In situ investigation for polarity-controlled epitaxy processes of GaN and AlN in MBE and MOVPE growth

Abstract The polarity-controlled growths of GaN and AlN on sapphire substrate by MOVPE and MBE were demonstrated. The effects of surface stoichiometry on the polarities of both GaN and AlN epilayers in RF-MBE growth were intensively investigated by in situ analysis of coaxial collision impact ion scattering spectroscopy. It was found that the polarity of the GaN epilayer could not change into another polarity for wide Ga/N surface stoichiometry. However, when the surface was modified by supplying Al, the polarity could be converted form N polarity to Ga polarity. On the other hand, AlN epilayer could be easily changed from N polarity to Al polarity under Al-rich surface stoichiometry. These results support our proposed-model for the polarity conversion mechanism based on the “two monolayers of Al”, i.e., the presence of excess Al on the surface plays an important role for the polarity conversion. It was concluded that the polarity conversion of GaN and AlN by both Al insertion layers and Al-rich AlN intermediate layer relied on the fact that they provided an Al platform on which the subsequent epilayer prefers to grow with Ga and/or Al polarity.