Temperature dependence of Raman scattering in hexagonal gallium nitride films

First-order Raman scattering of hexagonal GaN single crystal films deposited on sapphire substrate by low pressure metal organic chemical vapor deposition is studied between 78 and 870 K. The temperature dependence of the five GaN Raman modes is obtained. Both the linewidth and Raman shift exhibit a quadratic dependence on temperature in our measured temperature range. Excellent agreement was found between the experiment data and calculated results based on a model involving three- and four-phonon coupling. Our results indicate it is necessary to include the contributions of both the thermal expansion and four-phonon terms in the four-phonon anharmonic processes to explain the change of Raman shift and linewidth with temperature. In addition, a decrease in the splitting between the longitudinal optical and transverse optical phonons with increasing temperature was also observed. From these data a weak nonlinear decrease of the transverse effective charge with increasing temperature is derived. The comparison of the transverse effective charge eT* at room temperature was made between experimental data and theoretical calculations by a pseudopotential expression and bond orbital model. Good agreement between theory and experiment is achieved.