  We analyze the cross-plane miniband transport in n-doped [001] silicon
(Si)/germanium (Ge) superlattices using an effective mass approximation (EMA)
approach that correctly accounts for the indirect nature of the Si and Ge band
gaps. Direct-gap based EMA has been employed so far to investigate the
electronic properties of these superlattices, that does not accurately predict
transport properties. We use the Boltzmann transport equation framework in
combination with the EMA band analysis, and predict that significant
improvement of the thermopower of n-doped Si/Ge superlattices can be achieved
by controlling the lattice strain environment in these heterostructured
materials. We illustrate that a remarkable degree of tunability in the Seebeck
coefficient can be attained by growing the superlattices on various substrates,
and varying the periods, and the compositions. Our calculations show up to
~3.2-fold Seebeck enhancement in Si/Ge [001] superlattices over bulk silicon,
in the high-doping regime, breaking the Pisarenko relation. The thermopower
modulations lead to an increase of power factor by up to 20%. Our approach is
generically applicable to other superlattice systems, e.g., to investigate
dimensional effects on electronic transport in two-dimensional nanowire and
three dimensional nanodot superlattices. A material with high S potentially
improves the energy conversion efficiency of thermoelectric applications and
additionally, is highly valuable in various Seebeck metrology techniques
including thermal, flow, radiation, and chemical sensing applications. We
anticipate that the ideas presented here will have a strong impact in
controlling electronic transport in various thermoelectric, opto-electronic,
and quantum-enhanced materials applications.
