The thermal stability of +/- c- and m-plane GaN substrates and its dependence on the m-plane GaN off-cut angle were investigated at and above 1200 degrees C at atmospheric pressure in flowing N-2 with and without added NH3. It was clarified that decomposition of the +c-plane GaN surface is promoted by the addition of NH3, while decomposition of the -c- and m-plane GaN surfaces is suppressed by added NH3. It was found that -c- and m-plane GaN substrates can be heated at 1300 degrees C with an added NH3 input partial pressure of more than 0.2 atm, which is 100 degrees C higher than for +c-plane GaN substrates. Nevertheless, decomposition of m-plane GaN substrates became noticeable with increase in the off-cut angle of the substrate. It was revealed that the decomposition was better controlled by using m-plane GaN substrates with an off-cut towards [000 (1) over bar] than towards [0001]. (C) 2019 The Japan Society of Applied Physics