Role of peroxo O₂²⁻ and ozonide O₃⁻ ions in the conduction mechanism of a-ZnSnO films

This work explores how annealing influences the properties of ultrathin amorphous zinc tin oxide (38 nm thick a-ZTO) thin films fabricated by magnetron sputtering. The samples were subjected to post-deposition annealing at temperatures between 200 °C and 300 °C in an oxidizing environment, consisting of an O₂/N₂ gas mixture. During the annealing process, the sheet resistance of the films was continuously monitored using in situ measurements. In-situ monitoring reveals a transition from semiconducting to metallic-like transport as the resistivity reaches its minimum. Optimized annealing yields films with conductivity up to 430Scm⁻¹, carrier concentration 1.5×10²⁰cm⁻³, and mobility 19cm²V⁻¹s⁻¹, without compromising optical properties. Complementary density functional theory (DFT) calculations suggest that the formation of oxygen-related species such as peroxo (O₂²⁻) and ozonide (O₃⁻) groups play a role in modifying the electronic structure and contributing to carrier transport, while the dominant effect arises from defect reorganization and mobility enhancement.