Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D31: Properties of Semiconducting Nanostructures |
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Sponsoring Units: DCMP Chair: Steve Erwin, Navel Research Laboratory Room: Baltimore Convention Center 328 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D31.00001: Enhanced Raman Scattering from Individual Semiconductor Nanocones and Nanowires Linyou Cao, Bahram Nabet, Jonathan Spanier We report strong enhancement ($\sim $10$^{3})$ of the spontaneous Raman scattering from individual silicon nanowires and nanocones as compared with bulk Si. The observed enhancement is diameter ($d)$, excitation-wavelength ($\lambda _{laser})$, and incident polarization state-dependent, and is explained in terms of a resonant behavior involving incident electromagnetic radiation and the structural dielectric cross-section. The variation of the Raman enhancement with $d$, $\lambda _{laser}$ and polarization is shown to be in good agreement with model calculations of scattering from an infinite dielectric cylinder. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D31.00002: Magic structures of H-passivated $\langle110 \rangle$ silicon nanowires Tzu-Liang Chan, Cristian V. Ciobanu, Feng-Chuan Chuang, Ning Lu, Cai-Zhuang Wang, Kai-Ming Ho We report a genetic algorithm approach combined with ab initio calculations to determine the structure of hydrogenated $\langle 110\rangle$ Si nanowires. As the number of atoms per length increases, we find that the cross section of the nanowire evolves from chains of six-atom rings, to fused pairs of such chains, to hexagons bounded by \{001\} and \{111\} facets. Our calculations predict that hexagonal wires become stable starting at about 1.2 nm diameter, which is consistent with recent experimental reports of nanowires with diameters of about 3 nm. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D31.00003: Field Emission of Silicon Nanowires Baoquing Zeng, Guangyong Xiong, Shuo Chen, Sung-Ho Jo, Wenzhong Wang, Dezhi Wang, Zhifeng Ren Field emission of silicon nanowires (SiNWs), which were grown by chemical vapor deposition method with Au catalyst at a temperature of 480 C from silane, has been investigated. To obtain a current density of 1 mA/cm2, an electric field of 5.5 V/um is needed with a turn-on electric field of 2.9 V/um for a current density of 0.01 mA/cm2, which are the best ever reported values. The NWs were studied by scanning electron microscopy (SEM) and transmission electron microcopy (TEM). SEM showed that the length of the wires is about 100 um with diameters of around 100 nm. High-resolution TEM showed that the nanowires have high crystallinity. The low growth temperature makes the process applicable to glass substrates that are used as the backing of large area flat panel displays. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D31.00004: Growth of Pure and Long Silicon Nanowires on Table Salt Support Guangyong Xiong, Shuo Chen, Baoqin Zeng, Wenzhong Wang, Dezhi Wang, Zhifeng Ren Table salt was found to be a very good support for the growth of silicon (Si) nanowires in large quantity by a simple thermal decomposition of diluted silane gas in argon and hydrogen gases at less than 500$^{o}$C. Hydrogen gold tetrachloride was uniformly mixed into the salt and decomposed into gold nanoparticles at the growth temperature and acted as the catalyst particles to start the growth of Si nanowires. The scanning electron microscope images showed that the as-grown Si nanowires are about 50-100 nm in diameter and up to 200 micrometers long. X-ray diffraction patterns showed that the salt was completely removed by water rinse. The transmission electron microscope studies showed that these Si nanowires are highly crystallized single crystals with an amorphous silicon layer of less than a few nanometers. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D31.00005: Fabrication of single-crystal Si nanowires by ultrahigh vacuum magnetron sputtering. J. W. Knepper, X. W. Zhao, F. Y. Yang Semiconductor nanowires have attracted great interests due to the intriguing fundamental science and technological application they provide. Many semiconductor materials have been made into single crystal nanowires with superior crystal quality and high mobility. Among them, silicon is particularly interesting because silicon is the foundation of modern electronic technology. A majority of the nanowire synthesis used laser-assisted catalyst growth or chemical vapor deposition. Here we reported a different approach to the fabrication of semiconductor nanowires using ultrahigh vacuum magnetron sputtering. Using thin Au layers as catalyst via vopor-liquid-solid mechanism, single crystal Si nanowires have been grown on Si substrates at a temperature of $\sim $700\r{ }C. Electron microscopy revealed that most Si nanowires grew epitaxially on Si(111) surfaces. Si nanowires are perpendicular to the Si(111) surface with a Si/Au alloy sphere on the top of the nanowires. The growth of Si nanowires on Si wafers with other orientations and amorphous silicon oxide layers was also observed, but with much less probability. The diameter of the Si nanowires is about 200 nm using Au layers as catalyst. The nanowire diameter can be controlled to smaller size by patterning the Au layers into small dots to reduce the catalyst size. Si nanowires fabricated by ultrahigh vacuum sputter at a base pressure of 10\^{}-10 torr are high purity and can be easily doped to desirable carrier concentration. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D31.00006: Structures of Si and Ge nanowires in the sub-nanometer range Helio Chacham, Ricardo Kagimura, Ricardo W. Nunes We report [1] {\it ab initio} and tight-binding calculations of several structures of pristine Si and Ge nanowires with diameters D between 0.5 and 5.0 nm. For nanowires with D $<$ 2 nm, the calculations are performed in the framework of Kohn-Sham density functional theory, within the generalized-gradient approximation. Total-energy differences are converged to within 10 meV/atom. For nanowires with D $>$ 2 nm, an order-N density-matrix tight-binding methodology (DMTB) is employed. We consider nanowires based on the diamond structure, high-density bulk structures, and fullerene-like structures. Our calculations indicate a transition from $sp^3$ geometries to structures based on denser bulk phases and fullerene-like structures, for diameters smaller than $\sim$1.2 nm. We show that a continuum model is able to reproduce quantitatively this transition. According to the model, the transition originates from the larger surface energy density of the $sp^3$ wires as compared to those of the denser wires. We also find that diamond-structure nanowires are unstable for diameters smaller than 1 nm, undergoing considerable structural transformations towards amorphous-like wires. For diameters between 0.8 nm and 1 nm, filled-fullerene wires are the most stable. For even smaller diameters ($\sim0.5~{\rm nm}$), we find that a simple hexagonal structure is particularly stable for both Si and Ge. [1] R. Kagimura, R. W. Nunes, and H. Chacham, Phys. Rev. Lett. 95, 115502 (2005) [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D31.00007: Physics and Applications of Ge/Si Core/Shell Nanowires Jie Xiang, Wei Lu, Yongjie Hu, Yue Wu, Hao Yan, Charles Lieber We recently reported hole-gas formation and ballistic transport through 1D modes in band-structure engineered Ge/Si core/shell nanowire heterostructures at cryogenic temperatures, opening up a new platform to study low-dimensional transport phenomena and applications such as high performance room temperature field effect transistors. In this talk we report studies of single Ge/Si nanowire field effect transistors employing high-k dielectrics with top gate geometry. The clean hole-gas system and enhanced gate coupling from the high-k dielectric allow clear identification of discrete 1D subbands, as well as the observation of superconductivity proximity effect with superconducting contacts. Room temperature FET characteristics exhibit the best performance achieved in nanowire FETs, and the calculated intrinsic delay as a function of gate length for these nanowire FETs shows a clear scaling advantage over planar Si MOSFETs. Studies investigating the effect of novel gate structures to control ambipolar behavior and threshold voltage will also be discussed. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D31.00008: What are new when Si nanowires get small: magic numbers and square shape Ruqian Wu Through systematic density functional studies, we found the existence of ``magic numbers'' for Si nanowires grown along the $<$100$>$ axis. Strikingly, Si nanowires prefer the sharp square cross-section with corner atoms when the diameter is smaller than 1.7 nm. This is promoted by two facts: (1) the presence of the corner atoms permits formation of benign reconstruction pattern to maximally saturate the dangling bonds; and (2) the corner atoms develop pairs and strongly interact with each other across nanowires. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D31.00009: Effect of surface roughness, defects and phonon scattering on electron current through silicon nanowires Alexei Svizhenko, Paul Leu, Kyeongjae Cho Silicon nanowires (SiNW) can become an important building block of nanoscale devices and circuits. Many experimental groups have fabricated SiNWs and measured their current-voltage characteristics. While electron current was found to be very high by some measurements, it is also highly sensitive to chemical modification of NW surfaces. It is therefore important to understand the detailed mechanisms of electron transport in the presence of external factors (e.g., surface roughness, surface defects, dopants, or phonon scattering) and estimate the current carrying capacity of SiNW. In this talk we will discuss our theoretical calculations of current-voltage characteristics of SiNW based on non- equilibrium Greens function (NEGF) equations using an $sp^3d^5s^*$ tight-binding Hamiltonian. We will focus on the effect of surface roughness, random defects and inelastic electron-phonon scattering on electron transport. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D31.00010: Scanning Probe Microscopy of Semiconducting Nanowires A.C. Bleszynski, R.M. Westervelt, F.A. Zwanenburg, L.P. Kouwenhoven, A.L. Roest, E.P.A.M Bakkers We have used a liquid-He cooled scanning probe microscope (SPM) with a conducting tip to image electrical conduction through InAs nanowires. The charged SPM tip is scanned above the nanowire and the resulting change in nanowire conductance is recorded to form the image. These conductance images are used to study the behavior of electrons in the nanowire on a local scale. For example, the images reveal barriers to conduction at the contacts as well as sections of the wire that act as quantum dots. At 4K the wires exhibit Coulomb blockade oscillations in conductance versus backgate voltage that are indicative of multiple quantum dots in series. The images reveal the location of the quantum dots along the wire and the tip voltage can tune their charge state. The nanowires, grown catalytically from small gold particles, have diameters between 50 and 100 nm. Ti/Al source and drain contacts with a spacing of 1 to 2 $\mu $m were defined using e-beam lithography. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D31.00011: Tunable supercurrent through semiconductor nanowires Jorden van Dam, Yong-Joo Doh, Aarnoud Roest, Erik Bakkers, Silvano De Franceschi, Leo Kouwenhoven We have developed a high-yield approach to the fabrication of nanoscale superconductor-semiconductor hybrid devices [1]. The devices are assembled from InAs semiconductor nanowires with diameters ranging from 30-130 nm, individually contacted by aluminum-based superconductor electrodes. Below 1 Kelvin, the high transparency of the contacts enables proximity-induced superconductivity. A supercurrent flows though the nanowire that can be tuned by a global gate acting on the electron density. Furthermore, we have used top-gates in order to make tunable barriers in InAs nanowires. By creating two closely spaced barriers in an InAs nanowire, we can define a Quantum Dot (QD) with tunable coupling. This enables us to study a tunable QD connected to superconducting leads. In these devices the supercurrent can be controlled by only a small change in gate voltage. If the QD is tuned at a charge degeneracy point, a supercurrent can flow through the QD. However, away from charge degeneracy the supercurrent is blocked due to coulomb blockade. [1] Y.-J. Doh, J.A. van Dam, et al., Science 309, 272 (2005) [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D31.00012: Bandstructure Effects in Unstructured AlGaAs Nanowires Neerav Kharche, Clemens Heitzinger, Gerhard Klimeck, Mathieu Luisier, Timothy Boykin Atomic scale alloy disorders can have profound effects on the bandstructure and charge transport through nanowires. With decreasing diameters down to nanometer scales, alloy disorder can no longer be treated in an average manner using the virtual crystal approximation (VCA). Our zone unfolding algorithm along with supercell calculations can be used to treat alloy disorders even to atomic scales. We calculate bandstructures and transmission coefficients of AlGaAs random alloy nanowires with diameters ranging from 2nm to 6nm. Using the nanoelectronic modeling tool (NEMO3D), eigenstates of the alloy nanowire supercell are calculated with the 20-band sp3d5s* spin model. Small cell bandstructures are then projected out of the supercell eigenstates. Transmission coefficients are calculated with an atomistic NEGF simulation of the alloy wire embedded between two ideal reservoirs. These projected bandstructures and transmission coefficients both show reduced bandgaps and noisy behavior and provide significant insight into the physics of charge transport. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D31.00013: Ground states of a novel AlAs quantum wire Trinanjan Datta, Erica W. Carlson Using abelian bosonization and renormalization group, we analyze the ground states of a recently fabricated novel AlAs quantum wire [Moser et al, Appl. Phys. Lett. (2005)]. There are two degenerate lowest energy bands, separated by half an umklapp vector. This bandstructure arrangement leads to four Fermi points and umklapp-induced pair processes, independent of filling. The ground states are different from those of a conventional quantum wire, where acquiring four Fermi points requires occupying two successive bands, leading to a CDW instability via density reorganization. Such an instability is forbidden in the AlAs wire, which may help stabilize other exotic gapped ground states. [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D31.00014: Electrical Detection of Oscillations in Micro- and Nano-Cantilevers Gayatri Keskar, Jay Gaillard, Rasvan Ciocan, Malcolm Skove, Apparao Rao Electrical Detection of Oscillations in Micro- and Nano- Cantilevers We described a fully electrical actuation and detection method for measuring mechanical oscillations in ambient conditions for two systems: Si-based micro- and multi-walled carbon nanotube (MWNT) based nano-cantilevers. In our studies, a cantilever is either a silicon microstructure shaped like a diving board, or a cantilevered MWNT. The cantilever is placed parallel to and within 1-10 $\mu$m from a counter electrode and is forced into resonance by applying an ac voltage (V$_{ac}$) with a dc offset (V$_{dc}$) on the counter electrode under ambient conditions. We measure the magnitude and phase of the electrical signal due to the charge induced on the cantilever. The signal showing the resonance frequency of the cantilever is best measured at harmonics of V$_{ac}$. A model describing the response of our electrical actuation and detection of resonance oscillations will be presented. [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D31.00015: Position-dependent deflection (PPD) of a nanobeam: a new method to determine the Young's modulus of nanoscale matter N. Duarte, Qihua Xiong, Tadigadapa Srinivas, Peter Eklund In this approach, a nanowire beam is fixed at two ends and an AFM is used to apply a force F(x) where x locates the position along the beam and the beam deflection $\delta $z(x) is measured simultaneously. This situation is realized by placing a nanobeam over a trench fabricated on a Silicon substrate via photolithography followed by metal evaporation, lift-off and XeF$_{2}$ etching. The AFM tip force-distance curve is first obtained from experiments on the rigid substrate. The slope of the AFM force-distance obtained when the tip contacts the beam is then measured and the Young's modulus Y is obtained from the change in slope using the Euler-Bernoulli (E-B) equation. The beam dimensions are also required: the beam height and length via AFM and the beam width and length by SEM. We believe this method can be used in any other nano-beam systems to measure the Young's modulus. Results for rectangular ZnS beams ($\sim $100nm x 100 nm x 5$\mu $m long) will be presented that demonstrate the potential for this method. Values for Y lower than reported for the bulk are obtained (i.e. Y(nano) $\sim $ 70{\%} Y(bulk)). This work was supported, in part, by NSF-NIRT DMR-0304178 [Preview Abstract] |
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