Thermal oxidation of polycrystalline and single crystalline aluminum nitride wafers

Two types of aluminum nitride (AlN) samples were oxidized in flowing oxygen between 900°C and 1150°C for up to 6 h—highly (0001) textured polycrystalline AlN wafers and low defect density AlN single crystals. The N-face consistently oxidized at a faster rate than the Al-face. At 900°C and 1000°C after 6 h, the oxide was 15% thicker on the N-face than on the Al-face of polycrystalline AlN. At 1100°C and 1150°C, the oxide was only 5% thicker on the N-face, as the rate-limiting step changed from kinetically-controlled to diffusion-controlled with the oxide thickness. A linear parabolic model was established for the thermal oxidation of polycrystalline AlN on both the Al- and N-face. Transmission electron microscopy (TEM) confirmed the formation of a thicker crystalline oxide film on the N-face than on the Al-face, and established the crystallographic relationship between the oxide film and substrate. The oxidation of high-quality AlN single crystals resulted in a more uniform colored oxide layer compared to polycrystalline AlN. The aluminum oxide layer was crystalline with a rough AlN/oxide interface. The orientation relationship between AlN and Al₂O₃ was (0001) AlN//( ) Al₂O₃ and ( ) AlN//( ) Al₂O₃.