Effects of deposition and annealing atmospheres on phase transition of tungsten oxide films grown by ultra-high-vacuum reactive sputtering

A series of oxygen-contained tungsten films were grown on Si(100) substrates without intentional heating by ultra-high-vacuum reactive magnetron sputtering at a constant argon pressure (P_Ar) of 1.33 × 10 ^{- 1} Pa mixed with a wide range of O₂ partial pressures (P_O) from 1.33 × 10^{- 4} to 4 × 10^{- 1} Pa, equivalent to P_O-to-P_Ar ratios (P_{O / Ar}) from 1 × 10^{- 3} to 3. The effect of varying P_{O / Ar} on phase evolution was evaluated by annealing the films in a controlled atmosphere (argon or oxygen) at 500 or 700 °C for 1 h. Grazing incident X-ray diffraction and transmission electron microscopy, together with the data of electrical resistivity and deposition rate, reveal that gradually increasing P_{O / Ar} induces a sequence of phase transitions from nanocrystalline β-W(O) (P_{O / Ar} ≤ 0.1), amorphous WO₂ (P _{O / Ar} = 0.6) to amorphous WO₃ (P_{O / Ar} 2). When annealed in argon atmosphere, the amorphous WO₂ and WO ₃ exhibit a very different magnitude of crystallization temperature (T_c) and can be transformed, respectively, into monoclinic WO ₂ (T_c = 500 °C) and tetragonal WO₃ (T _c = 700 °C). However, the oxidizing atmosphere plays a role to accelerate significantly the crystallization of the amorphous WO₂ into a completely different phase (monoclinic WO₃) at a significantly reduced T_c of 500 °C.