Optical properties of GaN on sapphire substrates grown by plasma-assisted MOCVD

In this paper, we investigated the effect of reactive hydrogen plasma, used in the thermal cleaning process of the substrate, on the optical properties of gallium nitride (GaN) films deposited on c-plane sapphire substrates in the reststrahlen and near bandgap regions. The GaN films were grown at 700 C by plasma-assisted metal organic chemical vapour deposition. IR reflectance and absorption measurements were conducted to study the optical properties in the reststrahlen and near bandgap region respectively. Since the optical properties of a material greatly depend on its structural properties, X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements were also performed as a comparison to the optical results. In addition, Hall effect measurements using the van der Pauw configuration were conducted to obtain the carrier density and mobility of the GaN films.