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(A1) 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.