  GaN is a wide-bandgap semiconductor used in high-efficiency LEDs and solar
cells. The solid is produced industrially at high chemical purities by
deposition from a vapour phase, and oxygen may be included at this stage.
Oxidation represents a potential path for tuning its properties without
introducing more exotic elements or extreme processing conditions. In this
work, ab initio computational methods are used to examine the energy potentials
and electronic properties of different extents of oxidation in GaN. Solid-state
vibrational properties of Ga, GaN, Ga2O3 and a single substitutional oxygen
defect have been studied using the harmonic approximation with supercells. A
thermodynamic model is outlined which combines the results of ab initio
calculations with data from experimental literature. This model allows free
energies to be predicted for arbitrary reaction conditions within a wide
process envelope. It is shown that complete oxidation is favourable for all
industrially-relevant conditions, while the formation of defects can be opposed
by the use of high temperatures and a high N2:O2 ratio.
