Structures and optical properties of tungsten oxide thin films deposited by magnetron sputtering of WO3 bulk: Effects of annealing temperatures

Tungsten oxide thin films were deposited on glass substrates by the magnetron sputtering of WO3 bulk at room temperature. The deposited films were annealed at different temperatures in air. The structural measurements indicate that the films annealed below 300 degrees C were amorphous, while the films annealed at 400 degrees C were mixed crystalline with hexagonal and triclinic phases of WO3. It was observed that the crystallization of the annealed films becomes more and more distinct with an increase in the annealing temperature. At 400 degrees C, nanorod-like structures were observed on the film surface when the annealing time was increased from 60 min to 180 min. The presence of W= O stretching, W-O-W stretching, W-O-W bending and various lattice vibration modes were observed in Raman measurements. The optical absorption behaviors of the films in the range of 450-800 nm are very different with changing annealing temperatures from the room temperature to 400 degrees C. After annealing at 400 degrees C, the film becomes almost transparent. Increasing annealing time at 400 degrees C can lead to a small blue shift of the optical gap of the film.