Optical absorption, transmission, luminescence, and Raman scattering in n-type material were analyzed at hydrostatic pressures of up to 30GPa. The results revealed a freeze-out of free electrons, at pressures above about 20GPa, due to trapping at levels that were resonant at ambient pressures but became gap states at high pressures. The use of  ab initio  calculations showed that both the N vacancy and the Ga interstitial underwent this transition at about 20GPa, but the vacancy was expected to be more abundant. The pressure dependence of the yellow luminescence indicated that a transition between a shallow donor and a deep acceptor was involved.

P.Perlin, T.Suski, H.Teisseyre, M.Leszczynski, I.Grzegory, J.Jun, S.Porowski, P.Boguslawski, J.Bernholc, J.C.Chervin, J.A.Polian, T.D.Moustakas: Physical Review Letters, 1995, 75[2], 296-9