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 transit ions have been observed: Type I associated with electron-hole recombin ation in the Al0.25Ga0.75As layers and type II between electrons in th e 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. Unde r optical pumping the type-II transitions in undoped samples show a pr onounced blueshift due to band bending. Under high pumping intensity, the spectra from the undoped samples strongly resemble those from the modulation-doped structures.