Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Nanostructures |
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Sponsoring Units: DCOMP Chair: David Ferry, Arizona State University Room: D167 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D24.00001: Transient response of a quantum point contact due to the coupling with reservoirs Bozidar Novakovic, Irena Knezevic Transient response is important for better understanding of the DC and AC response of open quantum systems connected to large charge reservoirs. In this study we calculate the transient response of a quantum point contact (QPC) due to its coupling with reservoirs. The QPC, an open system, is modeled by a solution to the coupled, two-dimensional Schr\"{o}dinger and Poisson equations using a discrete subset of the normal modes basis. The normal modes are projected onto the traveling-wave solutions that match the incoming reservoir plane waves. The occupation of the open system states carries the information about the time evolution and is calculated by solving a coarse- grained quantum master equation with suitably defined open system/contact interaction Hamiltonians. The final electronic transient response is obtained by enforcing the current continuity across the open system/contacts boundaries through a time-dependent reservoir drift wavevector. We investigate the transient current response to a voltage step and its dependence on the gate bias and relaxation time in the contacts. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D24.00002: Open boundary-conditions using empirical pseudopotentials in quantum transport Bo Fu, Massimo Fischetti As device dimensions approach the 10 nm length-scale, the study of electronic transport requires the knowledge of an accurate band structure and of transport equations transcending the semiclassical Boltzmann picture. Having as our ultimate goal the study of dissipative quantum transport using the Pauli Master Equation, in this talk we address the numerical issue of how to formulate and implement numerically the open-boundary-condition Schr\"odinger equation within an empirical-pseudopotential full- band framework. Results regarding ballistic transport in Si nanowires will be presented. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D24.00003: Quantum Transport in Crossbar Devices Brandon Cook, Peter Dignard, Kalman Varga Electronic devices with crossbar geometries have recently been fabricated with nanoscale features (Zhong. et al, Science Vol. 302). Consisting of a two dimensional grid of wires, devices have been formed with a variety of components including carbon nanotubes and semiconductor nanowires. These devices are assumed to operate classically, but as the dimensions of the device shrink consideration of quantum effects becomes necessary. We consider a single junction between two wires up to a four by four grid of wires. Through a series of calculations with atomistic first-principles, tight-binding and analytic models of multi-terminal devices we demonstrate the presence of unique behavior, such as interference effects, not present in classical models. It is expected that exploitation of these effects will useful in the creation of circuit components. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D24.00004: Empirical Pseudopotential Approach to Semiclassical and Quantum Electronic Transport in Nanometer-scale Structures Invited Speaker: The study of electronic transport in semiconductor structures requires an accurate knowledge of the kinematics ($i.e$., band structure) and of the dynamics ($i.e$., transport equations and collision processes). As the VLSI technology looks at various sub-10 nm structures as alternatives to the traditional Si CMOS, neither the conventional bulk band structure of the semiconductor nor the semiclassical (Boltzmann) transport equations can be used with confidence to treat the kinematics and dynamics of electronic transport, the former because of size-dependent (quantum confinement) and interface/surface dependent band structure effects; the latter because of the possibility of quantum interference effects at this length scale. Here we will show that empirical pseudopotentials -- obtained from the literature and adjusted to yield correct workfunctions, band-alignment, and strain effects -- can be used to obtain a sufficiently accurate (as compared to first-principle results) band structure of several systems of technological interest ($e.g$., thin Si and Ge layers, III-V hetero-layers, nanowires, graphene nanoribbons and C nanotubes). Using this information, semiclassical transport is studied using a Monte Carlo technique and calculating the scattering rates consistently with the band structure information. In some cases, such as in considering scattering with interface and line-edge roughness, the pseudopotential themselves can be used to obtain accurate scattering potentials. The case of high-field transport in Si inversion layers is discussed, showing how the band-structure near the X symmetry point induces a lower saturated electron velocity. Finally, we discuss the wave equation and open boundary conditions which must be employed to handle ballistic quantum transport accounting for the full band structure. Dissipative transport is discussed in the context of a Master equation approach, illustrating this approach with examples ranging from double-gate FETs to Si nanowires. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D24.00005: Self-Consistent Monte Carlo Study of the Coulomb Interaction under Nano-Scale Device Structures Nobuyuki Sano It has been pointed that the Coulomb interaction between the electrons is expected to be of crucial importance to predict reliable device characteristics. In particular, the device performance is greatly degraded due to the plasmon excitation represented by dynamical potential fluctuations in high-doped source and drain regions by the channel electrons. We employ the self-consistent 3D Monte Carlo (MC) simulations, which could reproduce both the correct mobility under various electron concentrations and the collective plasma waves, to study the physical impact of dynamical potential fluctuations on device performance under the Double-gate MOSFETs. The average force experienced by an electron due to the Coulomb interaction inside the device is evaluated by performing the self-consistent MC simulations and the fixed-potential MC simulations without the Coulomb interaction. Also, the band-tailing associated with the local potential fluctuations in high-doped source region is quantitatively evaluated and it is found that the band-tailing becomes strongly dependent of position in real space even inside the uniform source region. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D24.00006: Oxygen vacancy mediated dielectric breakdown in ultrathin high-k gate dielectric stacks Blanka Magyari-Kope, Yoshio Nishi The reliability of the high-k gate stack becomes a significant challenge with the continuous scaling of the metal-oxide-semiconductor-field-effect-transistors, due to deposition techniques of ultrathin oxides and defects in the gate stack. One of the key problems associated with ultrathin oxide layers is the degradation of the gate oxides under electrical stress, due to traps generated by oxygen vacancies present in these materials. First principles methods based on density functional theory combined with non-equilibrium Green's function calculations are employed to calculate the tunneling current through ultrathin oxide layers of HfO$_{2}$ and SiO$_{2}$ in a gate stack structure with TiN metal electrode. Model systems that incorporate the atomistic description of a conductive filament formation due to ordering of oxygen vacancies in the oxide layers and the oxide-oxide interface of the gate stack were investigated. The microscopic effects of defects ordering on the electronic transport through the gate oxides are analyzed and discussed. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D24.00007: Electron-phonon coupling in semiconductors and their nanostructures: effect on transport properties Jelena Sjakste, Paola Gava, Nathalie Vast, Valeriy Tyuterev Parameter-free description of the electron-phonon coupling is crucial for the simulation of the electron and thermal transport in materials, especially nanostructured ones. Recently, we have developed an \emph{ab initio} approach which allows to calculate the electron-phonon constants and scattering times for collisions of carriers in the conduction band with short-wavelength phonons [1,2]. We will present our results on the electron-short-wavelength phonon interaction in silicon, which enables us, on one hand, to shed new light on the transitions between shallow donor levels in doped Si [2], and, on the other hand, to improve the description of its electronic mobility [3]. Finally, we will discuss the effect of the material nanostructuring on the electron-phonon coupling constants, e.g. in semiconducting superlattices.\\[4pt] [1] J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99, 236405 (2007).\\[0pt] [2] V. Tyuterev, J. Sjakste, N. Vast, Phys. Rev. B 81, 245212 (2010)\\[0pt] [3] Z. Wang, S. Wang, S. Obukhov, N. Vast, J. Sjakste, V. Tyuterev, N. Mingo, submitted (2010) [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D24.00008: Ballistic Spin Field Effect Transistor Based on Silicon Nanowires Dmitri Osintsev, Viktor Sverdlov, Zlatan Stanojevic, Siegfried Selberherr We investigate the properties of ballistic spin field-effect transistors build on silicon nanowires. An accurate description of the conduction band based on the {\bf k$\cdot$p} model is necessary in thin and narrow silicon nanostructures. The subband effective mass and subband splitting dependence on the nanowire dimensions is analyzed and used in the transport calculations. The spin transistor is formed by sandwiching the nanowire between two ferromagnetic metallic contacts. Delta-function barriers at the interfaces between the contacts and the silicon channel are introduced. The major contribution to the electric field-dependent spin-orbit interaction in confined silicon systems is due to the interface-induced inversion asymmetry which is of the Dresselhaus type [1]. We study the current and conductance through the system for the contacts being in parallel and anti-parallel configurations. Differences between the [100] and [110] orientated structures are investigated in details.\\[4pt] [1] M.O. Nestoklon {\it et al., Phys.Rev.B} {\bf 77}, 155328 (2008); M. Prada {\it et al.}, cond-mat 0908.2417. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D24.00009: Junctionless nanowire field-effect transistor versus inversion mode devices Bart Soree, Anh-Tuan Pham, Dries Sels, An De Keersgieter, Wim Magnus Several years ago, a novel device concept was proposed : the nanowire (NW) iJFET [1]. Today, this device concept is being explored by several research teams [1-3] and is also known as the pinch-off FET (POFET) or junctionless transistor. The most important advantage of the junctionless transistor is the uniform doping throughout source, channel and drain which greatly simplifies its fabrication. We have performed modeling and simulations to compare the performance of the junctionless pinch-off FET with that of inversion mode devices. In order to make the comparison, we address the regime of thick and long nanowires through analytical modeling of the current- voltage characteristics, while for long and thin nanowires we perform dissipative transport modelling to obtain the low-field mobility. Finally, ballistic transport modelling is performed using the sub band decomposition method for ultra- short nanowires. \\[4pt] [1] B. Sor\'ee, et al., Journal of Computational Electronics, vol.7, issue 3, 380-383 2008. \\[0pt] [2] B. Sor\'ee, et al., Nanoelectronics days 2010, Aachen, Germany. \\[0pt] [3] J.-P. Colinge, et al., Nature Nanotechnology 5, 225-229, 2010 [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D24.00010: Gap suppression and delocalization of 1D electron gas driven by a strong AC electric field Kathleen E. Hamilton, Leonid P. Pryadko We will argue that a strong AC electric field can coherently suppress a band gap in a high-mobility one-dimensional wire. At half-filling, the expected effect is delocalization of the carriers, in contrast to Stark localization at low frequencies, or Dynamical Destruction of Tunneling at frequencies exceeding the bandwidth. Another effect of the gap suppression is the doubling of the Bloch oscillations' period. We support these expectations with numerical simulations of the non-linear current response for several model systems driven by a combined high and low-frequency electric field. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D24.00011: Modeling of Phonon-assisted Zener Tunneling in Indirect Semiconductors William Vandenberghe, Bart Sor\'ee, Massimo Fischetti, Wim Magnus, Guido Groeseneken With the scaling in the semiconductor device dimensions, Zener tunneling has become an important source of leakage in conventional MOSFET devices but it could also provide drive current for a novel type of tunnel transistor. A good understanding of the process of Zener tunneling is therefore required and present-day one-dimensional semi-classical models fall short of explaining tunneling in devices with potential profiles with a pronounced two-dimensional shape. We have developed a formalism to calculate the phonon-assisted current under a given three dimensional external potential profile. The current is calculated from the transition probability for an electron to go from the valence to the conduction band. The transition probability is determined from the spectral functions corresponding to the valence and the conduction band. In the presence of a one-dimensional uniform low electric field, the Kane model is recovered. An example of the formalism is given for the case of an abrupt p-n diode and compared with existing semi-classical models. It is seen that the uniform field model is actually better than the WKB model but that none of the semi-classical models give good results at low bias conditions. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D24.00012: Zener Tunneling: Correspondence between Quantum and Semi-Classical Formalisms Kartik Ganapathi, Sayeef Salahuddin The resurgence of interest in band-to-band tunneling has been due to its usefulness in overcoming the 60 mV/decade limit in turn-on characteristics of a MOSFET thereby providing path for lowering the operating power. The expression due to \textit{Kane}, for calculating transmission coefficient and current due to Zener tunneling in a $p-n$ diode, has been extensively used over the years for explaining experimental tunneling characteristics. While this closed form expression relates tunneling probability with simple quantities like bandgap, effective mass, electric field etc., being a formula based on semi-classical approximation, it is valid strictly in the low-field regime. With finite size effects playing significant role in ultra small device dimensions, this approximation breaks down and one needs to have a full quantum mechanical treatment of the tunneling problem with a realistic band-structure. We report a numerical simulation of this problem within the NEGF formalism with a tight-binding Hamiltonian wherein the extent of validity of Kane's formula is examined. We also discuss how Kane's parameters should be altered and interpreted in high field region. The results are compared with experimental data in two different systems -- InAs $p^{+}-n^{+}$ and GaN/AlGaN heterojunction tunnel diodes. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D24.00013: Simulation of nanoscale four-probe resistance measurements under finite bias voltages Asako Terasawa, Keiji Tobimatsu, Tomofumi Tada, Takahiro Yamamoto, Satoshi Watanabe To understand the interesting features in nanoscale four-probe measurements such as the negative value and the oscillation of four-probe resistance [1], we investigate the behavior of nanoscale four-probe resistance theoretically [2-3]. In the present work, we examine the effect of bias voltage on four-probe resistance in nanoscale four-probe systems. For a set bias voltage between current probes, we first estimate the voltage between voltage probes when no current flows between them from the four-probe and two-probe resistances at the zero-bias limit, assuming the linear response. Then we calculate the dependence of currents in the voltage probes on the bias voltage applied to the current probes with applying the voltage thus estimated between the voltage probes. The calculated currents in the voltage probes have nonzero but much smaller values compared with those in current probes, and show the non-linear dependence on the bias voltage. This result indicates assumption of linear response is not valid for the bias voltage of the order of a tenth V, and that currents and voltages should be determined self-consistently to estimate four-probe resistance. [1] B. Gao et al., Phys. Rev. Lett. 95, 196802 (2005). [2] A. Terasawa et al., Phys. Rev. B 79, 195436 (2009). [3] A. Terasawa et. al., New J. Phys. 12, 083017 (2010). [Preview Abstract] |
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