Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P28: Focus Session: Thermoelectricity in Si-containing Materials |
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Sponsoring Units: FIAP Chair: Udo Pernisz, Dow Corning Corporation Room: 330 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P28.00001: Atomistic Simulations of Heat Transport in Silicon Nanowires Invited Speaker: Silicon is one of the best known materials of our age, cheap and readily available, being the basic constituent of semiconductor electronics. It would therefore be highly desirable to broaden its utilization for, e.g. renewable energy applications. Recently, it has been proposed that Silicon may be engineered to be an efficient thermoelectric material for use in solid state devices. Although a rather inefficient thermoelectric in its bulk form, at the nanoscale Si may become a poor heat conductor, while retaining good electronic conduction properties, and thus exhibit high efficiency in converting heat into electric current. However the fundamental reasons for the reported low heat conduction in Si nanowires (NW) are not yet understood, and different interpretations has so far appeared in the literature. Here we present atomistic simulations of heat conduction in Si NW of 1 to 3 nm diameter. Our results show that, depending on their surface structure, these wires may exhibit values of the thermal conductivity varying by two orders of magnitude, and as high as those of bulk Si. This clearly indicates that the increased surface to bulk ratio at the nanoscale may be only partially responsible for the decreased thermal conductivity observed experimentally. We also find that diffusive, yet extended, vibrational modes present in the case of wires with amorphous surfaces, are responsible for a dramatic decrease of a factor of 100 in the conductivity of purely crystalline NWs. Our findings suggest ways of engineering wires with even lower thermal conductivity, by increasing surface disorder, in particular by alloying Si with, e.g. Ge at the crystalline-amorphous interface. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P28.00002: Ultralow thermal conductivity in Electrolessly Etched (EE) Silicon Nanowires Kedar Hippalgaonkar, Renkun Chen, Bair Budaev, Jinyao Tang, Sean Andrews, Padraig Murphy, Subroto Mukerjee, Joel Moore, Peidong Yang, Arun Majumdar EE process produces single-crystalline Silicon nanowires with rough walls. We use suspended structures to directly compute the heat transfer through single nanowires. Nanowires with diameters less than the mean free path of phonons impede transport by boundary scattering. The roughness acts as a secondary scattering mechanism to further reduce phonon transport. By controlling the amount of roughness it is possible to push limits to the extent that nanowire conductance close to quanta of thermal conductance,${\pi k_B^2 T} \mathord{\left/ {\vphantom {{\pi k_B^2 T} {6\hbar }}} \right. \kern-\nulldelimiterspace} {6\hbar }$ is observed. Traditionally, the lower limit of conductivity is amorphous Silicon at 1 W/mK at room temperature. The measured conductivity of our nanostructures challenges even this amorphous limit pointing towards previously unstudied mechanisms of thermal resistance. We measure thermal conductivity of $\sim $150nm diameter EE wires to be $\sim $1 W/mK. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P28.00003: Thermoelectric Power of Silicon Nanowires Hyuk Ju Ryu, Deborah Paskiewicz, Shelley Scott, Max Lagally, Mark Eriksson Thermoelectric nanomaterials have been attracting considerable interest for the cooling of hotspots and the conversion of waste thermal energy into useful electrical energy. There is special interest in silicon thermoelectrics because of potential monolithic integration on microchips as well as opportunities in nanofabrication and bandstructure engineering. We present measurements of the thermoelectric power of silicon nanowires with different doping concentrations and the gate field effect. Because silicon heterostructures can have modulation in charge density and mobility along the charge path, we have fabricated and measured silicon/silicide, silicon/silicon-germanium, and hybrid orientation silicon heterostructures in the form of nanowires. Measurements of the thermoelectric power of these structures and the effects of the internal interfaces will be presented and compared with theoretical calculations. This work is supported by DOE, AFOSR, NZ-FRST, NDSEG, NSF, and SOITECH. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P28.00004: Thermoelectric Properties of Higher Manganese Silicide Nanowires Arden Moore, Jeremy Higgins, Feng Zhou, Song Jin, Li Shi Higher manganese silicides (HMS) have a relatively high thermoelectric figure of merit (\textit{ZT}) of about 0.7. HMS nanowires have been synthesized using a chemical vapor deposition method. In this work, the thermoelectric properties of individual HMS nanowires are measured and analyzed to determine the role of size effects on electron and phonon transport as well as potential \textit{ZT }enhancement. Measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity were performed using both suspended and substrate-based microdevices. Results show that the Seebeck coefficient of two as-synthesized 60 nm diameter nanowires between 300-400K is about 25-50{\%} lower than that of single crystal bulk parallel to the $c$-axis, while the electrical conductivity values are about 25{\%} lower than bulk single crystal in the same direction. The thermal conductivity of one 60 nm diameter nanowire at room temperature was found to be four times smaller than the bulk value along the $c$-axis. The large reduction in thermal conductivity and small to moderate impact on electrical transport may lead to HMS nanowires with enhanced \textit{ZT}. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P28.00005: Thermal conductivity of silicon-germanium alloys 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 strucure 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. The contribution of acoustic and optical phonons to the thermal conductivity is also presented, together with the phonon mean free paths. These calculations could be used to estimate the size of the nanostructures that could reduce the thermal conductivity below bulk values through increased scattering of phonons. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P28.00006: ABSTRACT HAS BEEN MOVED TO S1.00260 |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P28.00007: Thermoelectric Properties of Silicon Nanowires: a Computational Study E. B. Ramayya, I. Knezevic We present a detailed simulation of electronic and thermal transport in thin, highly doped silicon nanowires, surrounded by a native oxide. Electronic states are found from a self- consistent Poisson-Schr\"{o}dinger solver within the effective mass framework. Confined acoustic phonon dispersions are calculated from the elastic continuum equation with the free- standing boundary conditions, appropriate for Si surrounded by the acoustically softer SiO$_2$. Transport of charge and heat is described by solving the Boltzmann transport equations for both electrons and acoustic phonons using the ensemble Monte Carlo technique. We see little increase in the phonon-drag portion of the Seebeck coefficient over the bulk value, and obtain the total Seebeck coefficient in agreement with experiment. Boundary roughness scattering indeed proves to have a significant effect on both electronic and thermal transport, and we discuss a novel method to account for the phonon boundary scattering, which supplants the use of the phenomenological specularity parameter. We demonstrate that indeed the room-temperature figure of merit in thin wires reaches values close to 1, and discuss options for its further enhancement. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P28.00008: Lattice thermal conductivity of nanostructured semiconductors from atomistic simulations Yuping He, Davide Donadio, Joo-Hyoung Lee, Jeffrey C. Grossman, Giulia Galli We present an atomistic analysis of the thermal conductivity (k) of nanoporous silicon (np-Si) [1, 2], and we compare our results with those obtained for bulk crystalline (c-Si) and amorphous Si. We computed k using equilibrium molecular dynamics and Green Kubo relations; we then analyzed our results by solving the Boltzmann Transport Equation in the single mode relaxation time approximation, and by using an approach devised [3] to describe thermal transport in disordered semiconductors. We observe that in np-Si the phonon mean free path is reduced by up to a factor of 10 with respect to c-Si, yielding a reduction of the k of about 2 orders of magnitude. The predominant phonon scattering processes contributing to k can be modeled by the same non-perturbative [3] approach that describes thermal transport in a-Si. \\[0pt] [1] J-H. Lee, et al. Appl. Phys. Lett, 91, 223110 (2007)\\[0pt] [2] J-H. Lee, et al., Nano. Lett., 8(11), 3750 (2008)\\[0pt] [3] P, B. Allen and J. L. Feldman, Phys. Rev. B 48, 12581 (1993) [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P28.00009: Direct Measurements of Figure-of-Merit in Amorphous Silicon-based Thermoelectric thin films Rubina Sultan, Azure Avery, Barry Zink Thermoelectric materials may play an important role in the solution of the urgent global need for energy. The dimensionless figure of merit ($ZT=\sigma {\alpha}^2 T/\textit{k}$) depends on three fundamental transport properties (thermal conductivity, electrical conductivity and thermoelectric power) of the material and optimizing the efficiency relies on effective measurement techniques of these quantities. These material properties may change from bulk to thin film form. The primary challenge is to search for materials with optimized electrical transport while minimizing the thermal conductivity. Amorphous materials and their alloys are relatively new functionally important materials that demonstrate superior properties in wide range of applications such as in thermoelectricity because of their low thermal conductivity due the higher degree of disorder. Previously, we reported in plane thermal conductivity of amorphous Silicon Nitride (\textit{a}-Si-N) membranes. In this talk we present our measurement technique and recent results of thermoelectric properties of thin film amorphous Si and its alloys by direct measurement of in plane thermal conductivity, thermopower and electrical conductivity on one platform and discuss the thermoelectric figure of merit. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P28.00010: Enhancement in power factor in p-type bulk SiGe alloys Giri Joshi, Austin Minnich, Xiaowei Wang, Gaohua Zhu, Yucheng Lan, Dezhi Wang, Bed Poudel, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon Germanium (SiGe) alloys have been used for high temperature power generation in thermoelectric generators that provided the onboard electrical power to several US space vehicles. Since their performance is related to dimensionless figure-of-merit (ZT), material scientists have focused their attention on possible improvements in ZT of SiGe alloys through an increase in power factor and decrease in thermal conductivity. We have improved peak ZT to 0.95 in p-type SiGe bulk alloys by reducing thermal conductivity with nanostructuring approach. Now, we have been perusing modulation doping technique to improve ZT by increasing power factor in SiGe bulk alloys. We have observed significant improvement in power factor but ZT is not improved due to increase in thermal conductivity. The enhancement of power factor is mainly due to increase in mobility of carriers without much affecting the Seebeck coefficient. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P28.00011: Thermal Conductivity of Silicon/Germanium Nanostructures H.-Y. Chang, L. Tsybeskov, A. Sirenko, D.J. Lockwood, J.-M. Baribeau, X. Wu, M.W.C. Dharma-Wardana, T.I. Kamins, A.M. Bratkovsky The efficiency of thermoelectric devices can be enhanced by increasing electrical conductivity and lowering thermal conductivity. Semiconductor nanostructures, whose electrical and thermal conductivities could be optimized by changing their electronic and structural properties, are ideal candidates for such device applications. However, complete understanding of their device properties and limitations requires a technique allowing temperature measurements with a nanoscale spatial resolution. In this work, we studied the thermal conductivities of two groups of Si/Ge nanostructures: Si/SiGe multilayer samples prepared by molecular beam epitaxy, and Si/Ge nanowire heterojunctions prepared by chemical vapor deposition based vapor-solid-liquid process. Sample temperatures during irradiation by a laser beam were measured using Stokes and Anti-Stokes modes of Raman scattering of different vibration modes, and thermal conductivity was calculated by using the temperature gradient between different parts of SiGe nanostructures. We find clear correlations between samples' structural properties and their thermal conductivity. This work suggests a novel approach toward high-efficiency Si/SiGe nanostructure-based thermoelectric generators. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P28.00012: High Temperature Elastic Moduli Measurements and Phase Transition Studies of Novel Thermoelectric Materials Guangyan Li, Resheed Adebisi, Josh Gladden Thermoelectric (TE) materials can be used to convert heat including waste heat to electrical power. They are one component to energy savings and independence. Silicon germanium (SiGe) and Zintl phase compounds are excellent candidates for high temperature applications. The mechanical properties of these materials need to be known before their actual applications in high temperature (1000C) environments. The temperature dependent elastic moduli of five different SiGe alloys were successfully measured using a high temperature resonant ultrasound spectroscopy (RUS) technique. A linear trend is generally observed up to 600C, a downward curvature especially in two n-type samples is noticeable at higher temperatures. Hysteresis is only observed in one of the n-type SiGe samples. Phase transitions, indicated by shifts in the natural frequencies of a Zintl sample, were observed near 792, 892, 931C. The nature of these transitions will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P28.00013: Preparation and thermoelectric properties of Magnesium compounds Xiunu Lin, George Nolas, Dongli Wang We report on the synthesis and low temperature transport properties measurements of environmental-friendly thermoelectric materials Mg$_{2}$B (B=Si, Ge, Sn) and their alloys. These semiconductors are prepared through solid-state reaction of constituent elements. The effect of electrons doping and structure vacancies on thermoelectric properties are studied by substituting trivalent Sb for tetravalent Ge or Si on Mg$_{2}$Si and Mg$_{2}$Ge compounds. For Mg$_{2}$Ge system, both the Seebeck coefficient and electrical resistivity first decrease and then increase with increasing Sb content, whereas the thermal conductivity decrease monotonically. The Mg$_{2}$Si system displays similar tendency in seebeck coefficient, resistivity, and thermal conductivity but shows smaller magnitudes. Our Hall measurement at room temperature indicates that the modulation in these thermoelectric properties can be accounted for by the variance of electron concentration. The Mg$_{2}$Si$_{1-x}$Sn$_{x}$ solid solutions were prepared and investigated to study the dependence of thermoelectric properties on carrier types and carrier concentrations. [Preview Abstract] |
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