The thermal stabilities of Al- and N-polarity AIN layers grown on (0 0 0 1) sapphire substrates were investigated at temperatures ranging from I 100 to 1400 degrees C in various gas flows (He, H-2 and H-2 + NH3). Decomposition of AIN occurred in flowing H,, while it did not occur in He or H-2 + NH3 flow. The decomposition rate in H, increased with an increase in the temperature over 1200 degrees C, and the decomposition rate of the M-polarity AIN layer was found to be lower than that of the N-polarity AIN layer at each temperature. The decomposition reaction of AIN and the relationship between the polarities of the AIN layers and their different decomposition rates are discussed. (c) 2007 Elsevier B. V. All rights reserved.