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 $\sim 20\mathrm{GPa}$ 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 $\sim 20\mathrm{GPa}$, 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.