The mechanism of heavy and light hole mixing in GaAs/AlAs superlattices

Abstract

We show that by investigating the exciton magnetoluminescence of type II GaAs/AlAs superlattices one can establish the nature of the additional exchange splitting, observed experimentally in such superlattices. This splitting is attributed to the symmetry reduction from D 2d to C 2v. At present, there are two alternative models, explaining this phenomenon. One model assumes that the symmetry is reduced in the GaAs layers, for example, due to presence of local deformations, which lead to the mixing of the heavy hole (hh) and light hole (lh) states. In the other model the similar hh-lh mixing was attributed to the effect of heterointerfaces, which have symmetry lower than the structure itself. In this model the exchange splitting appears only if the exciton is localised near one GaAs/AlAs interface. Both models predict the existence of the two kinds of the excitons in nearly equal amounts, with splitting of the same magnitude but different sign. We have shown that the splitting of the excitonic states in a magnetic field, parallel to the plane of the superlattice, is very different in both models. As a result of the additional exchange splitting, the luminescence under the excitation, polarized along the directions (100) or (010), is non-polarized. However, the polarization of the luminescence reappears at certain values of the magnetic field, when the energies of two excitonic states became equal. We show that determination of those values allows one to choose between the two models.