Temperature dependence of the far-infrared reflectance spectra of Si2:P near the metal-insulator transition

We report on far-infrared reflection measurements on Si:P in the temperature range between 10 and 300 K for doping concentrations between N = 3.4 x 10(17) cm(-3) and 7.4 x 10(19) cm(-3), thus including the metal-insulator transition occurring near N-c = 3.5 x 10(18) cm(-3). At 300 K all samples show only the well-known free-carrier absorption where the carriers are thermally activated into the conduction band. At 10 K, for samples with very low-doping concentrations, optical transitions to donor 2p states and to the conduction band are observed. Metallic samples show at 10 K interband transitions giving evidence for the existence of central-cell effects in the metallic phase, i.e., transitions between the 1s(A(1)) ground state and the closely spaced 1s(E) and 1s(T-2) states. The occupation of the Is and the conduction-band states as a function of temperature can be deduced from the oscillator strengths of the corresponding interband transitions. These results strongly suggest that at the metal-insulator transition, the impurity band is still separated energetically from the conduction band.