Vibrational Properties of GaP and GaP1-xNx under Hydrostatic Pressures up to 30 GPa

We report on Raman scattering of the vibrational modes of GaP and the dilute nitride ternary GaP_0.79N_0.021 under hydrostatic pressures of up to 30 GPa. We measured the pressure induced shift of the TO and LO modes of GaP up to the phase transition (I-II) and determined mode Grüneisen parameters of 1.12 and 0.98. The phase transition was observed near to 24.5 GPa. We have also studied the dependence of the vibrational spectrum of the GaP_0.97N_0.021 up to 20 GPa. The zincblende optical phonons of the ternary alloy show great similarities to those of binary GaP, with no significant changes to the Grüneisen parameters or the phase transition. The pressure-induced shift of local nitrogen mode frequency in GaP_0.979N_0.021 was studied up to 10 GPa and found to be considerably larger than that of the phonon modes in GaN, indicating that the local mode does not show a GaN-like behaviour under pressure.