Far-Infrared Impurity Absorption in Highly Doped<i>n</i>-Type Silicon

Abstract

Far-infrared absorption spectra of uncompensated phosphorus-doped silicon with donor concentrations between 6.8×10 17 and 2.0×10 18 cm -3 have been measured at 6 K. These crystals show an absorption edge in the photon energy region lower than the lowest (1s→2p) excitation line of isolated phosphorus atoms. The position of this absorption energy shifts to lower energy with increasing donor concentration. The thermal donor ionization energy ε 1 determined from measurements of the electrical property is situated near the low-energy threshold in the absorption cross-section spectrum, and therefore the absorption is ascribed to the ionization transition of donor electrons into the conduction band states perturbed by interactions with impurities. The absorption cross-section spectrum shows no anomaly at the photon energy equal to the thermal activation energy ε 2 , which is characteristic of the intermediate concentration range of impurity conduction.