Diminished thermal conductivity of Si/SiGe multilayers established through heating current frequency
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1172-T06-07
Diminished thermal conductivity of Si/SiGe multilayers, established through heating current frequency A. Dooraghi, P.R. Bandaru Materials Science Program, Department of Mechanical Engineering, University of California, San Diego, La Jolla, CA, 92093-0411, USA D. Krommenhoek, N. Elsner Hi-Z Technology Inc., San Diego, CA
ABSTRACT We report on the measurement of the thermal conductivity of Si/Si0.8Ge0.2 multilayers on Si substrates through a variation of the 3 method. We exploit the frequency dependent variation of the thermal wave, through invoking the thermal penetration depth (TPD), which is inversely proportional to the frequency. Consequently, spectral measurements covering decades of frequency were used to finely probe the substrate and the overlying Si and Si0.8Ge0.2 thin film layers. Both in-phase and out-of phase measurements yielded comparable values of the thermal conductivity in the range of 3-5 W/mK, much lower than the reported bulk values. Our results provide proof of the potential of multilayered media to be used for reduced thermal conductance applications such as thermoelectrics, heat insulation etc.
INTRODUCTION Lower dimensional thermoelectrics, such as superlattices and nanowires have been proposed1,2 to have a higher thermoelectric figure of merit ZT(= ), - a prime determinant of the heat conversion efficiency where S is the Seebeck coefficient, , the electrical conductivity, and , the thermal conductivity. Generally, has contributions from both charge carriers and the lattice – in this paper we mainly consider the latter contribution, L, as it typically dominates the total thermal conductivity. The increase in ZT in nanostructures was then predicted to arise from both an increase of the power factor- S2 , due to a net increase in the magnitude3 of the density of states (DOS) and through a reduction of the L. While a few experiments4,5 indicate an increase in the power factor most of the understood enhancement in ZT seems to arise from a reduced L. The reduction seems mainly to arise through a reduction of the mean free path, l, as 6 L~ Cvl (C: specific heat, and v: phonon velocity) and is manifested, for example, in bulk 7,8 nanocomposite thermoelectrics , Bi2Te3/Sb2Te3 superlattice devices9 with the highest recorded ZT (of ~ 2.4), and Si nanowires10. It is therefore imperative that methodology for the measurement of L be well developed and understood. In this paper, we focus on planar thin film based Si/Si0.8Ge0.2 multilayer structures. In thin films, the L values are generally anisotropic whichcan be understood through the dependence of L on l and also confirmed through various experimental measurements11, e.g., on GaAs/AlGaAs thin film superlattice structures, where a smaller l in the superlattice growth direction implies a smaller value of L in that direction compared to the value in the film plane.
2 However, it was observed that while the L of AlAs/GaAs superlattices12, comprised of an equal thickness of AlAs and GaAs indeed decreases with a decreasing layer thickness and
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