Structural-crossover-induced optical band gap variation of Hf-doped ZnO films

The deposition-temperature-dependent physical properties of Hf-doped ZnO thin films grown on Al2O3(0 0 0 1) by pulsed laser deposition were examined. X-ray diffraction measurements showed that all the samples had the (0 0 2) orientation except for the sample grown at room temperature, which showed amorphous characteristics because of the lack of kinetic energy. The in-plane strain changed from compressive to tensile as the deposition temperature increased above 200 °C. Optical transmission data revealed that all the samples exhibited >90% transmittance regardless of the deposition temperature. However, the optical band gap decreased with increasing deposition temperature, which was related to variation in stress in the films. X-ray photoelectron spectroscopy also revealed an increase in Zn-O bonding but decreases in oxygen vacancies with increasing deposition temperature.