High-Temperature Heat-Treatment of c-, a-, r-, and m-Plane Sapphire Substrates in Mixed Gases of H2 and N2

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixed gas flow of H 2 and N 2 (H 2 /N 2 = 3/1) was investigated within the temperature range 980–1480 °C. AlN was formed on sapphire in the temperature range 1030–1430 °C for c -, a -, and m -plane sapphire, and 980–1430 °C for the r -plane sapphire. At 1480 °C, AlN was not formed, and only sapphire was decomposed by H 2 with the ranking of m - > r - > a - > c -plane. The ranking was contrary to that of the amount of AlN formation at 1380 °C, which occurred by the reaction of gaseous Al generated by the sapphire decomposition and N 2 . This discrepancy was due to the shape of AlN formed on sapphire; whisker-like AlN does not protect c - and a -plane sapphire from decomposing, while layer-like AlN protects r - and m -plane sapphire from decomposing.