Material considerations for thermoelectric enhancement via modulation doping

Abstract

Modulation doping has recently emerged as a method for improving the thermoelectric figure of merit using composite materials. By spatially decoupling charge carriers from their dopant impurities, the average carrier mobility can be improved by reducing the influence of ionized impurity scattering. However, as we show in the present work using a simple parabolic band model and effective medium approach, enhancement in such composites is effective only if ionized impurities dominate the scattering of the charge carriers, which is often not the case in many materials. For example, the enhancement is more significant at lower temperatures (T < 300 K for our model material) where acoustic phonon scattering begins to freeze out. Furthermore, for large dielectric constant, which is common for good thermoelectric materials, the ionized impurities are largely screened, and formation of the modulation-doped composite is likely to have little to no benefit, or possibly even degrade the thermoelectric performance of the material. The effective mass of the charge carriers also plays an important role in the relative strength of the scattering mechanism and the possibility for thermoelectric enhancement through modulation doping.