Γ-X Mixing in GaAs/AlAs Superlattices

Abstract

There is a growing interest towards understanding the effect of two-dimensional confinement on the symmetry properties of the Bloch wavefunctions of carriers in superlattices (SLs) and quantum wells (QWs). In particular, recent theoretical studies have evaluated the possibilities of obtaining direct or "pseudo-direct" Si/Ge superlattices although both constituents are indirect, through the combined effects of strain and superperiodicity [1]. Useful insights in this still scarsely documented field can be obtained from the more conventional GaAs/AlAs system, in which one of the constituents is direct and the other one is not. It has been reported recently [2,3] that such superlattices could be indirect provided that the GaAs layer was thin enough. We will show here that they can further be continuously tuned from indirect to direct by means of an externally applied electric field. This experiment and others have allowed us to demonstrate that in such superlattices the conduction minimum is a combination of X and Γ states, with a degree of admixture related to the superperiodicity. Possible mechanisms responsible for this mixing will be discussed.