Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q9: Semiconductor Thermodynamic & Transport Properties |
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Sponsoring Units: DCMP Chair: Selman Hershfield, University of Florida - Gainesville Room: 006D |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q9.00001: Anomalous pressure dependence of thermal conductivities of large mass ratio compounds L. Lindsay, D.A. Broido, J. Carrete, N. Mingo, T.L. Reinecke The lattice thermal conductivities ($k)$ of binary compound materials are examined as a function of hydrostatic pressure, $P$, using a first-principles approach. Compound materials with relatively small mass ratios, such as MgO, show an increase in $k$ with $P$, consistent with measurements. Conversely, compounds with large mass ratios ($e.g.$, BSb, BAs, BeTe, BeSe) exhibit decreasing $k$ with increasing $P$, a behavior that cannot be understood using simple theories of $k$. This anomalous $P$ dependence of $k$ arises from the fundamentally different nature of the intrinsic scattering processes for heat-carrying acoustic phonons in large mass ratio compounds compared to those with small mass ratios. This work demonstrates the power of first principles methods for thermal properties and advances the understanding of thermal transport in non-metals. * * * L. L. acknowledges support from the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division for work done at ORNL. D.A.B acknowledges support from the National Science Foundation under Grant No. 1402949. NM and JC acknowledge support from Institut Carnot through project SIEVE. T.L.R. acknowledges support from ONR and DARPA. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q9.00002: Numerical solution of the drift-diffusion equation for a p-i-p diode Angel Mancebo, Selman Hershfield In low carrier density semiconductors such as organic semiconductors the leads supply carriers to the sample. The extra charge flowing into the sample causes the phenomena of space charge buildup. In its simplest form this is characterized by the Mott-Gurney law, where the current is proportional to the voltage squared. The Mott-Gurney law as usually derived includes the drift term for the current but omits the diffusion term. The diffusion term cannot be neglected as the diffusion coefficient is proportional to the mobility by the Einstein relation. We numerically solve the drift diffusion equation and Poisson's equation for a p-i-p diode, where p refers to a p-type semiconductor and i to an intrinsic semiconductor with very few charge carriers. The model includes no charge traps. By including both the drift and the diffusion terms we find that the current is no longer proportional to the voltage squared but primarily linear with a slight upturn at higher voltages. Furthermore, by examining the carrier density vs. electric field in the sample, we are able to show that for fixed length, there is a maximum current for which there is a physical steady state solution. We will discuss the implications of our results for using the Mott-Gurney law to fit for carrier mobility. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q9.00003: Theory of multiphonon inelastic scattering and carrier capture by defects in semiconductors - Application to capture cross sections Georgios D. Barmparis, Yevgeniy S. Puzyrev, X.-G. Zhang, Sokrates T. Pantelides Inelastic scattering and carrier capture by defects in semiconductors are the primary causes of hot-electron-mediated degradation of power devices. At the same time, carrier capture is a major issue in the performance of solar cells and light-emitting diodes. First-principles, parameter-free calculations of inelastic-scattering and capture cross sections as functions of carrier energy can provide guidance for modeling device degradation based on atomic-scale physical mechanisms. Here we report the construction of a comprehensive theory of multiphonon inelastic scattering by defects, with carrier capture being a special case. We resolve the old debate about what constitutes a correct theory of capture cross sections and report first-principles density-functional-theory calculations of capture cross section for three prototype defects. A Monte Carlo algorithm has been developed to obtain converged sum over all possible phonon configurations. The results reveal that the capture cross section depends strongly on the energy of the incoming electron. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q9.00004: Giant thermal resistivity of interlaced nanoparticles Yevgeniy Puzyrev, Xiao Shen, Sokrates Pantelides We present a theoretical study of thermal resistivity of ``interlaced crystals,'' recently discovered in hexagonal-CuInS$_{2}$ nanoparticles [1]. Interlaced crystals exhibit a perfect global Bravais lattice with two cations and multiple ordering patterns within the cation sublattice. The interlaced crystal consists of interlaced domains and phases where the corresponding phase/domain boundaries are not uniquely defined. Since there are no structural defects or strain, the interlacing has a minimal effect on electronic properties, but causes a large increase in phonon scattering at the boundaries. The size of domains reaches down to one nanometer, resulting in a high density of the boundaries, making interlaced crystals an attractive candidate for thermoelectric applications. Large-scale molecular dynamics calculations show orders of magnitude increase in the thermal resistivity caused by a high density of boundaries. This is a general effect, arising due to a mass disparity of the cations present in interlaced crystals. \\[4pt] [1] ``Interlaced crystals having a perfect Bravais lattices and complex chemical order revealed by real-space crystallography.'' X. Shen, et. al, Nature Comm. 10.1038/ncomms6431. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q9.00005: First-principles calculation of phonon-limited mobility in silicon Yuning Wu, Xiaoguang Zhang, Sokrates Pantelides We introduce a new first-principles method to calculate phonon-scattering-limited electron mobilities. The lifetime of a Bloch state due to scattering is represented by an imaginary electron self-energy which is extracted from the complex band structure of a supercell that contains the phonon vibrations within the frozen-phonon approximation. The phonon vibrations are modeled by a set of configurations generated from a Monte Carlo simulation. Mobility contributions are dominated by electrons on the transverse ellipsoids with low effective mass and long lifetime. The results indicate that high-mobility channels form a conduction network above the percolation threshold. As a result the Matthiessen's rule does not hold for phonon scattering. The overall mobility is evaluated through the configurational average of the percolation-dominant resistor network. The calculated electron mobility agrees with available experimental data. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q9.00006: Three-dimensional Fully-coupled Electrical and Thermal Transport Model of Oxide Memristors Xujiao Gao, Denis Mamaluy, Patrick Mickel, Harold Hjalmarson, Brian Tierney, Matthew Marinella Accompanying the spectacular progress in experimental demonstration of oxide-based memristors, there has been significant modeling effort to aid in understanding of switching physics in memristive devices. However, existing models often simplify the treatment of electronic transport and the interplay of electrically and thermally driven processes. In this work, we present a fully-coupled electrical and thermal numerical model that treats the transport of electrons, holes, and vacancies, together with the Joule heating on an equal footing. Namely, we solve simultaneously the five coupled partial differential equations: the time-dependent drift-diffusion equations for electrons, holes, and vacancies, the time-dependent lattice heat equation, and the Poisson equation for all the charged species in three spatial dimensions. This fully coupled model allows us to investigate the microscopic interplay of field drift, Fickian diffusion, Soret effect, and Joule heating, and to facilitate the understanding of physical mechanisms responsible for the SET and RESET switching processes in tantalum oxide memristors. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q9.00007: Fast response time of electron plasma in high electron mobility transistor channels Greg Rupper, Sergey Rudin, Michael Shur, Meredith Reed We report on the theoretical studies of the response of the two-dimensional gated electron gas to a voltage step applied at the gate. The results are given for numerical solution for a non-linear hydrodynamic model as well as the analytical solution for the linear hydrodynamic model. Both models include the effects of pressure and viscosity and are valid for all mobilities. For low mobility samples, such that $\omega_{\mathrm{p}}$ $\tau $ \textless \textless 1, where $\omega_{\mathrm{p}}$ is the plasma frequency and $\tau $ is the momentum relaxation time, the solutions in the frame of the hydrodynamic model of non-linear plasma transport predict the decay time of 4L$^{\mathrm{2}}$/( $\pi^{\mathrm{2}}\mu $U), $\mu $ is the mobility, U is the gate voltage swing, and L is the channel length. For high mobility samples, such that $\omega_{\mathrm{p}}\tau $ \textgreater \textgreater 1, the drain voltage response oscillates with a decaying magnitude. The period of the oscillations is on the order of the plasma wave transit time (4L/s where s is the plasma velocity). The decay time of the oscillations is determined by the momentum relaxation time and the viscosity of the electronic two dimension gas (fluid). We find that the oscillating period is much shorter than the electron transit time L/v$_{\mathrm{d}}$ (v$_{\mathrm{d}}$ is the electron drift velocity), which is used as a conventional figure of merit for the device speed. These results are important for developing a new generation of terahertz electronic devices and comparing different materials systems for their potential of reaching ultimate switching speed and/or the highest frequency of operation [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q9.00008: Phonon Anharmonicity in Silicon from 100 to 1500 K Dennis Kim, H.L. Smith, J.L. Niedziela, C.W. Li, D.L. Abernathy, B. Fultz Silicon has widespread use in modern technology and understanding the thermodynamics and thermal transport is of great importance. As the phonons dominate the total entropy as well as thermal transport properties, it is essential to measure accurately the temperature-dependent lattice dynamics. Inelastic neutron scattering measurements of silicon were performed at temperatures ranging from 100 to 1500 K (previous neutron measurements of phonon dispersions of pure silicon extended only up to 700 K). The experiments were done on high-quality powder and on single crystals, which were shaped for optimal neutronics in the ARCS spectrometer at the SNS. Large phonon anharmonicities manifested by phonon energy shifts and broadenings at high temperatures were observed in measured phonon dispersions, and in the phonon density of states. At 1500 K the anharmonicity contributes approximately 80\% of the deviation from the harmonic vibrational entropy. These large effects are beyond the predictions of the quasiharmonic model and demonstrate that phonon anharmonicity is a major contributor to both vibrational entropy and phonon lifetimes. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q9.00009: Measurement of the hot electron mean free path in GaN Donald J. Suntrup III, Geetak Gupta, Haoran Li, Stacia Keller, Umesh K. Mishra We present a method for measuring the mean free path (MFP) and relaxation time of hot electrons in GaN using the hot electron transistor (HET). In this device electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge. After traversing the base, high energy electrons either surmount the base-collector barrier and become collector current or reflect off the barrier and become base current. We fabricate HETs with various base thicknesses and measure the common emitter transfer ratio ($\alpha$) for each device. The MFP is extracted by fitting a decaying exponential to $\alpha$ as a function of base width and the relaxation time is computed using a suitable injection velocity. For current devices with an injection energy of $\sim$1eV, we measure a hot electron MFP of 14nm and calculate a relaxation time of 16fs. These values are in agreement with theoretical calculations where longitudinal optical (LO) phonon scattering is the dominant relaxation mechanism. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q9.00010: Anisotropic lattice thermal conductivity for chain tellurium Hua Peng, Nicholas Kioussis, Derek Stewart Trigonal tellurium is elemental crystal consisting of two kind of boding: weak interchain van der Waal's forces and strong covalent intrachain boding. Using the ab initio calculations combined with the phonon Boltzmann transport equation we calculate the temperature and pressure dependent lattice thermal conductivity of chain tellurium. The heat transport along and perpendicular to the chain has large anisotropic character, and also for the gruneisen parameter and phonon velocity. The three-phonon scattering rate of the acoustic branches and the phonon mean free path of tellurium are also investigated, which are important for the nanostructure thermoelectric material developing. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q9.00011: Transport Properties of InAs/GaSb Bilayers with a Tunneling Barrier Ruiyuan Liu, Lingjie Du, Rui-Rui Du, Gerald Sullivan Inverted InAs/GaSb bilayers have been shown to support the quantum spin Hall effect, characterized by an insulating 2D bulk and a pair of counter - propagating helical edge states around the perimeter of the sample. Here we investigate the transport properties of InAs/GaSb bilayers in similarly inverted bands, but with an AlGaSb tunneling barrier placed between InAs and GaSb quantum wells. While the interlayer tunneling are essentially suppressed by increasing the barrier width, we found that the 2D bulk can still be turned into an insulating state that is characterized by a temperature-dependent resistance peak as a function of gate biases. On the other hand, we found no evidence for edge states in this regime. In this talk, we will present sample structures, transport data, as well as a brief discussion in connection to possible coherent exciton states in InAs/GaSb system proposed in Naveh and Laikhtman, Phys Rev. Lett. 77, 900 (1996). Work in Rice was supported by DOE DE-FG02-06ER46274. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q9.00012: Estimation of in-plane g-factor and disorder of InSb quantum wells via magnetoconductance mapping J.T. Mlack, M.B. Santos, C.M. Marcus We show the magnetoconductance mapping of an InSb quantum well(QW) and use the mapping to estimate material properties. Measurement and fitting of the suppression of weak antilocalization by an in-plane field oriented along in-plane crystal axes provides estimates of the QW's g-factor and disorder. By comparing measurements along different crystal axes, a variation in the estimated parameters is observed. The observed variation is consistent across different hall bars and is dependent on the direction of the applied field with respect to the crystal axes. These results show the utility of such measurements in better understanding the material properties of QW's. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q9.00013: Temperature Dependent Electrical Conductivity and 1/f Noise in Boron Doped Amorphous Silicon Kiran Shrestha, Vincent Lopes, Dale Whitfield, A.J. Syllaios, Chris Littler We report on temperature dependent electrical conductivity and noise measurements made on boron doped a-Si:H thin films prepared by plasma enhanced chemical vapor deposition. Samples were grown at various boron concentrations and hydrogen dilution of the silane precursor. Measurements were made at temperatures ranging from 200 $^{\circ}$K to 400 $^{\circ}$K. We found that in this temperature range the electrical conductivity generally follows the Mott variable range hopping conduction model $\sigma $ $= \quad \sigma $exp[-(T$_{\mathrm{0}}$/T)$^{\mathrm{m}}$]where m $=$ 1/4. For hopping conduction it is found that the noise has a 1/f$^{\mathrm{n}}$ component. The exponent, n, in this temperature range is n $\approx $ 1, i.e., the noise is 1/f and follows the Hooge model. The normalized Hooge parameter, $\alpha_{\mathrm{H}}$/p, where p is the carrier density, is correlated to the Mott hopping parameters $\sigma _{\mathrm{o}}$, and T, and in turn, to the material dopant boron concentration, hydrogen content, structural disorder as determined by Raman spectroscopy. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q9.00014: Ab initio study of complex defects and optical transitions in MgAl$_{2}$O$_{4}$ J.C. Garcia, P.D. Borges, F.G. Pinto, J. Tronto, L. Scolfaro The excellent optical properties of the Magnesium aluminate (MAO) spinel makes it an important material for novel technological applications. Considering that a presence of native defects can promote important changes in those properties, we present in this work a study of the structural, electronic and thermodynamic properties of the MAO. The calculated formation energy for isolated defects, such as the vacancies of manganese, aluminum and oxygen, oxygen interstitial, manganese and aluminum antisites, as well as the complexes in the most stable charge states are shown. In good agreement with experimental data, we obtained that complex centers, such as oxygen vacancies in conjunction with oxygen interstitial, manganese or aluminum antisites at different charge states are good candidates for the observed optical transitions at 4.75, 5.3, and 6.4 eV. Our findings were obtained from ab initio electronic structure calculations performed within the Density Functional Theory. For the exchange-correlation potential, the generalized gradient approximation was used. Furthermore, a modified Becke-Johnson correction to the exchange potential was applied to obtain a suitable value for the band gap energy, 7.40 eV, in good agreement with the experimental one of 7.8 eV. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q9.00015: Native point defects and conductivity of ZnRh$_{2}$O$_{4}$: a GGA + U study Piotr Boguslawski, Oksana Volnianska ZnRh$_{2}$O$_{4}$ spinel belongs to the family of transparent conducting oxides, which are promising for applications in optoelectronic technology. To assess essential material properties, we analysed energy levels and formation energies of native point defects, i.e., vacancies (V), interstitials, and cation antisites in ZnRh$_{2}$O$_{4}$. Calculations were based on generalized gradient approximation (GGA) to the Density Functional Theory supplemented by the +U corrections. The value of U was treated as a free parameter, which allowed for the systematic study of the U-induced changes of the electronic structure. The experimental band gap of ZnRh$_{2}$O$_{4}$ is reproduced only when the +U term is imposed on both d(Rh) and p(O) orbitals, and the pronounced distortions of the oxygen sublattice are included. Zn:Rh is the dominant acceptor that can be responsible for the observed p-type conductivity in ZnRh$_{2}$O$_{4}$. The low formation energy of Zn:Rh can make the intentional n-doping difficult. In the O-rich conditions, the second important acceptor is V:Zn. The two dominant donors in Zn-rich and O-rich conditions are V:O and Rh:Zn, respectively. Growth conditions leading to the lowest concentrations of native defects were identified. [Preview Abstract] |
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