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

Abstract

Photoluminescence spectra from AlxGa1−xAs/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 band gap 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 AlxGa1−xAs heavy-hole valence band. This transition also provides a direct and highly accurate measure of the valence-band offset without requiring precise knowledge of other system parameters such as the Fermi energy, effective masses, and exciton or dopant binding energies. The confirmation of a staggered band alignment has important implications for future device applications.