Effect of band anisotropy on phonon-drag thermopower in AlAs quantum wells: Strong enhancement of phonon drag

Abstract

Wepresentadetailedtheoreticalframeworkforthecalculationofthermopower, S,intwo-dimensionalelectron gases confined in (001) AlAs quantum wells (QWs) taking into account the band anisotropy. Particular emphasis is given on the phonon-drag contribution S g that is related to the momentum exchange between acoustic phonons and electrons in the presence of a weak in-plane temperature gradient via the electron-phonon coupling. Our model is based on the semiclassical Boltzmann formalism and is a generalization of previous models that are applicable only for an isotropic energy spectrum. We find that the electron anisotropy in AlAs affects strongly the electron-acoustic phonon coupling and S g . Considerable enhancement of S g is reported in relation to isotropic GaAs QWs. More interestingly, we find that strong anisotropy in phonon-drag thermopower occurs by tuning the valley occupancy. Namely, the predicted S g and S g can differ by over a factor of 5 while a giant magnitude for S g can be achieved in a particular direction exceeding 10 mV/ Ka tT = 5 K. The diffusion thermopower is also calculated for comparison.