M-plane GaN epilayers have been directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy using a low temperature GaN nucleation layer. The m-plane GaN surface is optically smooth and mirror-like, with rms roughness similar to 2.0 nm (5 x 5 mu m) measured by atomic force microscopy. We have found that the sapphire surface treatments prior to the growth determine the crystal orientations of the GaN epilayers. In particular, in situ thermal cleaning in H-2 ambient, followed by NH3 nitridation at low temperature, reproducibly results in single m-plane GaN orientation, while other surface preparations lead to semi-polar GaN (10 over bar over bar) plane.
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