Effect of strain on the thermoelectric properties of silicon: an ab initio study

On the basis of detailed first-principles calculations the anisotropic thermoelectric transport properties of biaxially strained silicon were studied with the focus on a possible enhancement of the power factor. Electron as well as hole doping was examined in a broad doping and temperature range. In the low temperature and low doping regime an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case, and for compressive strain in the hole-doped case. In the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found for the hole-doped case under small biaxial tensile strain. The results are discussed in terms of band structure effects. An analytical model is presented to understand the fact that the thermopower decreases if degenerate bands are energetically lifted due to a strain-induced redistribution of states.