Infrared Reflectivity of Doped InSb and CdS

Abstract

The coupling of free carriers and lattice-vibrational modes in CdS and InSb was investigated by using reflectance measurements. When the free-carrier concentration is such that the longitudinal-optical-mode frequency is at or near the plasma frequency, the longitudinal mode takes on a mixed character and is shifted in a manner similar to that described by Varga in 1965, while the transverse mode remains fixed. The electron effective masses in such a situation can be determined with the lattice effects excluded. In the case of CdS, the polaron mass and additional absorption in the plasma mode above the longitudinal-opticalphonon frequency must be taken into account.