Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q29: Focus Session: Thermoelectrics IV: Group IV's & Nanostructures |
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Sponsoring Units: DMP FIAP GERA Chair: Rama Venkatasubramanian, RTI International Room: C123 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q29.00001: Atomistic simulations of thermal transport in Si and SiGe based materials: From bulk to nanostructures Ivana Savic, Natalio Mingo, Davide Donadio, Giulia Galli It has been recently proposed that Si and SiGe based nanostructured materials may exhibit low thermal conductivity and overall promising properties for thermoelectric applications. Hence there is a considerable interest in developing accurate theoretical and computational methods which can help interpret recent measurements, identify the physical origin of the reduced thermal conductivity, as well as shed light on the interplay between disorder and nanostructuring in determining a high figure of merit. In this work, we investigate the capability of an atomistic Green's function method [1] to describe phonon transport in several types of Si and SiGe based systems: amorphous Si, SiGe alloys, planar and nanodot Si/SiGe multilayers. We compare our results with experimental data [2,3], and with the findings of molecular dynamics simulations and calculations based on the Boltzmann transport equation. [1] I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008). [2] S.-M. Lee, D. G. Cahill, and R. Venkatasubramanian, Appl. Phys. Lett. 70, 2957 (1997). [3] G. Pernot et al., submitted. [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q29.00002: Atomistic Analysis of Thermoelectric Properties of Ultra Narrow Nanowires Neophytos Neophytou, Hans Kosina The progress in nanomaterials' synthesis allows the realization of thermoelectric devices based on low dimensional nanostructures. In these confined systems the electrical and thermal conductivity, and the Seebeck coefficient can be designed to some degree independently, providing enhanced ZT values compared to their bulk material's value. We calculate the electrical conductivity, the Seebeck coefficient, and the electronic thermal conductivity of scaled silicon and germanium nanowires using an atomistic sp3d5s*-spin-orbit-coupled tight-binding model. This atomistic model accurately captures the electronic structure of the nanowires while being computationally affordable. We examine n-type and p-type nanowires of diameters/thicknesses from D=3nm to 12nm with various aspect ratios, in [100], [110] and [111] transport orientations, at different doping levels and temperatures. Using experimentally measured values for the lattice thermal conductivity, the expected ZT values of the nanowires are estimated. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q29.00003: Thermal conductivity of silicon-germanium alloys and superlattices from first-principles Jivtesh Garg, Nicola Bonini, Nicola Marzari Thermoelectric materials will become commercially viable for converting heat into electricity and for refrigeration once their figure of merit (ZT) is improved. One key approach to increase performance is to reduce thermal conductivity - e.g. in alloys it is lower than the binary endpoints due to increased scattering induced by strain and disorder. Understanding the thermal conductivity of complex materials is also important in other applications from reducing hot-spot temperatures in electronic chips to better thermal-insulation materials. Here, we have calculated the thermal conductivity of silicon-germanium alloys using ab-initio density functional perturbation theory. The electronic structure of the alloy is studied with the virtual crystal approximation and the single mode relaxation time approximation; perturbation theory up to the third order provides phonon lifetimes, and disorder effects are taken into account by ensemble averages over configurations with random mass disorder. We find that first-principles calculations lead to excellent qualitative agreement with experiments. The thermal conductivity of Si/Ge superlattices has been measured to be lower than Si/Ge alloys. Here we present the first principle calculations of the thermal conductivity of Si/Ge superlattices as a function of layer thickness. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:27PM |
Q29.00004: Thermoelectric Properties and the Benefits of Nanostructuring and Electronic Structure Modifications Invited Speaker: Recent experimental and theoretical research has shown that there are several routes with promising results for obtaining materials with improved thermoelectric characteristics. In particular, it has been demonstrated that nanostructured composites, such as bulk thermoelectrics with nanoinclusions and granular composites, offer the possibility to decrease the thermal conductivity and increase the power factor. The key ingredient for such an enhancement is the carrier scattering from the interfaces. The possibility of increasing the power factor through electronic structure modifications has also been recognized. In this case, this is due to resonant dopant levels located in the band gap region of the thermoelectric material. In this talk, I will discuss recent developments in our fundamental understanding of thermoelectric transport when nanostructuring and electronic structure modifications due to dopant resonant levels are present. Examination of theoretical advancements as well as important experimental results will be presented. The importance of the characteristics of the specific thermoelectric materials will also be discussed. \textit{In collaboration with A. Popescu, A. Datta, and G.S. Nolas, University of South Florida, Tampa, FL 33620. } [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q29.00005: Nano-engineering thermoelectrics in silicon Slobodan Mitrovic, Jen-Kan Yu, Douglas Tham, Joseph Varghese, James R. Heath Though silicon is a poor thermoelectric (TE) material, silicon nanowires have been demonstrated to achieve the TE figure-of-merit ZT=1, comparable to commercially applied materials. This enhancement comes predominantly from the nearly two orders of magnitude reduction in thermal conductivity possibly due to the dimensional crossover or/and the increased boundary scattering. Here we present the study of the TE properties of a novel hole-bar-like silicon thin film structure (25 nm thick). The hole-bar consists of a two-dimensional array of 14 nm-wide holes at a pitch of 34 nm, made by Superlattice Nanowire Pattern Transfer technique. This phonon-crystal material exhibits thermal conductivity as low as 2 W/mK at room temperature, about 3 times smaller than that of similarly dimensioned nanowires. This is the first such demonstration of the TE enhancement mediated by a nanoscale phonon crystal. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q29.00006: Phonon Transport in one-dimensional Silicon Nanowires Kedar Hippalgaonkar, Jinyao Tang, Renkun Chen, Baoling Huang, Karma Sawyer, Peter Ercius, Peidong Yang, Arun Majumdar Traditionally, heat transfer in crystalline solids has been understood to follow the diffusive equation of conduction where the thermal conductivity of the material is an intrinsic property. Silicon, being a semiconductor material, has a significant portion of its thermal conductivity arising from phonons. Decrease in dimensionality coupled with secondary scattering from rough surfaces can decrease the thermal conductivity by as much as two orders of magnitude (k = 1.6 W/mK) compared with bulk single crystal (k = 140 W/mK) making it interesting for thermoelectric applications. The additional scattering from rough surfaces introduces another length scale to scatter the phonons and we find that this might make it possible to break Fourier's Law stemming from diffusive heat transfer from the phonons. This might result in the dependence of the thermal conductivity of the Silicon Nanowires on dimensions and roughness. In this work we have characterized the roughness of different kinds of Silicon Nanowires with controlled geometry and correlate these characteristics with transport properties. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q29.00007: Silicon Nanowire Thermoelectrics: Surface Roughness and Quantum Confinement Edwin Ramayya, Jie Chen, Irena Knezevic The thermoelectric figure of merit (ZT) of silicon nanowires (SiNWs) can be almost two orders of magnitude higher than that in bulk silicon, holding promise for all-silicon on-chip cooling. However, the physics behind the increase in ZT is not clear. We calculate the ZT using a detailed Monte Carlo technique that accounts for the localization of phonons at the rough boundaries and the 2D confinement of carriers. We show that the ZT enhancement in SiNWs is primarily because of strong phonon-boundary scattering that degrades the lattice thermal conductivity. In extremely small wires, contrary to the conventional belief, decreasing the cross section does not necessarily result in an increase in ZT because of the rapid decrease in electrical conductivity owing to strong electron-surface roughness scattering. Roughness also smears the high peaks in the 1D density of states, negating potential benefits that quantum confinement could have on the Seebeck coefficient. We also calculate the optimal roughness for maximal ZT in wires of different cross sections. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q29.00008: Temperature dependence of the thermal conductivity of thin silicon nanowires Davide Donadio, Giulia Galli We report extensive atomistic simulations aimed at understanding the variation of the lattice thermal conductivity of silicon nanowires as a function of temperature (T). We consider the range between 150 and 600 K, where experimental results are available. We find that the thermal conductivity of crystalline wires is of the same order of magnitude as that of the bulk and decreases as 1/T at high temperature. In wires with amorphous surfaces, the thermal conductivity may reach values close to that of amorphous silicon, and it is nearly constant in the temperature range examined here. The low value of kappa in core-shell wires, and its apparently anomalous behavior as a function of T, are determined by the presence of a large majority of diffusive, non-propagating modes in the vibrational spectrum. A parameter free model is presented that accounts for the T dependence observed in crystalline and core-shell systems, and provides a qualitative explanation of recent experiments. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q29.00009: Nanoscale control of thermal transport via SiGe quantum dots F. Pezzoli, P. Chen, M. Stoffel, C. Deneke, A. Rastelli, O.G. Schmidt, A. Malachias, A. Jacquot, G. Pernot, S. Dilhaire, I. Savic, N. Mingo A study of cross plane thermal transport in SiGe/Si quantum dot multilayers (QD ML) is presented. Recent advances in heteroepitaxial growth allowed the fabrication of nanostructures, initiating intriguing investigations into low dimensional physics. Previous reports on thermal transport demonstrated that nanostructuring can reduce the thermal conductivity of a material even below the amorphous limit. Yet, the fundamental reasons why nanostructuring reduces thermal conductivity in crystalline materials are not fully understood. In this work we investigate cross-plane thermal transport through SiGe QD ML grown by means of MBE on Si. Measurements of the thermal properties were carried out along with a detailed AFM, TEM and x-ray characterization. Our findings demonstrate that quantum dots provide a means to tailor the thermal conductivity to extremely low values, about 1 W/m K. The highly diffusive interfaces achieved in SiGe/Si systems may be relevant to the development of integrated miniaturized energy harvesting or thermal management devices, in view of the integrability of SiGe in novel nanoscale devices. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q29.00010: A comparative study of thermal transport in crystalline,amorphous and nano-porous silicon Y.P. He, D. Donadio, G. Galli Recent theoretical predictions [1] suggest that nanoporous Silicon(np-Si) is a promising material for thermoelectric applications, with a figure of merit(ZT) close to 1. One of the reasons for the increased ZT with respect to crystalline Si is a smaller thermal conductivity k. By using a combination of techniques (equilibrium and non-equilibrium molecular dynamics, and the Boltzman transport equation),we analyze the origin of the predicted small k in np-Si, and the interplay between disorder at the mesoscopic scale(the presence of nanometer sized pores) and at the atomic scale(the presence of amorphized pore surfaces). We find that for pores with ordered surfaces, the reduction in k--observed only in the plane perpendicular to the pores-- is due to a reduction of both phonon group velocities and lifetimes. Upon amorphization of the pore surfaces, k is dominated by contributions from diffusive modes and it may decrease to values close to that of amorphous Si even for modest sizes of the amorphized regions. The decrease is observed both in the direction parallel and perpendicular to the pores. Our results are compared to those obtained for Si nanowires[2] with similar surface structures. [1]J-H.Lee, et al., Appl.Phys.Lett. 91, 223110(2007);J-H.Lee,et al., Nano.Lett., 8(11), 3750(2008) [2]D.Donodio and G.Galli, Phys.Rev.Lett. 102, 195901(2009), Work supported by DOE/SciDAC-DE-FC02-06ER25794 [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q29.00011: Diffusion and Phonon-drag Thermopower in Gated Silicon Nanoribbons Hyuk Ju Ryu, Zlatan Aksamija, Deborah Paskiewicz, Shelley Scott, Max Lagally, Irena Knezevic, Mark Eriksson Thermoelectric devices are attracting interest for the targeted cooling of local hotspots in integrated circuits and the harvesting of waste heat to generate power. Special interest in silicon as a thermoelectric material arises from the possibility of monolithic integration, significant reduction in thermal conductivity for nanopatterned silicon wires, and the potential to use bandstructure engineering to improve the power factor. We present measurements of the thermopower in gate-tunable silicon nanoribbons. The gate voltage effectively modulates the thermopower by changing both the 2D electron density and the confinement electric field. The thermopower varies by almost a factor of four in the density range studied. We understand much of this variation and its temperature dependence by considering the roles of the carrier diffusion and phonon-drag contributions to the thermopower. The data are well fit by theoretical calculations based on the Boltzmann transport equation and self-consistent modeling of the confinement electrostatics. We discuss the optimization of the power factor and the cooling efficiency in gated silicon nanoribbons. This work is supported by AFOSR, DOE, NSF, and NDSEG. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q29.00012: Anisotropy and boundary scattering in the lattice thermal conductivity of silicon-on-insulator nanomembranes Zlatan Aksamija, Irena Knezevic Silicon-on-insulator (SOI) membranes and membrane-based nanowires and ribbons show promise for application as efficient thermoelectrics, which requires low thermal conductivity. Thermal conductivity in thin SOI layers, as well as in thin wires and ribbons, is dominated by boundary scattering even at room temperature. Therefore, surface orientation and the direction of heat flow are expected to play a significant role in thermal transport and offer additional control of thermal conductivity in confined systems. In this paper, we demonstrate the sensitivity of the lattice thermal conductivity in thin SOI to the surface crystalline orientation and the direction of heat flow. In this work, we employ a momentum-dependent specularity parameter p(q)=exp (-16$\pi ^{3}\Delta ^{2}$q$^{2})$; that allows us to connect the specularity parameter p directly to the rms magnitude ($\Delta )$ of surface roughness. Results for 20 nm SOI with different surface and transport orientations show a strong anisotropy due to the directional dependence of both the phonon velocity and boundary scattering rates. [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q29.00013: Improved Thermoelectric Behavior of Nanotube-Filled Polymer Composites Jaime Grunlan, Choongho Yu, Yeon Seok Kim, Kyungwho Choi, Dasarayong Kim The thermoelectric properties of single-walled carbon nanotube (SWNT)-filled polymer composites can be enhanced by modifying junctions between SWNTs using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) The resulting composites exhibit electrical conductivities up to $\sim $40,000 S/m without significantly altering thermopower (or Seebeck coefficient). On the other hand, thermal transport remains comparable to typical polymeric materials due to the dissimilar bonding and vibrational spectra between CNT and PEDOT:PSS. This behavior is very different from that of typical semiconductors whose thermoelectric properties are strongly correlated. SWNT-filled composites, made with an aqueous poly(vinyl acetate) emulsion (dried at room temperature followed by 80$^{o}$C) exhibited the best thermoelectric performance in this study. The highest thermoelectric figure of merit (\textit{ZT}) in this study is $\sim $0.02 at room temperature, which is at least one order magnitude higher than most polymers and higher than that of bulk Si. Further studies with various polymers and nanoparticles with high thermoelectric performance could result in economical, light-weight, and efficient polymer-based thermoelectrics. [Preview Abstract] |
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