Modeling of thermoelectric properties of SiGe alloy nanowires and estimation of the best design parameters for high figure-of-merits
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© 2015 AIP Publishing LLC. We present comprehensive thermoelectric transport models of SiGe nanowires based on the Boltzmann transport theory with the relaxation time approximation to calculate electrical conductivity, thermopower, and thermal conductivity at a wide range of temperature up to 800 K. Our model does not only accurately reproduce the experimental data of SiGe nanowires but also predict the best possible thermoelectric performance and the optimum conditions. In particular, non-ionized impurities, whose concentration is often significant in heavily doped (or degenerate) semiconductors, were introduced to correct the discrepancies between the experimental electrical conductivity of SiGe nanowires and calculated values obtained from earlier models. Our models also considered bipolar thermal conductivity and separate longitudinal and transverse phonon modes as well as employed adjusted cutoff frequencies to minimize the errors associated with the linear approximation of the phonon dispersion. With optimal ionized impurity concentrations without non-ionized impurities, ZT of a Si0.73Ge0.27 nanowire was found to be as high as 1.3 at 800 K. In case that the diameter of the Si0.73Ge0.27 wire is reduced down to 10 nm, it may be possible to have an even larger ZT of 1.9 at 800 K. We believe our comprehensive models are useful for predicting thermoelectric properties of various semiconductor nanowires at a wide range of temperature, which can guide experiments to develop high performance thermoelectric materials at desired temperatures.
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