Effects of annealing temperature, thickness and substrates on optical properties of m-plane ZnO films studied by photoluminescence and temperature dependent ellipsometry

A series of m-plane ZnO films grown on sapphire and Si substrates with different thicknesses (186–371 nm) and different annealing temperatures (150–200 °C) were studied in detail by comparing X-ray diffraction, photoluminescence and spectroscopic ellipsometry measurements. The impact of thickness, annealing temperature and substrates on optical properties was investigated by the systematical analysis of the highly correlated stain, grain size, exciton effect, absorption tailing and crystallinity. Our finding indicates that the improvement of crystal quality along with the increase of grain size and refractive index for m-plane ZnO can be observed with increasing annealing temperature, increasing thickness or by introducing sapphire instead of Si substrate. The refractive index and bandgap have stronger dependence on annealing temperature than on thickness, causing the blue-shift of bandgap with rising annealing temperature despite thicker film. This bandgap shift is more pronounced for ZnO in ZnO/Si system. Photoluminescence combined with ellipsometry analysis demonstrate that higher annealing temperature can strengthen the excitonic feature. Bose-Einstein equation, employed to fit the dependence of bandgap on experimental temperature (300–583 K) illustrates that, the reduction of bandgap at elevated temperatures is more significant for ZnO in ZnO/sapphire system due to the corresponding stronger electron/exciton-phonon interactions involved with larger longitudinal optical phonon energies.