Ga₂O₃ is a promising material for use in “solar-blind” UV-detectors which can be produced efficiently by oxidation of GaN. In this study we focus on the evolution of the oxide layer when GaN is heated in air. The experimental method applied is the perturbed angular correlation (PAC) spectroscopy of γ-rays emitted by radioactive nuclides, here ¹¹¹Cd and ¹⁸¹Ta, whose parent nuclei are ion implanted into films of GaN grown on sapphire. As the emission pattern for nuclei in GaN is clearly distinct from that of nuclei in Ga₂O₃, the fraction of probe nuclei in the oxide layer can be directly measured and allows to follow the time dependent growth of the oxide on a scale of less than 100 nm. Additional measurements were carried out with the oxidized sample held at fixed temperatures in the temperature range from 19 K to 973 K showing transitions between the hyperfine interactions of ¹¹¹Cd in the oxide matrix both at high and low temperatures. A model for these transitions is proposed.Instituto de Física La Plata