Comparison of nanostructure characteristics of ZnO grown on GaN and sapphire

The immature developments of p-type ZnO and ZnO-related ternary or quaternary compound and the small lattice mismatch between ZnO and GaN stimulate interest in the hybrid growth of ZnO and GaN. In this research, we compared the nanostructures and optical properties of two ZnO thin-film samples grown under the same conditions but on different underlying materials (sapphire and GaN). With the high growth temperature of 450 degrees C, both samples show domain structures. However, in contrast to the sample of ZnO on sapphire, the sample of ZnO on GaN starts with smaller domains (a few tens of nanometers in width) of almost vertical boundaries at the interface and evolves into larger domains (around 100 nm in width). The 30 degrees twist of the basal plane in ZnO with respect to its underlying sapphire does not occur in the sample of ZnO on GaN. Within a domain of ZnO on sapphire, misfit and threading dislocations can be periodically observed along the interface with the separation of about 8 nm. The transition of the lattice structure across the interface between ZnO and GaN was quite smooth, indicating the high heterojunction quality in this sample. The x-ray diffraction results showed that the crystalline quality of ZnO on GaN is slightly better than that of ZnO on sapphire. However, based on the thermal quenching behavior of integrated photoluminescence intensity, we estimated that the optical quality of the two samples is about the same.