Angular behaviour of the polar optical phonons in AIIIBVI layered semiconductors

The behaviour of the infrared‐active lattice vibrations of GaS, GaSe, InSe are studied by measuring the room‐temperature reflectivity in the 100 to 500 cm −1 frequency range when the incident wave vector is inclined at an angle θ to the crystallographic Z ‐axis. The experiments are performed in polarized light. The transverse and longitudinal mode frequencies are obtained by Kramers‐Kronig analysis of the reststrahlen bands observed in these crystals with θ between θ and 90°. The experimentally observed changes of the lattice frequencies with an angle θ are interpreted on the basis of the theory of long‐wavelength lattice vibrations in anisotropic crystals. Good agreement between the experimentally observed dependence and the calculated one according to the theory gives the possibility to elucidate the nature of a fine structure observed in the reststrahlen region of the layered GaS, GaSe, InSe crystals.