Low-temperature AlN templates were grown directly on a sapphire c-plane (001) by a two-step growth method. The different AlN morphologies were observed, and the influence of the ammonia and trimethylaluminum flowrate on the AlN morphology was investigated separately. We found that only one parameter of V/III ratio is not enough for the discussion of AlN morphology. An increase of either the V or III precursor flowrate will lead to a decrease in the surface mobility of Al atoms and possibly cause the same kind of change in AlN morphology. A smooth AlN surface is acquired by adjusting the Al mobility to a suitable value that is neither too high nor too low.

The mechanisms of AlGaN device buffer layer growth were studied. Gallium residues in the reactor chamber may be harmful to the quality of the AlN strain modulation layer, which eventually worsens the AlGaN buffer layer. By restraining the gallium residues, the crystalline quality of the AlGaN layer is markedly improved. In addition, enhancing stress relief in nucleation and coalescence stages will reduce the edge dislocations induced by strain relaxation in the 2D growth stage. A slower precursor flow rate can promote the stress relief in nucleation and coalescence stages. By comparison, a suitable suppression of Al atoms’ surface migration can decrease surface roughness, which can be realized by increasing the precursor flow rate. Eventually, we obtained a AlGaN buffer layer having both low edge dislocation density and a flat surface using a two-step growth method.