The impact of the oxidation time on the structures of thermal oxides formed on AlN was determined by high resolution transmission electron microscopy (HRTEM). Oxidation of AlN single crystals was performed for 2 to 6 h at 1000 °C. Oxidation for 2 h produced mostly amorphous oxide layers whereas oxidation for both 4 and 6 h produced partially crystalline oxide layers. The oxide layer thickness varied from 205 to 600 nm for oxidation times of 2 and 6 h respectively. The crystalline oxide was mostly single phase α-Al2O3 except at the surface where it was a mixture of γ-Al2O3 and α-Al2O3. Based on the different structures produced for different oxidation times, we speculate that the oxide formed changes with thickness: first an amorphous oxide, then γ-Al2O3, and finally α-Al2O3 as the oxide thickness increases. The AlN crystal was nearly defect- and oxygen-free for oxidation at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at a high temperature leading to a point-defect equilibrium throughout the nitride. A faceted interface between Al2O3 and AlN could be attributed to the surface diffusion to minimize energy.
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