Surface thermal stability of free-standing GaN substrates

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 (1010), and semipolar (2021) and (2021) 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 (1010) plane, but the surface of the (20-21) plane became rough with increasing cleaning temperature.