Tuning Optoelectronic Properties of WO3 Thin Films Through Reduction Annealing

Tungsten oxide (WO3) thin films have been a research focus in recent years because of its tunable bandgap, functional structural and optical property and potential application in optoelectronics, gas sensing and energy storage. In this study, we have explored the effects of reduction annealing on structural and electrical property of few-layer WO3 thin films deposited by using Pulsed Laser Deposition with vacuum pressure of 10-4 mbar on crystalline Si/SiO2 substrate. X-ray diffraction data indicated a nanocrystalline structure of as-deposited WO3 films with a preferred (0 0 2) texture orientation. Moreover, highly crystalline orthorhombic WO3 with progressive peak shifts were realized after annealing for 15 hours indicating a reduction of unit cell volume. Microstructural and elemental composition analyses were performed using a Field Emission Scanning Electron Microscopy combined with Energy Dispersive Spectroscopy. Specific Raman peaks at 270 cm-1, 714 cm-1, 807 cm-1, and 950 cm-1 were characteristic to O-W-O bending modes, W-O stretching vibrations, O-W-O stretching modes, and terminal W=O bond stretching vibrations, respectively. UV-Vis spectroscopy measurements indicated that the energy band gap of the WO3 films was approximately 3.2 eV and approximately 10% decrease can be observed after the annealing. Reduction annealing can introduce oxygen vacancies by tuning carrier concentration which leads to a significant improvement in electrical conductivity by several order of magnitudes. Although this increase in conductivity is reasonably significant, preliminary field-effect transistor (FET) characterization demonstrated retained semiconducting properties, further studies are currently underway. Our data demonstrate that structure and electric properties of WO3 thin film can be modified by reduction annealing. This may pave the prospect of WO3 based devices for applications in nanoelectronics and optoelectronics.