Thermodynamics, structure and kinetics in the system Ga–O–N

Within the ternary system Ga–O–N we performed experimental and theoretical investigations on the thermodynamics, structure and kinetics of new stable and metastable compounds.We studied the ammonolysis of β-Ga2O3 at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction, and in situ X-ray absorption spectroscopy (XAS). From total diffraction pattern refinement with the Rietveld method we analyzed the anionic occupancy factors and the lattice parameters of β-Ga2O3 during the reaction. Within the detection limits of these methods, we can rule out the existence of a crystalline oxynitride phase that is not derived from wurtzite-type GaN. The nitrogen solubility in β-Ga2O3 was found to be below the detection limit of about 2–3 at.% in the anionic sublattice. The kinetics of the ammonolysis of β-Ga2O3 to α-GaN and of the oxidation of α-GaN to β-Ga2O3 was studied by means of in situ X-ray absorption spectroscopy. In both cases the reaction kinetics could be described well by fitting linear combinations of β-Ga2O3 and α-GaN spectra only, excluding that other crystalline or amorphous phases appear during these reactions. The kinetics of the ammonolysis can be described well by an extended Johnson–Mehl–Avrami–Kolmogorow model with nucleation and growth of GaN nuclei, while the oxidation kinetics can be modeled by a shrinking core model where Ga2O3 grows as a layer. Investigations by means of TEM and SEM support the assumptions in both models.To investigate the structure and energetics of spinel-type gallium oxynitrides (γ-galons) we performed first-principles calculations using density-functional theory. In addition to the ideal cubic γ-Ga3O3N we studied gallium deficient γ-galons within the Constant-Anion-Model.In highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO1.2 we found at a temperature around 670 K an insulator–metal transition, with a conductivity jump of seven orders of magnitude. We demonstrate through experimental studies and density-functional theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric β-Ga2O3 within the metastable oxide matrix. By doping with nitrogen the critical temperature and the conductivity in the highly conducting state can be tuned.