Nano-structure study of ZnO thin films on sapphire grown with different temperature conditions

We compared the nano-structures of three samples of ZnO thin film on sapphire under different growth temperature conditions. Although disconnected domain structures (on the scale of 100 nm in size) were observed in the samples of high-temperature (450 °C) growth, their crystal quality is generally better than the one grown at a low temperature (200 °C), either near or away from the sapphire interface. Lattice misfits and threading dislocations were observed within a domain with the separation of around 8 nm. The sample grown at the low temperature showed a continuous structure through the ZnO layer although void-like structures might exist inside. However, its crystal quality is relatively poorer. Of the two samples with high-temperature growth, the one with initial low-temperature growth had a larger domain structure (around 150 nm in size) and relatively lower crystal quality. In particular, strong strains existed near the interface of this sample. The samples of high-temperature growth generally have higher photon emission efficiencies. Temperature-dependent integrated photoluminescence intensities of the high-temperature-growth samples show that the exciton trapping by either intrinsic donors or acceptors leads to a higher thermal quenching rate in comparison with free excitons.