Infrared absorption in superlattices: A probe of the miniband dispersion and the structure of the impurity band.

Infrared spectroscopy is used to study optical transitions between the two lowest minibands in strongly coupled n-type GaAs/Al x Ga 1-x As superlattices. Two absorption maxima, related to the critical points of the mini-Brillouin zone, are clearly observed. The asymmetric shape of the spectrum can be reproduced theoretically, when the variation of the oscillator strength over the mini-Brillouin zone is taken into account. In a superlattice which is doped not too heavily, but metallic, a third absorption line appears at low temperatures. It can be identified with the 1s-2p z donor transition and provides a sensitive probe to investigate the density of states near the metal-insulator transition