Infrared Absorption of Localized Longitudinal-Optical Phonons

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Abstract
A new type of localized vibrational mode is shown to occur near donors and acceptors in polar semiconductors. In general, the mode appears in a solid with infrared-active lattice vibrations when an impurity center with appropriate electronic transition energies is introduced by doping. The polarizability of the center perturbs the dielectric function locally, shifting the longitudinal-optic phonon. Infrared-reflection data showing the new localized mode are presented for GaP and GaAs. A macroscopic Clausius-Mosotti-type theory is developed for the effective dielectric function of the solid including the spheres. This theory yields good fits to the Raman data of Dean et al. and to the present infrared data.