Strain effect analysis on the thermoelectric figure of merit in n-type Si/Ge nanocomposites

Abstract

In this paper, the effect of strain on the thermoelectric figure of merit is investigated in n-type Ge nanowire-Si host nanocomposite materials. The Seebeck coefficient and electrical conductivity of the Si–Ge nanocomposites are calculated using an analytical model derived from the Boltzmann transport equation (BTE) under the relaxation-time approximation. The effect of strain is incorporated into the BTE through the strain induced energy shift and effective mass variation calculated from the deformation potential theory and a degenerate k·p method at the zone-boundary X point. The effect of strain on the phonon thermal conductivity in the nanocomposites is computed with a model combining the strain dependent lattice dynamics and the ballistic phonon BTE. The electronic thermal conductivity is computed from the electrical conductivity using the Wiedemann-Franz law. Normal and shear strains are applied in the transverse plane of the Si–Ge nanocomposites. Thermoelectric properties, including the electrical conductivity, thermal conductivity, Seebeck coefficient, and dimensionless figure of merit, are computed for Si–Ge nanocomposites under these strain conditions.

Significant enhancement in the thermoelectric performance of strained nanoporous Si

“…On the other hand, Xu and Li investigated the effect of the strain types on the thermoelectric properties of Si-Ge nanocomposites based on the deformation potential theory and a degenerate kÁp method. 23 This study showed that the room-temperature ZT decreases by as large as 40% under 1% of shear or biaxial strain, whereas it exhibits around 10% increase when orthorhombically strained, but the origin of the difference was not identified.…”

“…However, Xu and Li demonstrated that k l under external strain deviates from the unstrained value only by 2% or less, and that the deviation is smaller for S o than S b or S s . 23 Based on this observation, k l is approximated as the unstrained value (0.63 W m À1 K À1 (ref. 10)), and the results are presented in Fig.…”

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…A theoretical study of such an approach was performed for Si/Ge composites in which an ordered array of nanoscale Si wires was embedded into a Ge matrix, and it was found to be rather promising. 185,186 Some of the other parameters mentioned above, in particular electric and magnetic fields, have rarely been considered, even in ambient-pressure studies of thermoelectric materials. However, both of these parameters have great potential to enhance the Seebeck coefficient, via the two-dimensional electron gas induced by an applied electric field [187][188][189] or via the strong magnetothermoelectric effects induced by an applied magnetic field.…”

Thermoelectric Transport in Nanocomposites

“…Other methodologies to engineer the band structure or scattering mechanism in NCs that do not require rigorous periodic geometric structures are also proposed. For example, Xu and Li [ 193 ] used the deformation potential theory and a degenerate k·p method to investigate the strain effect of n -type Si/Ge NCs which could induce energy band shift and effective mass variation. Popescu and Woods [ 194 ] investigated electronic structure modified by the locally distorted DOS induced by spherical nanoinclusions embedded in bulk PbTe.…”