Temperature dependence of the far-infrared reflectance spectra of Si: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×{}10¹⁷ cm^-3 and 7.4×{}10¹⁹ cm^-3, thus including the metal-insulator transition occurring near Nc=3.5×{}10¹⁸ 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(A₁) ground state and the closely spaced 1s(E) and 1s(T₂) states. The occupation of the 1s 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.