Far-infrared reflectance spectra of Si:P near the metal-insulator transition

Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the metal-insulator transition (MIT) were performed at low temperatures. In the metallic regime free-carrier absorption and, additionally, absorption peaks due to interband transitions from the impurity band to the conduction band and to transitions between the broadened valley-orbit split 1s states are observed. This gives clear evidence that the impurity band is formed by the overlap of the 1s(${\mathit{A}}_1$) ground states and is well separated from the conduction band when the MIT occurs. Only far beyond the metallic limit the impurity band merges completely with the conduction band.