Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D11: Thermal Properties in Semiconductors and Nanostructures |
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Sponsoring Units: FIAP Chair: Hongping Zhao, Lehigh University Room: D222 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D11.00001: Thermoelectric Properties of Granular Materials Andreas Glatz, Igor Beloborodov I will present our recent studies of thermoelectric properties of mono-phasic nanocrystalline semiconductors and metals in the weak coupling regime. The focus is in particular on the thermopower and figure of merit for temperatures less than the charging energy. I will show that the dimensionless figure of merit $ZT$, which is a measure for the performance of thermoelectric materials, has a maximum at certain temperatures and grain sizes which can be in the range of technological relevant values $ZT>3$. The talk is based on: Phys. Rev. B {\bf 80}, 245440 (2009) and EuroPhys. Lett. {\bf 87}, 57009 (2009). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D11.00002: Thermal Conductivity Characteristics of Three-Layer Superlattices Jing Zhang, Hua Tong, Nelson Tansu For thermoelectric applications, the thermal conductivity of the material needs to be reduced as low as possible in order to achieve higher thermoelectric efficiency of devices, as the device efficiency depends on the thermoelectric figure of merit ($Z*T)$. Both theoretical and experimental data show that the cross-plane thermal conductivity of superlattices is much lower than that of the bulk materials The cross-plane thermal conductivity of three-layer superlattices is calculated by a numerical method, which is developed from the lattice dynamical theory. The phonon mean free path is included into the calculation, thus the minimum thermal conductivity occurs at the crossover of the particle-like model and wave-like model of the phonons. The studies focus on the effect of mass ratio, layer thickness, and mean free paths on the minimum thermal conductivity of the three-layer superlattice design. The minimum thermal conductivity of the three-layer superlattice structure is approximately half of that of the conventional two-layer superlattice structure. This finding indicates that the thermoelectric figure of merit for superlattice structure can further be enhanced by 2 times from the use of the three-layer superlattice design. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D11.00003: Lattice thermal conductivity with first-principles anharmonic lattice model Terumasa Tadano, Yoshihiro Gohda, Shinji Tsuneyuki First-principles calculation of lattice thermal conductivity is important to design new devices such as high-efficiency thermoelectric materials. For lattice thermal conductivity calculations of complex materials and nanostructures, non-equilibrium molecular dynamics (NEMD) is more suitable than widely used Boltzmann transport theory. However, a combination of NEMD and FPMD is almost impossible because of its high computational cost, so that NEMD has been performed only with classical model potentials for specific materials. In order to overcome this limitation in materials, we have developed a new methodology for calculating lattice thermal conductivity without relying on any experimental values. In this method, the potential energy of a system is expressed as a many-body anharmonic model, that is, a Taylor expansion of the total energy with respect to displacements of atoms up to 4th order. Parameters of the anharmonic lattice model are determined with Hellmann-Feynman force of FPMD by least-square fitting. We performed thermal conductivity calculations with the anharmonic lattice model combined with NEMD and obtained reasonable agreements with experimental values. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D11.00004: Surface roughness and phonon transport in thin Si nanowires: an atomistic study Jesus Carrete, Luis Javier Gallego, Luis Miguel Varela, Natalio Mingo Good thermal insulation is much harder to achieve than electrical insulation. Thus, the astonishingly low thermal conductivities recently reported on Si nanowires came as a surprise, since the displayed values were an order of magnitude lower than predicted by the diffuse boundary limit of Casimir's theory. Recent theoretical work has employed the Born approximation to predict a very much enhanced boundary scattering rate that would lead to a thermal conductivity well below the Casimir limit. We present a Green's function calculation that answers the question of whether the Casimir limit to the phonon mean free path can be overcome by roughness. Our results show that the mean free path (MFP) and the thermal conductivity of a nanowire are very close to the Casimir limit for shallow disorder, and can only be pushed below it using very deep surface roughness, well beyond previous estimates. We also explore the limits of the Born approximation in this context using vacancies and isotopic impurities as defects. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D11.00005: Quantal Heating of 2D electrons in strong magnetic fields Sergey Vitkalov, Alexey Bykov Usually heating of conducting electrons by $dc$ electric field increases electron temperature and effects weakly the electron transport. In this report we show that the $dc$ heating of 2D electrons with a quantized spectrum is very peculiar and violates strongly the Ohm's Law [1]. The quantal heating establishes nontrivial electron distribution, which has the same broadening or an effective ``temperature'' as the unbiased system. The heating reduces significantly the dissipative electron transport, forcing the quantum conductors into a state with zero differential resistance (ZDR). Furthermore an apparent $dc$ driven metal-insulator transition is found, which correlates with the transition into the ZDR state. This interesting correlation is unexpected and is not understood.\\[4pt] [1] J.-Q. Zhang, S.A. Vitkalov and A.A. Bykov, Phys. Rev. B 80, 045310 (2009); S. A. Vitkalov, International Journal of Modern Physics B, 23, 4727 (2009). [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D11.00006: Light-induced thermodynamic metastability in amorphous silicon Daniel Queen, Julie Karel, Frances Hellman, Qi Wang, Richard Crandall, Eugene Iwaniczko The efficiency of amorphous silicon photovoltaics is limited due to the generation of dangling bond defects upon light soaking which leads to a decrease in their efficiency known as the Staebler-Wronski Effect. These defects act as recombination centers for photoexcited electron-hole pairs and can be reversibly removed by annealing above 150\r{ }C. The electrical properties of these defects are well documented but the mechanism that gives rise to them is still an open question. It is known that hydrogen plays a crucial role in their formation and recovery but it is not clear if hydrogen participates in the defect formation. We present heat capacity data for a-Si:H films grown by the Hot-Wire CVD (HWCVD) technique and a-Si films grown by e-beam evaporationand measured using our MEMS based nanocalorimeter. Both materials have an excess heat capacity observed upon light soaking that is reversibly removed by annealing at 200\r{ }C. This excess is found to be independent of H content in the HWCVD films and is present but at a smaller magnitude in the e-beam evaporated a-Si. The lack of dependence on H content and the presence in the e-beam films indicates the light induced metastability is intrinsic to the amorphous silicon matrix. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D11.00007: Thermodynamic properties of ZrSiO4 polymorphs from DFT based \textit{ab initio} phonon calculations Jincheng Du, Mrunal Chaudhari Zircon and Reidite are the polymorphs of ZrSiO$_{4 }$minerals that are natural hosts of various radioactive elements in the crust of the earth. Its high permittivity also makes it a promising material for the gate dielectric material in metal-oxide semiconductors. Knowledge of the thermodynamic properties at high temperature and high is very important to consider its application as an effective natural storage for the radioactive wastes and high technology ceramics. These properties are thoroughly studied both computationally and experimentally for zircon, while significantly less attention was paid to reidite in the literature. We report studies of thermodynamic properties of Zircon and Reidite from phonon spectra calculations using \textit{ab initio} based periodic density-functional theory (DFT) calculations. Various thermodynamic properties such as free energy, internal energy, entropy, enthalpy, heat capacity and thermal displacement as a function of temperature are calculated. Phoon dispersion curves and density of states are calculated and compared with the experimental data. Calculated bulk properties agree very well with the experimental data in the literature. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D11.00008: Vibrational and thermal properties of ternary semiconductors and their isotopic dependence: chalcopyrite CuGaS2 Aldo Romero, M. Cardona, R. Kremer, R. Lauck, A. Mu\~noz The availability of \textit{ab initio} electronic calculations and the concomitant techniques for deriving the corresponding lattice dynamics have been profusely used in the past decade for calculating thermodynamic and vibrational properties of semiconductors, as well as their dependence on isotopic masses. The latter have been compared with experimental data for elemental and binary semiconductors with different isotopic compositions [1]. Here we present theoretical and experimental data for several vibronic and thermodynamic properties of a canonical ternary semiconductor of the chalcopyrite family: CuGaS2 [2]. Among these properties are the lattice parameters, the phonon dispersion relations and densities of states (projected on the Cu, Ga, and S constituents), the specific heat and the volume expansion coefficient. The calculations were performed with the ABINIT and VASP codes within the LDA approximation for exchange and correlation.\\[4pt] [1] Cardona {\it et al.}, PRB81, 075202 (2010)\\[0pt] [2] Gibin {\it et al.}, Solid State Commun Solid State Commun 133, 569 (2005); Sanati {\it et al.} S.S. Commun 131 229 (2004). [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D11.00009: Temperature dependence of band gap of highly confined CdSe and PbSe nanocrystals Jason Bylsma, Prasenjit Dey, Jason Rejman, Aaron Zaubi, Sarath Witanachchi, Pritish Mukherjee, Denis Karaiskaj We have recorded fluorescence spectra from PbSe and CdSe quantum dots in hexane/toluene respectively between 5K and 300K in order to investigate the temperature dependence of the electronic band gap of these highly confined nanostructures. The band gap for CdSe follows the known blue shift with decreasing temperature (dE/dt = -225 $\mu $eV/K). Olkhovets et. al. first reported a red shift of the band gap energy with decreasing temperature for small (d $<$ 4 nm) PbSe and PbS quantum dots [1]. Such behavior would contradict the expected blue shift of the band gap with decreasing temperature. We have measured the temperature dependence of the band gap of PbSe quantum dots for two different diameters below 4 nm and indeed observe a red shift of the band gap with decreasing temperature (dE/dT = 58 $\mu $eV/K), which is stronger for the smaller size quantum dots (dE/dt = 82 $\mu $eV/K). The origin of this peculiar behavior is not well understood and we are pursuing further theoretical and experimental studies in order to elucidate the mechanism behind it. [1] A. Olkhovets, et. al. Phys. Rev. Lett. 81, 3539 (1998). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D11.00010: Temperature and polarization dependent photoluminescence studies of WO$_{3}$ and WO$_{3-x }$single crystals Prasenjit Dey, Justin Easley, Denis Karaiskaj, Satyen Deb, Ted Ciszek, Daniel Dessau WO$_{3}$ is an important material not only due to its interesting electronic properties but also for applications in electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades.\footnote{Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein.} We have studied two WO$_{3}$ single crystals, a transparent and a doped WO$_{3-x}$. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not follow the band gap energy with temperature. However polarization dependent studies reveal at least two polarization dependent component of the center. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D11.00011: Mapping free-carrier diffusion in GaAs with radiative and heat-generating recombination Tim Gfroerer, Ryan Crum, Mark Wanlass We use a tightly focused laser along with optical and thermal imaging to measure the diffusion-driven, free-carrier distribution in a GaAs/GaInP heterostructure. We find that temperature profiles are broader than their luminescence counterparts. This observation is consistent with how the underlying recombination mechanisms depend on carrier density: the rate of heat generation should be approximately proportional to the density of carriers, while the radiative rate should scale with the density squared. We show that the square root of the light signal follows the heat profile, giving consistent, independent measurements of the local carrier density. [Preview Abstract] |
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