Thermal activation of carriers from a metallic impurity band

Transport and far-infrared transmission measurements are reported on Si-doped GaAs epitaxial films (n the Mott density n0). The spectra display a 1s→2p resonance and a Drude response that persists to low temperatures. An analysis of the temperature dependence of the spectra gives the activation energy for thermal excitation of carriers from the metallic impurity band to the conduction band. The effective mass of the impurity band mi is obtained from optical sum-rule considerations. mi appears to diverge at the metal-insulator transition. The large mi implies a correlation-induced narrowing of the impurity band. The activation energy Ea and effective mass mi obtained from optical measurements are consistent with the results obtained from transport measurements. The transmission spectra of Si:P and planar Si-doped GaAs/AlxGa1-xAs superlattices are also discussed.