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 H2 and N2 (H2/N2= 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 H2 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 N2. 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.