Barrier-width dependence of optical transitions involving unconfined energy states in GaAs-AlxGa1−xAs superlattices

Abstract

Optical transitions between confined electron and hole states in GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As superlattices have been studied extensively. Transitions involving unconfined states, i.e., states above the conduction- or below the valence-band barriers, have not received much attention. Here, we report the results of photoluminescence-excitation spectroscopy in the energy range of the unconfined transitions. We find a variety of structures in the spectra, which can be interpreted with the aid of theoretically generated absorption spectra. The strength of the transition involving the first unconfined heavy-hole to first unconfined conduction state is found to depend strongly on the barrier width. For superlattices with 150-Å wells and barriers with aluminum concentrations of 20%, the transition is very strong for 150-Å barriers, but insignificant for barrier widths of 70 and 30 Å. Transitions involving the split-off valence bands are also observed.