Optical Phonons in GaS 1‐x Se x Layer Mixed Crystals

Measurements are made of polarized Raman scattering and infrared reflection spectra of twenty two compositions of GaS 1 ‐ x Se x layer mixed crystals grown by the Bridgman method. Besides the usual one‐mode and two‐mode behaviours of phonon modes, a number of more complex dependences characterized by the appearance of additional bands in the spectra of mixed crystals are observed. Their appearance is accounted for by the uncovering of the Brillouin zone due to the disturbance of the periodicity of the crystal lattice of mixed crystals. Specifically, for modes with a low dispersion in the Brillouin zone the additional modes are interpreted as vibrations of cells having mixed compositions. A number of anomalies are revealed in the concentration dependences of intensities and frequencies of optical phonons, which are attributed to the β → γ and γ → ϵ polytype transformations at x ≈ 0.6 and x ≈ 0.97, respectively.