Diffusion of oxygen in bulk GaN crystals at high temperature and at high pressure

• Oxygen diffusion in GaN investigated experimentally for the first time.
• Oxygen diffusion in GaN is an anomalously weak even at very high temperature.
• Theoretically predicted stability of the nitrogen sublattice of GaN is confirmed.

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration (NO=(2–4)×1019 cm−3) than that in the bulk matrix (NO<1×1017 cm−3). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients DO(T, P). The obtained values of DO(T, P) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.