Far infrared magneto-absorption of hydrogenic donors in GaAsAl_0.3G_0.7As quantum wells

Far infrared magneto-absorption measurements have been performed on a 375 Å GaAsAl_0.3Ga_0.7As multiple quantum well in which shallow donors have been selectively doped at the well edges. The measurements were made with a Fourier spectrometer in conjunction with a 9 T superconducting magnetic. With the applied magnetic field parallel to the superlattice growth direction, reductions in the 1s-2p intraimpurity transition energy relative to the three-dimensional limit are observed, in agreement with recent theoretical predictions. However, measurements made with the magnetic field slightly inclined from the growth direction deviate from the expected behavior near magnetic fields where the energy of the 2p⁺ state crosses the second subband. These deviations are attributed to mixing of the 2p⁺ impurity state associated with the lowest subband with impurity states derived from the second subband. The experimental results are compared with a simple calculation which treates the coupling of the impurity states perturbatively.