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

Abstract

The deposition-temperature-dependent physical properties of Hf-doped ZnO thin films grown on Al2O3(0001) by pulsed laser deposition were examined. X-ray diffraction measurements showed that all the samples had the (002) 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.