The thermal stability of GaN surfaces was investigated with respect to homo-epitaxy on free-standing GaN substrates. Morphologies and etching rates of the GaN surfaces for free-standing polar (0001), nonpolar
, and semipolar
and
planes were studied before and after thermal cleaning. In the case of the polar (0001) plane, polishing scratches disappeared after thermal cleaning at temperatures above 1000 °C. The surface morphology depended on not only the cleaning temperature, but also the substrate off-angle. The surface after thermal cleaning became rough for the substrate with off-angle less than 0.05°. In the case of nonpolar and semipolar planes after thermal cleaning, surface morphologies and etching rates were strongly dependent on the planes. A flat surface was maintained at cleaning temperatures up to 1100 °C for the
plane, but the surface of the
plane became rough with increasing cleaning temperature.