Formation of stoichiometric, sub-stoichiometric undoped and hydrogen doped tungsten oxide films, enabled by pulsed introduction of O2 or H2 during hot-wire vapor deposition

Tungsten oxide films with various stoichiometries, undoped and hydrogen doped were deposited by heating a W wire at 660 °C and at a base pressure of 13 Pa set by various gasses (O2, N2, H2) or gas mixtures (N2-O2 10% in O2, forming gas, FG) and pulsed injection of O2 or H2. Using this method and dependent on the deposition conditions four classes of hot-wire (hw) tungsten oxide films were synthesized: i) stoichiometric (hwWO3), ii) oxygen deficient (sub-stoichiometric, hwWOx with x < 3), iii) stoichiometric and hydrogen doped (hwWO3:H), and iv) sub-stoichiometric and hydrogen doped (hwWOx:H). Due to the pulsed injection of O2 the W wire re-oxidizes during deposition thus creating WO3 vapors continuously, so films deposited by this method do not suffer by thickness limitations and, moreover, exhibit high porosity. The optical properties of these classes of films, studied with spectroscopic ellipsometry (SE) measurements, differ substantially between them indicating corresponding differences in their electronic structures. So, hwWO3 films were semiconducting exhibiting a band gap near 3 eV. Sub-stoichiometric hwWOx deposited with up to 2 injections of O2 were semi-metallic preserving some features of the electronic structure of the pure metal, while further injection of O2 leads to stoichiometric films. Fourier transform infrared spectroscopy and SE measurements indicated that hwWOx films contain H bonded with the O ions while in hwWO3:H and hwWOx:H the H is incorporated in films by direct bonding with the W ions.