Interband transitions iAs/AlAs quantum-well structures

We have studied the interband transitions from undoped and modulation-doped (Formula presented)(Formula presented)As/AlAs multiple-quantum-well structures (the latter were doped n type in the (Formula presented)(Formula presented)As layers). Two types of transitions have been observed: Type I associated with electron-hole recombination in the (Formula presented)(Formula presented)As layers and type II between electrons in the (Formula presented) and (Formula presented) valleys of the AlAs layers and holes confined in the (Formula presented)(Formula presented)As layers. In undoped samples the luminescence associated with type-II transitions is dominated by phonon replicas. In contrast, the intensities of the zero-phonon (Formula presented)(Formula presented) and (Formula presented) 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.