Post-deposition annealing control of phase and texture for the sputtered MoO₃ films

Thin films were sputtered on Si substrates from a metallic Mo target at room temperature and then annealed in air at the temperatures between 200-550 °C to form MoO₃ of various phases and textures. The films annealed at 450 °C for 1 h showed a pure α phase and their texture was dependent on the ambient pressure during the initial sputtering deposition. The films sputtered under 3 mTorr were b-axis oriented with a broad in-plane alignment after annealing, while the films sputtered under higher pressures showed a texture with the Mo-O₆ double-layers upright to the substrate. The films annealed between 350-400 °C were composed of both α and β phases. Below 350 °C, nearly a pure β phase was obtained, which showed a preferred a-axis orientation if the annealing temperature was in the range of 350-300 °C. Ammonia gas sensing properties were tested for the films. The β phase showed best sensitivity, while the α phase with the planar texture showed shortest recovery time. MoO ₃ films have wide-ranging applications, each of which prefers a specific phase and texture. This simple but productive method is therefore of practical importance.