Raman study of the vibrational modes in ZnGeN2 (0001)

A Raman spectroscopy study was carried out for ZnGeN2 with direction of propagation along the (0001) crystallographic direction on hexagonal single crystal platelets obtained by reaction of gaseous ammonia with a Zn-Ge-Sn liquid alloy at 758 degree C. The sample geometry allowed measurement of the a2 and a1 Raman modes. First-principles calculations were carried out of the spectra. Measurements with crossed polarizers yielded spectra that agreed well with first-principles calculations of the a2 modes. Measurements with parallel polarizers should in principle provide the a1L modes. However, for most of the Raman modes, the LO-TO splitting was calculated to be very small, and for the few modes which were predicted to have larger LO-TO splittings, the LO mode was not observed. This absence is tentatively explained in terms of overdamped LO-plasmon coupling. LO-TO mode crossing was identified by comparing the calculated eigenvectors and intensities of individual modes. Some features in the experimental spectra were identified as arising from critical points in the phonon density of states and are indicative of the degree of disorder on the cation lattice.