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 degrees C to form MoO3 of various phases and textures. The films annealed at 450 degrees C for 1 h showed a pure a 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-O-6 double-layers upright to the substrate. The films annealed between 350-400 degrees C were composed of both alpha and beta phases. Below 350 degrees C, nearly a pure beta phase was obtained, which showed a preferred a-axis orientation if the annealing temperature was in the range of 350-300 degrees C. Ammonia gas sensing properties were tested for the films. The beta phase showed best sensitivity, while the 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.