Stress dependence on N/Ga ratio in GaN epitaxial films grown on ZnO substrates

GaN films were grown on ZnO substrate under various beam equivalent pressure ratios by plasma-assisted molecular beam epitaxy (P-MBE). We theoretically calculated the thermal stress caused by the difference of thermal expansion coefficients between GaN and ZnO. The changes of stress and critical thickness were evaluated by measurement of XRD for HT GaN and LT GaN buffer grown under Ga-rich and N-rich conditions. From this study, we observed that GaN grown under Ga-rich condition causes GaN film to under compressive-stress, while GaN grown under N-rich condition was tensile-stressed. Consequently, interdiffusion has no effect on the variation of the critical thickness.