Polarity determination for GaN/AlGaN/GaN heterostructures grown on (0001) sapphire by molecular beam epitaxy

In order to study the formation of polarization-induced two-dimensional electron gases (2DEGs), GaN/AlGaN/GaN heterostructures with different polarity were grown on sapphire (0001) substrates by plasma-assisted molecular-beam epitaxy. The polarity of GaN layers can be changed from the normal N-polarity to a Ga-polarity surface by inserting a thin Al metal layer prior to the growth of AlN buffer layer. The surface stability of each lattice polarity film was studied by in situ reflection high-energy electron diffraction. The polarity of the films and the location of 2DEGs in the heterostructure have been evaluated by investigating the carrier concentration profiles in the GaN/AlGaN/GaN heterostructures. A simple model to explain the polarity change by a thin Al metal layer is proposed.