Observation of a metallic impurity band inn-type GaAs

Magnetotransport and far-infrared spectroscopy are combined to probe the electronic states in the vicinity of the magnetic-field-induced metal-insulator transition in n-type GaAs. Resonant absorption lines, identified as the shallow-donor 1s-2p transitions by their selection rules, magnetic-field dependence, and temperature dependence are observed even in the metallic state. These results demonstrate that, near the metal-insulator transition, the electrons are in the donor impurity band, which is split off from the conduction band.