Optical evidence of staggered band alignments in (Al,Ga)As/AlAs multi-quantum-well structures.

Photoluminescence spectra from Al(x)Ga(1-x)As/AlAs multi-quantum- well structures are presented which provide the first direct optical confirmation of staggered band alignments in this technologically important material system. Above a critical Al concentration, the X minima are expected to become the lowest-energy conduction- band states in the heterostructure, so that photoexcited electrons and holes are separated and are trapped in alternate layers. In this regime we observe photoluminescence spectra which occur below the bandgap of either of the individual materials in the samples and exhibit properties characteristic of band-to- band recombination. We attribute this emission to the spatially- indirect recombination of electrons confined to the AlAs X minima and holes in the Al(x)Ga(1-x)As heavy-hole valence band.