Localized vibrational modes in GaN:O tracing the formation of oxygenDX-like centers under hydrostatic pressure

Vibrational modes are observed at ambient pressure in O-doped GaN at $544{\mathrm{cm}}^{-1}$ using Raman spectroscopy. Investigation of these modes with applied hydrostatic pressure reveals the existence of three closely spaced modes that shift in relative intensity with increasing pressure. Notably, transitions between the different modes occur at previously observed electronic transitions associated with the $\mathrm{DX}$-like center behavior of substitutional O on the N site. A simple one-dimensional oscillator model is used to extract approximate force constants; these are consistent with the assignment of the $544{\mathrm{cm}}^{-1}$ mode to $O_{\mathrm{N}}$ and with force constants for C and B dopants in GaP. The relative intensity changes observed at 11 and 17 GPa are assigned to changes in the charge state due to the merging of the +/0 ionization level and the Fermi energy and the transition to ${\mathrm{DX}}^{-}$ that causes a previously observed drop in free electron concentration, respectively.