Towards the Identification of the Dominant Donor in GaN

We analyze optical absorption, transmission, luminescence, and Raman scattering in n-type GaN at hydrostatic pressures up to 30 GPa. The results show freeze-out of free electrons above 20GPa due to trapping at levels that are resonant at ambient pressure but become gap states at high pressures. Ab initio calculations show that both the N vacancy and the Ga interstitial undergo this transition at 20GPa, but the vacancy should be more abundant. The pressure dependence of the yellow luminescence indicates that a transition between a shallow donor and a deep acceptor is involved.