Stabilization of amorphous GaN by oxygen
  Ion assisted deposition (IAD) has been investigated for the growth of GaN, and the resulting films studied by x-ray diffraction and absorption spectroscopy and by transmission electron microscopy. IAD grown stoichiometric GaN consists of random-stacked quasicrystals of some 3 nm diameter. Amorphous material is formed only by incorporation of 15% or more oxygen, which we attribute to the presence of non-tetrahedral bonds centered on oxygen. The ionic favourability of heteropolar bonds and its strikingly simple constraint to even-membered rings is the likely cause of the instability of stoichiometric a-GaN. 
F. Budde, B.J. Ruck, A. Koo, S. Granville, H.J. Trodahl (1), A.   Bittar, G.V.M. Williams (2), M.J. Ariza (3), B. Bonnet, D....