Photoreflectance investigations of GaAs/AlGaAs complex structures

Photoreflectance spectroscopy has been applied to investigate MBE grown GaAs/AlGaAs low dimension structures. The first is an HEMT type structure with buried 10 periods of 2.5 nm GaAs/2.5 nm AlGaAs superlattice. Oscillation-like signals associated with this SL have been observed and analysed in terms of the Franz-Keldysh oscillations (FKO) related to the presence of free carriers in the miniband structure. On the basis of these FKO we have evaluated the electric field in the SL region of the structure. This value is almost identical to the value obtained from FKO for the GaAs channel. The second structure, consisting of a sequence of 10 quantum wells, differs in the width of well from 3 up to 30 monolayers (ML) and with the same barrier thickness (96 ML). For the PR spectra at 300 K and 90 K, transitions in almost all wells have been observed. Energies of the observed transitions are in good agreement with the results of numerical calculations, based on the envelope function approximation for rectangular wells. Transitions associated with above barrier states of amplitude comparable to the transitions involving only bound states are also observed in our PR spectra. (C) 1997 Elsevier Science Ltd.