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

Vibrational modes are observed at ambient pressure in O-doped GaN at 544 cm (-1) using Raman spectroscopy. Investigation of these modes with applied hy drostatic 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 transi tions associated with the DX-like center behavior of substitutional O on th e N site. A simple one-dimensional oscillator model is used to extract appr oximate force constants; these are consistent with the assignment of the 54 4 cm(-1) mode to ON and with force constants for C and B dopants in GaP. Th e relative intensity changes observed at 11 and 17 GPa an assigned to chang es in the charge state due to the merging of the +/0 ionization level and t he Fermi energy and the transition to DX- that causes a previously observed drop in free electron concentration, respectively.