Raman study of the vibrational modes in ZnGeN_2 (0001)

摘要

A Raman spectroscopy study was carried out on ZnGeN_2 hexagonal single crystal (0001)-oriented platelets obtained by reaction of gaseous ammonia with a Zn-Ge-Sn liquid alloy at 758 °C. The sample geometry allowed measurement of the A_2 and A_1 Raman modes. First-principles calculations of the spectra were carried out using an improved pseudopotential. Measurements with crossed polarizers yielded spectra that agreed well with first-principles calculations of the A_2 modes. Measurements with parallel polarizers should in principle provide the A_(IL) modes. However, for most of the Raman modes, the LO-TO splitting was calculated to be smaller than could be resolved experimentally, 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.