Thesis (M.Sc.)--Chulalongkorn University, 2008

Strain and hexagonal phase generation in c-GaN layers grown by metalorganic vapor phase epitaxy on (001) GaAs and on (311) GaAs substrates were investigated using HRXRD and Raman scattering. The results show that c-GaN films are compressive. This contrasts with results which should be under tensile strain. In addition, c-GaN films have a cubic structure which is generated on the (001) and (311) planes and contain some amount of hexagonal phase generated on plane. From HRXRD results, it is found that the relief of strains in the c-GaN layers has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5045 ± 0.0021 Å, which is in agreement with the theoretical prediction of 4.503 Å. The c-GaN layers contain 25-85% hexagonal phase inclusion. Although, the hexagonal phase inclusion in c-GaN layers on GaAs (311) cannot be determined by HRXRD measurement. On the other hand, Raman scattering is a very sensitive method for measuring an existence of hexagonal phase. The hexagonal phase inclusion determined by Raman spectroscopy technique, HRaman, exhibits a linear dependence on the HXRD, providing a useful calibration method to determine the hexagonal phase inclusion in c-GaN layers.