Strain Controlled Modulations and Anomalies in the Thermopower of Si/Ge Superlattices: A First-Principles Study

The thermopower/Seebeck coefficient (S) of metals and heavily doped semiconductors usually decreases monotonically with increasing carrier concentration, n_e, following the Pisarenko relation (PR). Heterostructures, such as III-V semiconductor superlattices (SLs), have been shown to display oscillatory S, deviating from the PR. Interestingly, this behavior has not been observed in highly technologically relevant n-doped Si/Ge SLs. Here, we demonstrate a strong oscillatory behavior of the cross-plane S of n-doped Si/Ge SLs with n_e, deviating from the PR, using symmetry, composition, and strain engineering. We use the density functional theory (DFT) and the effective mass approximation, independently, in combination with the Boltzmann transport equation framework, to establish the results. We predict 5.4- and 1.8-fold enhancements in the cross-plane S and the power factor of Si/Ge SLs, respectively, in the high doping regime, compared to bulk Si. In addition, our DFT study shows that cross-plane S displays anomalous sign-changing nature within the conduction miniband regime. This study will open up research directions to use strain-engineered bands to control electronic properties for various heterostructures.