Complemental theory for vertical transport in semiconductor superlattices

In this paper, we present a unified picture which can describe transport phenomena independent on applied electric field. Based on the unified picture and by means of the Monte Carlo simulation, we calculate drift velocities of electrons in a superlattice, taking LO phonon scattering and impurity scattering into account. When impurity density is low or electric field is high, electrons are easily accelerated to the edge of the Brillouin zone, and Bloch oscillation is realized. As a result of the Bloch oscillation, localized Stark-ladder states are formed and hopping transport between the Stark-ladder states is realized. With increasing impurities, electrons are scattered frequently and delocalization of Stark-ladder states takes place. In this case, band transport described in the momentum-space becomes important. Crossover between band transport and hopping transport is investigated and discussed.