Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y37: Thermodynamic and Transport Properties of Semiconductors |
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Sponsoring Units: FIAP Chair: Terry Alford, Arizona State University Room: Morial Convention Center 229 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y37.00001: Impact ionization rate calculation based on $GW$ approximation Takao Kotani, Mark van Schilfgaarde Impact ionization (IMI) means the electron-hole pair production due to high energy electron (or hole). It can control the performance of devices related to the high-field transport process. For solar cell, it was proposed to use IMI for efficient energy conversion. The IMI rate can be identified as the lifetime of an electron (imaginary part of the self-energy). Since the electron-hole pair spectrum is included in the screened Coulomb interaction $W$, it should be important to use full $W$ for the calculation of the lifetime. However, to our knowledge, no such calculation has been presented for semiconductors until now. For example, Ref.[1] uses a model $W$. In addition, the conventional formalism of Ref.[1] contains some double-counting problem. We will show our calculation for the IMI rate based on our recently-developed quasi-particle self-consistent $GW$ method for semiconductors. Our calculation predicts a rather smaller IMI rate. [1] A. Kuligk, N. Fitzer, and R. Redmer, PRB71, 085201(2005) [2] T.Kotani, M. van Schilfgaarde, and S.V. Faleev, PRB76, 165106 (2007) [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y37.00002: First-principles calculations of mobilities in ultra-thin double-gate MOSFETs Oscar D. Restrepo, Kalman Varga, Blair Tuttle, Sokrates T. Pantelides Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO$_{2}$. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005). [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y37.00003: Composition Dependence of the Hole Mobility in Dilute GaSb$_{x}$As$_{1-x}$ Kirstin Alberi, O.D. Dubon, K.M. Yu, W. Walukiewicz, J.A. Gupta, J.-M. Baribeau Highly p-type doped GaSb$_{x}$As$_{1-x}$, long considered a promising component for III-V-based double heterojunction bipolar transistors, exhibits an unusually abrupt reduction in the hole mobility in the dilute Sb alloy composition range (x $<$ 0.2), which cannot be completely explained by conventional carrier scattering models. We show that this behavior is due to the reconfiguration of the alloy's valence band structure through an anticrossing interaction between the localized and extended $p$-states of the Sb impurities and GaAs host as described by a valence band anticrossing model [1]. Our model suggests that the drop is due to the significant upward movement of the valence band edge as well as an increase in the heavy hole effective mass that enhance the scattering processes in this composition range. K. Alberi, \textit{et al.}, \textit{Phys. Rev. B}, \textbf{75}, 045203 (2007). [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y37.00004: Enhanced Hall coefficient in InAs/AlSb $\mu $-Hall bars induced by ballistic electron scattering from interfacial impurities Goran Mihajlovic, John E. Pearson, Axel Hoffmann, Samuel D. Bader, Mark Field, Gerard J. Sullivan We fabricated micrometer-sized Hall bar channels of variable width $w$ from an InAs/AlSb quantum-well, two-dimensional electron system and studied their electrical response in perpendicular magnetic fields. For the narrowest channels ($w \quad \sim $ 1 $\mu $m) at low fields ($<$0.5 T) and 5 K, we observed that the Hall coefficient increases above its classical value. This increase persists up to temperatures of order 100 K, but its magnitude decreases with increasing channel width and disappears for $w \quad \sim $ 4 $\mu $m. At the same time, the longitudinal resistance decreases with increasing magnetic field. The strong negative magnetoresistance is present even for the widest channels, suggesting that boundary scattering is only partially responsible for its observation. We show that both results can be explained by a mechanism of large-angle scattering of ballistic electrons from non-ionized impurities residing at the InAs/AlSb interfaces. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y37.00005: Superradiance And Electron Transport Through Nanosystems Luca Celardo, Lev Kaplan Electron transport through a sequence of potential wells is investigated. Using the effective non hermitian Hamiltonian approach to open systems, the transition to a superradiance regime is shown to occur. The consequences of the superradiance transition on the conductance, including negative differential conductance, are investigated. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y37.00006: Thermal conductivity of amorphous silicon films Joseph Feldman, Xiao Liu, R. Crandall, N. Bernstein, M. Mehl, D. Papaconstantopoulos We measured the thermal conductivity of an 80 $\mu $m thick amorphous silicon film from 80K to room temperature. The amorphous silicon sample was prepared by hot-wire chemical-vapor deposition with 1 at. {\%} hydrogen, which was found previously to contain almost no atomic tunneling states that is common in amorphous solids. The value of the thermal conductivity is about a factor of two larger than previous results. To explain this unusually large thermal conductivity, we report on a Kubo theory that makes use of a tight binding electronic structure of a 1000 atom model. We include the low frequency modes that the Kubo model does not take into account because of its finite size. By considering Rayleigh and boundary scattering, and scattering of tunneling states, our theory can explain not only our result but also previous ones as well. We conclude that the large thermal conductivity of our film is attributed to the lack of scattering of the low frequency modes by the tunneling states. Therefore, low frequency modes can make significant contribution to heat transport even at near room temperature. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y37.00007: Thermoelectric properties of nanoporous Si Joo-Hyoung Lee, Jeffrey C. Grossman, John Reed, Giulia Galli Improvements in thermoelectric (TE) materials could lead to efficient solid-state energy-conversion for environmentally benign power generation and refrigeration. This realization would require a large increase to $\sim 3$ in the thermoelectric figure of merit $ZT$ at room temperature. Recent experiments have shown promise for practical applications of TE materials such as Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices and PbSeTe/PbTe quantum dot superlattices, yielding $ZT$ of $2.4$ and $1.3-1.6$, respectively. In addition, there have been recent attempts to use Si for TE pplications due to its structural simplicity and the possibility of utilizing existing Si-based manufacturing processes. In the present work, we report theoretical studies on thermoelectric properties of Si with periodically arranged nanometer-sized pores ({\it nanoporous Si}). Specifically, we calculate the electrical conductivity, Seebeck coefficient and figure of merit of nanoporous Si for a range of configurations using a combined {\it ab inito} electronic structure calculation and Boltzmann transport approach at room temperature. The results show a substantial increase in $ZT$ compared with that of bulk Si, similar to recent findings for $ZT$ in Si nanowires. Approaches for increasing $ZT$ further in this porous material will also be discussed. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y37.00008: ac conductivity in low dimensional structures: acoustic study A. Suslov, I. Drichko, I. Smirnov, A. Dyakonov, Yu. Galperin, V. Vinokur Surface acoustic waves (SAWs) were used for contactless measurements of ac conductivity in low dimensional structures. The value of complex ac conductivity was extracted from simultaneous measurements of the sound attenuation and velocity. The measurements were performed in the frequency range 17-300MHz, at temperatures down to 0.3K and in the magnetic fields up to 18T. Such measurements allowed to study, for example, mechanisms of conductivity in a dense array of SiGe quantum dots and localization of the 2D carries in GaAs/AlGaAs and Si/SiGe heterostructures in the ultraquantum limit. An extended list of coauthors will be presented during the presentation. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y37.00009: Effects of capture, escape and confinement on SAW-dragged photocurrents in a single QW Godfrey Gumbs, Danhong Huang, Michael Pepper A dual-plasma model is developed for studying the steady-state transport of SAW-dragged photocurrents of 1D confined-state carriers. This model includes the effects of the quantum confinement and the tunneling escape of SAW-dragged 1D carriers, as well as the effects of the inelastic capture of 2D continuous-state carriers and the space-charge field. The numerical results uncover a high photocurrent gain due to suppressed recombination of 1D carriers in a crossover region of the sample between an absorption strip and a surface gate. Based on a discrete model, responsivities for the SAW-dragged photocurrents in a quantum well are calculated as functions of the gate voltage, photon flux, SAW power and frequency and temperature, respectively. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y37.00010: Evidence of Real-Space Transfer in Buried-Channel Ge$_{x}$C$_{1-x}$ Devices En-Shao Liu, David Kelly, Joseph Donnelly, Emanuel Tutuc, Sanjay Banerjee We present experimental evidence of real-space transfer (RST) in buried-channel Ge$_{x}$C$_{1-x}$ p-type metal-oxide-semiconductor field effect transistors (MOSFET) containing a Si cap layer. The output characteristics of these devices reveal a negative differential resistance (NDR) below 150K, at the onset of the saturation regime. This observation indicates a charge transfer from Ge$_{x}$C$_{1-x}$ layer into the Si cap at sufficiently large drain bias values. The lower hole mobility of the Si cap with respect to the Ge$_{x}$C$_{1-x}$, translates into a drain current reduction, hence the observed NDR. Our low-field, temperature-dependent mobility measurements indeed reveal a higher effective carrier mobility in the buried-channel Ge$_{x}$C$_{1-x}$ layer with respect to a Si-reference sample, which suggests that the observed NDR is caused by RST of holes from the Ge$_{x}$C$_{1-x}$ into the Si layer. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y37.00011: Tunable Channel Interference in an Aharonov-Bohm Ring Yiping Lin, Pei-Jung Wu, Kuan Ting Lin, J. C. Chen, T. Ueda, S. Komiyama We have investigated the Aharonov-Bohm effect in a quasi one-dimensional ring on a GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure, which is defined by two metallic arc gates coupled to each branch of the ring. Each gate can be separately biased to uniformly squeeze the channel width of electrons, thereby externally tuning the transverse modes in the interference paths. The oscillatory magnetoconductance of the device is systematically studied by varying the number of channels in each path. We have observed the evidence of phase shifts in the magnetoconductance oscillations due to the suppression of the mode numbers on the ring path. Though the periodicity is not well resolved, qualitatively our data support the random phase shifts between the successive modes. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y37.00012: Energy Relaxation in the Bloch-Gruneisen Regime Probed by Weak Antilocalization (WAL) Measurements in GaN Heterostructures Hailing Cheng, Cagliyan Kurdak, Necmi Biyikli, Jinqiao Xie, Hadis Morkoc Electron-phonon (e-p) interaction was investigated in wurtzite Al$_{0.15}$Ga$_{0.85}$N/AlN/GaN and Al$_{0.83}$In$_{0.17}$N/AlN/GaN heterostructures with polarization induced two dimensional electron gases in the Bloch-Gruneisen regime. WAL and Shubnikov-de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. With gate voltage, we cover a carrier density range from 3.41$\times $10$^{12}$cm$^{-2}$ to 4.92$\times $10$^{12}$cm$^{-2}$. Moreover, we used the WAL as a thermometer to measure the electron temperature T$_{e}$ as a function of the bias current. We find the power dissipated per electron P$_{e}$ is proportional to T$_{e}^{4 }$ due to piezoelectric acoustic phonon emission by hot electrons. We calculated P$_{e}$ as a function of T$_{e}$ without using any adjustable parameters for both static and dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment the static screening case is expected to be applicable; however, our data are in better agreement with the dynamic screening case. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y37.00013: Bloch Oscillations and Saturation to a Steady-State Current of an Electron Gas in a Modulated Quantum-Wire Superlattice in a High Electric Field S.K. Lyo, D. Huang, W. Pan We present rigorous theoretical results for the time-dependent and steady-state nonlinear DC current of an electron gas in a periodically modulated one-dimensional semiconductor quantum wire in a high electric field. The theoretical model considers electron-phonon and impurity scattering microscopically in the degenerate and the nondegenerate regime in a tight-binding model. The time-dependent oscillatory and saturation (i.e., steady-state) currents are studied as a function of the field, the radius of the wire, the elastic scattering rate, the lattice period, and the temperature. The radius controls the inelastic scattering rate. The distinctive roles of elastic and inelastic scattering for the current are contrasted and examined. Finally, we compare the results with those from an exact analytic formalism based on a relaxation-time model. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y37.00014: Shot Noise in SU(N) Quantum Dot Kondo Effects Pavel Vitushinskiy, Karyn Le Hur, Aashish Clerk We study effect of interactions on transport properties of mesoscopic conductors whose low-temperature behavior corresponds to SU(N) Kondo model where cases of N=2 and N=4 describe spin-1/2 and carbon nanotube quantum dots and thus are experimentally relevant. Unlike previous studies, we find that there exist two distinct physical mechanisms via which two-particle interactions modify shot noise: scattering process with N-dependent effective charge and enhancement of coherent partition noise. We also account for possible deviation from perfect models such as asymmetry of couplings to source and drain as well as presence of residual potential scattering at low temperatures. The method we propose is not specific just to Kondo effect quantum dots and can be applied to a wide variety of different mesoscopic systems. [Preview Abstract] |
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