  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.
