Amorphous hafnium oxide thin films for antireflection optical coatings

Hafnium oxide (HfO2) thin films were gown on silicon and quartz substrates by radio frequency reactive magnetron sputtering at temperature < 52 degrees C. X-ray diffraction of the films showed no structure, suggesting that the films grown on the substrates are amorphous. The optical properties of these films have been investigated using spectroscopic ellipsometry with wavelength range 200-1400 nm and ultraviolet-visible spectrophotometer techniques. Also, the effects of annealing temperatures on the structure and optical properties of the amorphous HfO2 (a-HfO2) have been investigated. The films appeared to be monoclinic structure upon high temperature (1000 degrees C) annealing as confirmed by X-ray diffraction. The results show that the annealing temperature has a strong effect on the optical properties of a-HfO2 films. The optical bandgap energy of the as-deposited films is found to be about 5.8 eV and it increases to 5.99 eV after the annealing in Ar gas at 1000 degrees C. The further study shows that the measurement of the optical properties of the amorphous films reveals a high transmissivity (82%-99%) and very low reflectivity (<8%) in the visible and near-infrared regions at any angle of incidence. Thus, the amorphous structure yields HfO2 film of significantly higher transparency than the polycrystalline (68%-83%) and monoclinic (78%-89%) structures. This means that the a-HfO2 films could be a good candidate for antireflection (AR) optical coatings. (C) 2007 Elsevier B.V. All rights reserved.