Barrier - Width - Dependence of optical transitions involving unconfined energy states in GaAs/Al(x)Ga(1-x)As superlattices.

Optical transitions between confined electron and hole states in GaAs/Al(x)Ga(1-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 150angstrom wells and barriers with aluminum concentrations of 20%, the transition is very strong for 150angstrom barriers, but insignificant for barrier widths of 70angstroms and 30angstroms. Transitions involving the split-off valence bands are also observed.