Far-infrared studies of doped AlGaAs/GaAs multiple-quantum-well structures

Far infrared spectroscopic studies of MBE-grown, selectively Si-doped multiple-quantum-well (MQW) structures have been carried out between 4.2K and 70K in magnetic fields up to 9T. Magnetotransmission measurements on a sample doped both in the centers of the barriers and the centers of the wells show strong features attributed to electrons in the wells bound to their parent donor ions in the barriers. Samples doped at the edges of the wells at the two different interfaces exhibit dramatically different absorption line profiles. This provides evidence for redistribution of impurities along the growth direction on a scale of 30 - 40 A°. Optical pumping experiments on center-doped MQW samples show large excess free electron concentrations at modest pump intensities while the density of neutral donors remains unchanged. These results are interpreted in terms of the existence of substantial densities of compensating acceptor-like states. Experiments on a center-doped 450 A° well-width sample with the magnetic field in the plane of the sample (Voigt geometry) have permitted measurements of the 2p state associated with the first excited confinement subband.