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RESEARCH PAPERS: Micro/Nanoscale Heat Transfer

# Modeling of Thermoelectric Properties of Semi-Conductor Thin Films With Quantum and Scattering Effects

[+] Author and Article Information
A. Bulusu

Interdisciplinary Program in Materials Science, Vanderbilt University, Nashville, TN 37235

D. G. Walker1

Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235greg.walker@vanderbilt.edu

1

Corresponding author.

J. Heat Transfer 129(4), 492-499 (Jul 29, 2006) (8 pages) doi:10.1115/1.2709962 History: Received March 03, 2006; Revised July 29, 2006

## Abstract

Several new reduced-scale structures have been proposed to improve thermoelectric properties of materials. In particular, superlattice thin films and wires should decrease the thermal conductivity, due to increased phonon boundary scattering, while increasing the local electron density of states for improved thermopower. The net effect should be increased $ZT$, the performance metric for thermoelectric structures. Modeling these structures is challenging because quantum effects often have to be combined with noncontinuum effects and because electronic and thermal systems are tightly coupled. The nonequilibrium Green’s function (NEGF) approach, which provides a platform to address both of these difficulties, is used to predict the thermoelectric properties of thin-film structures based on a limited number of fundamental parameters. The model includes quantum effects and electron-phonon scattering. Results indicate a 26–90 % decrease in channel current for the case of near-elastic, phase-breaking, electron-phonon scattering for single phonon energies ranging from 0.2 meV to 60 meV. In addition, the NEGF model is used to assess the effect of temperature on device characteristics of thin-film heterojunctions whose applications include thermoelectric cooling of electronic and optoelectronic systems. Results show the predicted Seebeck coefficient to be similar to measured trends. Although superlattices have been known to show reduced thermal conductivity, results show that the inclusion of scattering effects reduces the electrical conductivity leading to a significant reduction in the power factor ($S2σ$).

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## Figures

Figure 1

Schematic representation of the nanodevice modeled in the simulation. E is the electron energy in the device while Σs is the scattering self-energy.

Figure 2

Self-consistent iterations between the Green’s function, scattering, and the potential. The inner process is integrated over energy.

Figure 3

Current versus voltage characteristics for incoherent, near-elastic scattering in a silicon thin film for n=5×1018cm−3

Figure 4

Temperature-dependent current-voltage characteristics of a silicon thin film for a channel temperature difference of 0–30K without scattering: n=5×1018cm−3

Figure 5

Predicted versus measured (39) values of Seebeck coefficient for silicon at 300K as a function of doping levels

Figure 6

Predicted versus measured Seebeck (5-6,41-43) coefficient for ballistic Si(10nm)∕Ge(10nm) superlattice with doping

Figure 7

Change in S2σ with incoherent electron-phonon scattering for silicon thin films and Si∕Ge heterojunctions: Nd,Si=5×1018cm−3, Nd,SiGe=5×1018cm−3

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