Influence of oxygen partial pressure on microstructure, optical properties, residual stress and laser induced damage threshold of amorphous HfO2 thin films

HfO₂ thin films were deposited by electron beam evaporation technique at different oxygen (O₂) partial pressures. The films are characterized using x-ray diffraction, x-ray reflectivity, spectrophotometry, atomic force microscopy, substrate curvature measurement and laser induced damage threshold (LIDT) measurement techniques. All the films show amorphous structure and are optically homogeneous. The film density decreases from 8.53 to 6.99 gm/cm³, while both the grain size and surface roughness increase with increasing oxygen partial pressure. Film refractive index decreases from 1.96 to 1.82, and the band gap increases from 5.59 eV to 5.67 eV with increasing O₂ partial pressure. The variation of refractive index is correlated with the grain size and density of the films. The measured residual stress for all the films is tensile in nature, and its variation with oxygen pressure is correlated with the film microstructure. The measured LIDT value decreases from 9.89 to 8.83 J/cm² for 1064 nm laser and 5.97 to 4.52 J/cm² for 532 nm laser, with increasing O₂ partial pressure. The variation of laser damage threshold with oxygen partial pressure has been explained through density of the film.