THERMAL-ACTIVATION OF CARRIERS FROM A METALLIC IMPURITY BAND

Transport and far-infrared transmission measurements are reported on S i-doped GaAs epitaxial films (n greater than or similar to the Mott de nsity no). The spectra display a 1s --> 2p resonance and a Drude respo nse that persists to low temperatures. An analysis of the temperature dependence of the spectra gives the activation energy for thermal exci tation of carriers from the metallic impurity band to the conduction b and. The effective mass of the impurity band m(i) is obtained from opt ical sum-rule considerations. m(i) appears to diverge at the metal-ins ulator transition. The large m(i) implies a correlation-induced narrow ing of the impurity band. The activation energy E(a) and effective mas s m(i) obtained from optical measurements are consistent with the resu lts obtained from transport measurements. The transmission spectra of Si:P and planar Si-doped GaAs/AlxGa1-xAs superlattices are also discus sed.