Controlling Defect Formation in WO3 and MoO3 Thin Films through Deposition and Interface Engineering

This thesis investigates the mechanisms governing defect formation in oxygen-deficient transition- metal oxide thin films, with a focus on WO3 and MoO3 grown on metallic and non-metallic substrates. Particular attention is given to how di!erent substrates, deposition kinetics, and thermal post-treatments collectively influence oxygen vacancy formation, metal di!usion, and interfacial phase evolution at oxide–metal heterojunctions. In WO3 thin films deposited on aluminium, copper, nickel, and silicon, the defect chem- istry is shown to depend strongly on the metal–oxide interface. Reactive metallic substrates promote oxygen removal from the WO3 lattice, resulting in tungsten reduction and the for- mation of oxygen-deficient WO3→x species, while inert silicon preserves near-stoichiometric compositions. Spectroscopic analyses demonstrate that these interface-driven redox processes extend beyond the immediate interface, significantly modifying the electronic structure and photoelectrochemical response. The role of growth kinetics is further investigated by comparing MoO3 thin films fabri- cated by thermal evaporation and pulsed laser deposition. The results show that the kinetic energy of incoming species critically determines the balance between oxide di!usion and metallic element di!usion within amorphous interfacial regions. Subsequent thermal treat- ments provide an additional degree of control, enabling the modulation of defect concentra- tion and ionic redistribution at the heterojunction. A phenomenological di!usion model is proposed to rationalise these observations. Overall, this research reveals that defect formation in transition-metal oxide thin films arises from the interplay between substrate reactivity, growth kinetics, and post-deposition thermal evolution. The findings provide a physically grounded framework for engineering interfaces and defect structures in oxide-based heterostructures, enabling improved perfor- mance in photoelectrochemical, electrochromic, and sensing applications.