Molecular beam epitaxy grown III-nitride materials for high-power and high-temperature applications: Impact of nucleation kinetics on material and device structure quality

The objective of this research is to investigate the effect of the initial growth conditions on characteristics of subsequent GaN epitaxial layers on sapphire substrates using molecular beam epitaxy (MBE) equipped with an rf-plasma nitrogen source. Two pretreatments are used at the initial growth of GaN: nitridation and buffer layers. The purpose of nitridation of sapphire is to convert the surface of sapphire (Al₂O₃) into a thin layer of AlN, suggesting the reduction of lattice mismatch (approximately 3%) with GaN. Growth of a GaN (or AlN) nucleation buffer layer of proper thickness under Ga (or Al)-rich conditions after nitridation is necessary to improve the electrical, structural, and optical characteristics of the subsequent GaN epitaxial layers. Moreover, the buffer design and synthesis provide control of GaN polarity whose difficulty is due to the non-centrosymmetric material of sapphire substrates.; New techniques have been developed to determine polarity: atomic hydrogen etching and surface potential electric force microscopy (SP-EFM), which provide the method for determining inversion domains as well as polarity of GaN epitaxial layers. These technologies, with proper buffer design, enable the achievement of single polar GaN epitaxial layers. For Mg-doped GaN, we investigated Mg incorporation behaviors into GaN epitaxial layers. We observed a reduction in Mg concentration with increasing Mg flux, which leads to a decreased hole concentration at high doping concentrations.