Interplay between GaN polarity and surface reactivity towards atomic hydrogen

Abstract

GaN epitaxial layers have been grown by molecular beam epitaxy (MBE). Low (200 °C) and high (700 °C) temperature nitrided sapphire substrates, and GaN and AlN buffer layers have been used as the platforms for investigating polarity selection processes of GaN grown by RF-MBE, aimed at presenting a comprehensive understanding of the issues of GaN polarity and growth conditions. The results show that Ga-polar GaN results from the use of AlN buffer layers, and when the substrate/bulk interface has a Ga- or Al-rich interface. In contrast, a large density of N-polar domains is found when GaN epitaxial films are grown on GaN buffer layers on sapphire when nitrided at 200 °C. The impact of the polarity of GaN on the interaction of GaN surfaces with atomic hydrogen is also studied. A different reaction rate and reaction extent is found for N- and Ga-polar GaN with atomic hydrogen, with N-polar GaN exhibiting greater reactivity. It is also demonstrated that the reactivity of GaN with atomic hydrogen coupled with surface potential measurements exploiting Kelvin probe microscopy is an alternative approach to determining the polarity of GaN films, and identifying the presence of inversion domains. Finally, the impact of GaN polarity on electrical properties of AlGaN/GaN heterojunction structures is also shown.