Thin film deposition and characterization of pure and iron-doped electron-beam evaporated tungsten oxide for gas sensors

Pure tungsten oxide (WO3) and iron-doped (10 at.%) tungsten oxide (WO3:Fe) nanostructured thin films were prepared using a dual crucible Electron Beam Evaporation (EBE) technique. The films were deposited at room temperature under high vacuum onto glass as well as alumina substrates and post-heat treated at 300 °C for 1 h. Using Raman spectroscopy the as-deposited WO3 and WO3:Fe films were found to be amorphous, however their crystallinity increased after annealing. The estimated surface roughness of the films was similar (of the order of 3 nm) to that determined using Atomic Force Microscopy (AFM). As observed by AFM, the WO3:Fe film appeared to have a more compact surface as compared to the more porous WO3 film. X-ray photoelectron spectroscopy analysis showed that the elemental stoichiometry of the tungsten oxide films was consistent with WO3. A slight difference in optical band gap energies was found between the as-deposited WO3 (3.22 eV) and WO3:Fe (3.12 eV) films. The differences in the band gap energies of the annealed films were significantly higher, having values of 3.12 eV and 2.61 eV for the WO3 and WO3:Fe films respectively. The heat treated films were investigated for gas sensing applications using noise spectroscopy. It was found that doping of Fe to WO3 produced gas selectivity but a reduced gas sensitivity as compared to the WO3 sensor.