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 [degree]C to form MoO3 of various phases and textures. The films annealed at 450 [degree]C for 1 h showed a pure [small alpha] 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 [degree]C were composed of both [small alpha] and [small beta] phases. Below 350 [degree]C, nearly a pure [small beta] phase was obtained, which showed a preferred a-axis orientation if the annealing temperature was in the range of 350-300 [degree]C. Ammonia gas sensing properties were tested for the films. The [small beta] phase showed best sensitivity, while the [small alpha] phase with the planar texture showed shortest recovery time. MoO3 films have wide-ranging applications, each of which prefers a specific phase and texture. This simple but productive method is therefore of practical importance.