ZnO thickness due to significant tunneling resistance from the depletion width of the low-doped ZnO layer. Annealing the Au/Ti/ZnO/n-Ge contacts at 400 Â°C for 60s gives nearly identical, Ohmic I-V characteristics that are independent of ZnO thickness. This can be attributed to the formation of a highly doped ZnO layer at the Ti/ZnO interface due to accumulation of oxygen vacancies during anneal. The increased doping reduces the thickness of the depletion tunneling barrier at the Ti/ZnO interface resulting in Ohmic contacts that are lower in resistance than the as-deposited devices. Additionally, the tunneling depletion width, and hence the I-V curves, are nearly independent of ZnO thickness for the highly doped interface. The Ohmic I-Vs on annealed Au/Ti/ZnO/n-Ge contacts show 500x higher current densities at low bias (Â± 0.1 V) versus control Au/Ti/n-Ge devices. This project was funded with a research grant from Applied Materials, Inc., USA. References: 1] K. Martens et al., APL 98, 013504 (2011) 2] D. Lee et al., APL 96, 052514 (2010) 3] L. J. Brillson et al., JAP 109, 121301 (2011) 4] C. A. Mead, Sol. St. Elect. 9, 1023 (1966) 5] K. Vanheusden et al., APL 68, 403 (1996)
5:00 PM - D3.20
Interfacing Ag Nanoparticles with 1D Semiconductor Micro/Nanostructures via Joule Heating for Transfer Printing Nanodevices at Room Ambient
Ombaba1, M. Saif
Electrical & Computer Engineering, University of California-Davis, Davis, California, USA; 2,
Nanoelectronics and Nanophotonics, Sandia National Laboratories, Livermore, California, USA.Show Abstract
In-situ imaging and characterization of ohmic contact formation mechanism between metal nanoparticles and semiconductor micro/nanoscale wires/pillars is important for several emerging device applications. We describe an experiment to interface and characterize Ag nanoparticle aggregates that are self-assembled and plastically deformable on Au film deposited on glass substrate. The electrical characterization is done using an electrical nanoprobe attached to a nano-manipulator inside a scanning electron microscope (SEM). Electrical current-voltage (I-V) measurements are made between the electrical nanoprobe in contact with the nanoparticle and the Au film. The Ag nanoparticles have diameters ranging between ~600-800nm and are self-assembled on a thiolated 100nm Au film. Application of contact force via the nanoprobe even after substantial particle deformation reveals initially only a small non-linear current. Upon current annealing through Joule heating, significant improvement in the electrical contact at the AgNP/substrate interface was observed. This is most likely based on the particle bonding to the substrate after passing a high current. The need for such an annealing step might be critical in forming good ohmic contacts at ambient conditions during transfer printing of semiconductor micro/nanopillars.