Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y9: Transport in Semiconductors II |
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Sponsoring Units: DCMP Chair: Sudarshan Karki, University of Missouri Kansas City Room: A105 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y9.00001: Fundamental Studies of p-type Boron Carbide to n-type Silicon PN Junctions Sudarshan Karki, Joseph Sandstrom, Chad Clayton, Saad Janjua, M. Sky Driver, K. I. Pokhodnya, Anthony Caruso Boron carbide -to silicon PN junction diodes are of present interest to radiation detection, yet the diode properties are far from optimized because of a lack of processing correlation to the electronic and transport properties. This talk will discuss and contrast the p-n junction transport studies within the context of the diode processing conditions for both B$_{4}$C and a-B$_{5}$C:H$_{x}$ forms of boron carbide. Of particular interest, we have found a correlation between the leakage current, turn on voltage and depletion region width as a function of elemental and defect impurity concentrations. Post growth thermal treatment helps to separate out the defect from elemental sites that could be causing deleterious transport and will be discussed as a function of both growth and annealing temperatures. The depletion region width on the boron carbide side was also determined by measuring the turn on voltage as a function of film thickness. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y9.00002: Transport Properties of Low Temperature Deposited Boron Carbide Chad Clayton, M. Sky Driver, Saad Janjua, Sudarshan Karki, Joseph Sandstrom, Anthony Caruso Amorphous B$_{x}$C:H$_{y}$ (x$\sim $4.5, y$\sim $2) is an important radiation hard semiconductor with little known about its electrical transport character. To move forward in using these materials systems, systematic carrier mobility and lifetimes need to be determined. This talk will present Hall transport measurements of thin film B$_{4}$C and B$_{5}$C:H$_{x,}$ as a function of various pre-growth and post-growth sample preparation conditions. Specifically, we have found that as deposited B$_{5}$C:H$_{y}$ films yield hole and electron concentrations near 1E$^{18}$/cm$^{3}$ resulting in a low mobility and ill defined majority carrier while the B$_{4}$C films are dominant in their hole majority carrier at 1E$^{17}$/cm$^{3}$. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y9.00003: Influence of surface and interface properties on the electrical conductivity of silicon nanomembranes X-F. Zhao, S. Scott, M. Huang, W. Peng, D. Savage, M. Eriksson, M. Lagally We investigate the electronic transport properties of silicon nanomembranes (SiNMs) on oxide by the van der Pauw method. SiNMs, thin sheets of single-crystal Si, feature an extreme sensitivity of electronic transport properties to surface and interface condition, because of the large surface-to-volume ratio. Removing the top oxide with HF reduces the sheet resistance four orders of magnitude for the thinnest NMs ($\sim $20nm) [1], a value much greater than can be accounted for by simply reducing interface traps. We compare SiNMs prepared with HF to H terminations prepared via CVD, to identify which factors control this change in conductivity. We also perform a forming gas (5{\%} H$_{2}$ in N$_{2})$ anneal on oxidized NMs. The sheet resistance drops relative to unannealed NMs, demonstrating the influence of states at Si/SiO$_{2}$ interfaces. A qualitative model that includes these several factors influencing the sheet resistance is described. \\[4pt] [1] Scott S. \textit{et al}, ACS Nano 3 (2009) 1683. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y9.00004: Transport and electronic properties of a polar semiconductor junction M.-H. Tsai, T.-H. Lu Besides molecular junctions, polar semiconductor
junctions may also be suitable candidates for nm-scale
electronic active devices. Here, the current density-voltage
(J-V) characteristics and partial densities of states (PDOS)
have been calculated for an Au/AlN/Au junction with a
two-bilayer thick AlN[0001] layer. Results show that when
$V_{bias}$, which is defined as the electric potential at the
Al-side electrode relative to that at the N-side electrode,
is positive, the J-V curve is approximately ohmic up to
$V_{bias}\approx 0.4V$. Then J drops suddenly to a small value.
Beyond $V_{bias}\approx 0.4V$, J exhibits approximately three
steps at J$\approx$3, 20, 39 nA/$a_{0}^{2}$ in $0.4V |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y9.00005: Phonon Transport in Semiconductor interface: An atomistic approach Yann Chalopin Thermal dissipation and thermal insulation are among crucial issues encountered in high speed electronics devices and thermoelectrics systems. Both applications rely on controlling the transport properties of the heat carriers at semiconductor interfaces. From microscopic perspective, it is of fundamental interest to understand how the transport of phonons is impacted by an interface formed by two semiconductor layers. In a typical junction, the mechanism of reflection/transmission of vibrational energy causes strong modifications in the conductance regime. Thus, it is important to address the thermal transport at a contact junction in the framework of phonon wave propagation. Our approach is based on the fluctuation/dissipation theorem in order to calculate the thermal conductance of an interface. Using molecular dynamics simulations, we address the problems associated to using Si/Ge Si/SiGe junctions. We propose a methodology that enables the recovery of the transmission of the phonon modes by correlating the atomic motions of the phonon modes. Furthermore, we conclude that the phonon transmission function can be reconstructed such that it is integrated in the spectral expression of the conductance. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y9.00006: Coulomb gap variable range hopping in graphitized polymer surfaces Yuri Koval, Irina Lazareva, Paul M\"{u}ller Polymer surfaces were graphitized by low-energy ion irradiation. We show that the conductance of the graphitized surfaces gradually increases with the energy of ions and the temperature of irradiation. At rather modest ion energies ($\sim $1000) eV and irradiation temperatures ($\sim $400$^{\circ}$C) the transition to a metallic state was observed. We investigated electric transport on the insulating side of metal-insulator transition (MIT). Temperature dependences of conductance and current-voltage characteristics at low temperatures were measured and analyzed. We found that electric transport in the graphitized surfaces can be described by 2D Coulomb gap variable range hopping. Similar to low-temperature results in crystalline 2D systems, the pre- factor is temperature independent and has a unique value ${e^2} \mathord{\left/ {\vphantom {{e^2} h}} \right. \kern-\nulldelimiterspace} h$. In the activationless regime of hopping, the pre-factor of current-voltage characteristics has a significantly smaller but also unique value $\sim {e^2} \mathord{\left/ {\vphantom {{e^2} {5h}}} \right. \kern-\nulldelimiterspace} {5h}$. We show that the localization length is constant for all samples. The dielectric constant gradually increases approaching MIT from the insulating side. Due to an extremely high bare density of states, the Coulomb gap persists up to high temperatures. We explain this by a strong inhomogeneity of density of states in the graphitized surfaces. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y9.00007: Effect of Bi incorporation on the carrier mobility in the dilute bismide alloy, GaAsBi Rajeev Kini, Aaron Ptak, Ryan France, Angelo Mascarenhas Results from our study of carrier mobilities in doped GaAs1-xBix epilayers will be presented and compared with the dilute nitride alloy, GaAsN. We observed no significant degradation in the electron mobility with Bi incorporation in GaAs, up to a concentration of 1.2{\%}. At higher Bi concentration ($\ge $1.6{\%}) some degradation of the electron mobility was observed, although there is no apparent trend. Temperature dependant Hall measurements of the electron mobility suggest neutral impurity scattering to be the dominant scattering mechanism. We will also present data on the effect of Bi alloying on the hole mobility. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y9.00008: Hybridized Electron-Hole States in Inverted InAs/GaSb Composite Quantum Wells Ivan Knez, R.R. Du, J. Kono, G. Sullivan, S. Sasa, M. Inoue Recently, it has been proposed that inverted InAs/GaSb composite quantum wells (CQWs) will exhibit the quantum spin Hall effect, characterized with a bulk gap and an odd number of gapless Kramers pair states at the edges. We have successfully fabricated a double-gated device on high-quality MBE-grown InAs/GaAs CQWs in the inverted regime, in which we were able to fine-tune the band structures via an electrical field. At $T =$ 20 mK, and zero front and back gate bias, only electrons are present in the CQWs, with a typical density of 7x10$^{11}$cm$^{-2}$ and mobility of 90,000 cm$^{2}$/Vs. We observed clear evidence for an energy gap in the inverted regime, with values of the gap consistent with those theoretically predicted. In addition, our preliminary data analysis suggests that bulk conductivity does not vanish even at $T =$ 20 mK for the CQWs studied. We propose a semi-classical model of hybridized electron-hole states to explain our data. Ref. C. Liu et al. Phys. Rev. Lett. 100, 236601 (2008), Y. Naveh and B. Laikthman, Euro. Phys. Lett. 55, 545 (2001). [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y9.00009: Thermopower of parallel conducting structures N. Miller, J.W. Ager III, H.M. Smith III, K.M. Yu, E.E. Haller, W. Walukiewicz, W.J. Schaff, C. Gallinat, G. Koblm{\"u}ller, J. Speck Thermopower measurements can be a powerful tool for characterizing structures with parallel conducting layers such as intentional heterostructures or p-n junctions (such as solar cells) or materials with unintentional surface accumulation or inversion layers (such as InN, InAs, In$_{2}$O$_{3}$, and CdO). In such cases, the macroscopically observed Seebeck coefficient depends on the contributions of various layers and the nature of the internal junctions between them. Thermopower measurements of such structures in group IV and III-V materials are presented and analyzed in terms of a parallel conduction model in which the observed Seebeck coefficient depends on the conductance-weighted Seebeck coefficients of each layer. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y9.00010: A Modified Monte Carlo Method for Carrier Transport in Germanium, Free of Isotropic Rates Kyle Sundqvist We present a new method for carrier transport simulation, relevant for high-purity germanium $<$ 100 $>$ at a temperature of 40 mK. In this system, the scattering of electrons and holes is dominated by spontaneous phonon emission. Free carriers are always out of equilibrium with the lattice. We must also properly account for directional effects due to band structure, but there are many cautions in the literature about treating germanium in particular. These objections arise because the germanium electron system is anisotropic to an extreme degree, while standard Monte Carlo algorithms maintain a reliance on isotropic, integrated rates. We re-examine Fermi's Golden Rule to produce a Monte Carlo method free of isotropic rates. Traditional Monte Carlo codes implement particle scattering based on an isotropically averaged rate, followed by a separate selection of the particle's final state via a momentum-dependent probability. In our method, the kernel of Fermi's Golden Rule produces analytical, bivariate rates which allow for the simultaneous choice of scatter and final state selection. Energy and momentum are automatically conserved. We compare our results to experimental data. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y9.00011: Breakdown of the Kane model for Zener tunneling William Vandenberghe, Bart Sor\'ee, Wim Magnus, Guido Groeseneken With the introduction of ever smaller dimensions in modern day semiconductor devices, Zener tunneling can no longer be neglected and is affecting device performance. On one hand Zener tunneling is responsible for a leakage current in classical devices such as the MOSFET, on the other hand it provides the drive current for some new devices under investigation such as the tunnel field-effect transistor (TFET). A popular model to calculate the Zener tunneling probability is the Kane model but this model is only valid for Zener tunneling in weak uniform fields. The Kane model can be extended to the case of a non-uniform field using a WKB approximation. But the proper way to calculate the tunneling probability is to derive it directly from the transmission probability of an incoming electron. Using a two-band model we compare the results of a direct calculation of the transmission probability with that calculated using the WKB approximation or the Kane model. We conclude that the Kane model breaks down in the case of high fields and low junction bias. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y9.00012: SymGF: A Symbolic Green's Function Derivation Method Zimin Feng, Qingfeng Sun, Hong Guo Keldysh non-equilibrium Green's function (NEGF) formalism is widely used in quantum transport theory. For transport problems involving interacting electrons, it can be a tedious and error prone process to derive the transport formula and various NEGF. We have developed a symbolic method called SymGF, based on Mathematica, for computer aided derivation tasks of NEGF in quantum transport theory. By inputting second quantized Hamiltonian of the transport model, the commutation and truncation rules for the operators and correlators, SymGF automatically derives various NEGF that enters transport formula such as conductance and current-voltage characteristics. SymGF is useful to deal with transport structures having multiple spin-resolved quantum dots with on-site Coulomb repulsion, connected to external non-interacting leads. It achieves overwhelming efficiency compared with manual derivation. We will present several examples of SymGF. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y9.00013: Molecular dynamics simulation of transport and separation of aqueous solutions in model nanopores Craig Martens, Jacob Goldsmith, Ramsay Brown We present non-equilibrium molecular dynamics simulations of transport of water and aqueous solutions through model nanopores induced by a pressure gradient. We treat simple model systems consisting of pores through rigid ice-like membranes and pores based on carbon nanotubes with surface charge patterns. The TIP3P water model is employed in the simulations presented. The flux of water and ions through the pores are determined as a function of pore size, surface characteristics, and pressure difference across the membrane. We find that flow rates are significantly higher for the hydrophobic systems for a given pore diameter and pressure gradient, in agreement with recent experiment. For aqueous salt solutions, the phenomenon of ion rejection is quantified as a function of pore size and the nature of the surface. It is found that bipolar charged pores, where positive and negative surface charge regions meet at a sharp boundary, can reject ions virtually completely for pore sizes that show no rejection based on size alone. The ultimate goal of this research is to develop rational design principles at the molecular scale for nanoscale separation and purification. [Preview Abstract] |
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