Pressure and temperature dependences of the raman-active phonons in CuGaS 2

Abstract

The ternary compound, CuGaS 2 is a direct energy gap semiconductor ( E g ~ 2.50 eV at 300 K) that crystallizes in the tetragonal chalcopyrite ( I 4 ̄ 2d ) structure and its isoelectronic cubic analog is ZnS. The temperature dependence (10–300 K) of the Raman active phonons has been studied up to 18 GPa in a membrane diamond anvil cell, using helium gas as the pressure transmitting medium. These measurements allowed us to determine the temperature dependence of the mode Grüneisen parameters and to separate the isobaric temperature dependence of the optical modes into pure-volume and pure-temperature contributions. By this procedure, the cubic and quartic anharmonicities responsible for the pure-temperature contributions to the mode frequencies were determined. The mode Grüneisen parameters of the center-of-the-zone Γ 5 1 mode (which corresponds to the edge-of-the-zone TA in ZnS) is negative throughout the whole temperature range, reaching − 0.64 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors.