Growth kinetics of AlN and GaN films grown by molecular beam epitaxy on R-plane sapphire substrates

This paper reports the growth by molecular beam epitaxy of AlN and GaN thin films on R-plane sapphire substrates. Contrary to previous findings that GaN grows with its (11̄02) A-plane parallel to the (11̄20) R-plane of sapphire, our results indicate that the crystallographic orientation of the III-nitride films is strongly dependent on the kinetic conditions of growth for the GaN or AlN buffer layers. Thus, group III-rich conditions for growth of either GaN or AlN buffers result in nitride films having (11̄20) planes parallel to the sapphire surface, and basal-plane stacking faults parallel to the growth direction. The growth of these buffers under N-rich conditions instead leads to nitride films with (11̄26) planes parallel to the sapphire surface, with inclined c -plane stacking faults that often terminate threading dislocations. Moreover, electron microscope observations indicate that slight miscut (∼0.5°) of the R-plane sapphire substrate almost completely suppresses the formation of twinning defects in the (11̄26) GaN films.