Structural analyses of MBE grown cubic and hexagonal GaN epilayers by X-ray diffraction

Detailed X-ray analyses have been carried out on the epilayers of cubic and hexagonal GaN grown on (001) GaAs and (0001) sapphire substrates, respectively, by plasma assisted molecular beam epitaxy (MBE). In order to study the existence of secondary crystallographic phases in the cubic GaN epilayers, X-ray pole figures were generated on (001) surfaces of the samples. The exact locations of misoriented cubic and secondary hexagonal phases have been identified and their relative intensities were estimated. Threading dislocations were observed despite the exhibition of excellent structural properties by the hexagonal GaN epilayers as measured by high resolution X-ray diffraction. A comparison of crystal coherence and mosaicity for the epilayers grown under different growth conditions has been made. A possible explanation is given for the poor electrical characteristics exhibited by the structurally high quality epilayers.