Phonon Structures in Optical Spectra of Layer Compounds GaSe and GaS

The infrared and far-infrared absorption spectra of β-GaSe and GaS have been measured in the temperature range from 50 to 300°K for the radiation polarized perpendicular to the c -axis in the hexagonal layer structure of these compounds. The reststrahlen frequencies measured at room temperature are 211.0 cm -1 in β-GaSe and about 310 cm -1 in GaS, respectively. In addition, the weak absorptions in the wave number range from 400 cm -1 to 750 cm -1 are identified with the multiphonon absorptions in the light of group theoretical analysis and of the experimental results on the Raman effect and on the electro-luminescence available up to date. The absorption line at 512 cm -1 and band around 450 cm -1 in β-GaSe are explained in terms of the localized mode of lattice vibrations induced possibly by an oxygen impurity. This analysis is the first preliminary investigation on the localized mode in a diatomic crystal of hexagonal structure.