Correlation of vibrational modes and DX-like centers in GaN :O

Vibrational modes in O-doped GaN have been observed at 544 cm~1 in Raman spectroscopy. Under perturbation of large hydrostatic pressure the mode appears as a set of three di!erent lines Q 123 whose relative intensities change by pressure. A switching between the modes occurs near 10 and 20 GPa and is found to correlate with the electron capture process to the DX-like state of O. We employ a simple oscillator model to predict the vibrational frequencies of O N . A localization energy of 23 cm~1 with respect to the optical phonon band is predicted. This is in reasonable agreement with the observed vibrational frequencies. Therefore, we assign the Q modes to the local vibration of O on N site in GaN. Modes Q 123 are tentatively assigned to three di!erent charge states of the O defect center. ( 1999 Elsevier Science B.V. All rights reserved.