Ⅲ nitrides have become one of the forefront research topics in semiconductor materials fields due to their successful application to fabricating of highly efficient optoelectronic devices operating in the ultraviolet and visible spectral regions. In this thesis, we grow hexagonal GaN on c-plane sapphire substrates in a horizontal MOCVD reactor equipped with an in situ normal incidence reflectance monitoring, and the focus has been turned to improve the quality of unintentionally doped GaN epilay...