Role of substrate strain to tune energy bands–Seebeck relationship in semiconductor heterostructures

In doped semiconductors and metals, the thermopower decreases with increasing carrier concentration, in agreement with the Pisarenko relation. Here, we demonstrate a new strain engineering approach to increase the thermopower of [001] Si/Ge superlattices (SLs) beyond this relation. Using two independent theoretical modeling approaches, we show that new bands form due to the structural symmetry, and, the SL bands are highly tunable with epitaxial substrate strain. The band shifts lead to a modulated thermopower, with a peak $\sim$5-fold enhancement in strained Si/Ge SLs in the high doping regime.