Impact of high-temperature annealing of AlN layer on sapphire and its thermodynamic principle

The N₂-CO gas annealing technique was demonstrated to improve the crystalline quality of the AlN layer on sapphire. 300-nm-thick AlN layers were fabricated on sapphire substrates by a metal-organic vapor phase epitaxy method. The AlN layers were annealed in N₂ and/or N₂-CO gas atmosphere at 1923-1973K for 0.5-4 h. Many pits and voids were observed on the AlN surface annealed in N₂ atmosphere at 1973K for 2 h. The rough surface was, however, much improved for the AlN annealed in N₂-CO gas atmosphere. The thermodynamic principle of the N₂-CO gas annealing technique is explained in this paper on the basis of the phase stability diagram of the Al₂O₃-AlN-C-N₂-CO system. Voids and γ-aluminum oxynitride (γ-AlON) at the AlN/sapphire interface formed during the annealing, which is also explained on the basis of the phase stability diagram. The in-plane epitaxial relationships among AlN, γ-AlON, and sapphire are presented, and misfits among them are discussed.