Interband transitions in AlxGa1-xAs/AlAs quantum-well structures

We have studied the interband transitions from undoped and modulation-doped Al0.25Ga0.75As/AlAs multiple-quantum-well structures (the latter were doped n type in the Al0.25Ga0.75As layers). Two types of transitions have been observed: Type I associated with electron-hole recombination in the Al0.25Ga0.75As layers and type II between electrons in the Xxy and Xz valleys of the AlAs layers and holes confined in the Al0.25Ga0.75As layers. In undoped samples the luminescence associated with type-II transitions is dominated by phonon replicas. In contrast, the intensities of the zero-phonon X(xy)h(1) and X(z)h(1) transitions in modulation-doped samples are comparable to those of the replicas. Under optical pumping the type-II transitions in undoped samples show a pronounced blueshift due to band bending. Under high pumping intensity, the spectra from the undoped samples strongly resemble those from the modulation-doped structures.