Vacuum Ultraviolet-Enhanced Topotactic Phase Transition and Physical Properties at the Surface of Cobalt Oxide Epitaxial Thin Films

Epitaxial CoO (111) thin films were grown on atomically stepped α-Al₂O₃ (0001) single-crystal substrates via pulsed laser deposition. Subsequently, the films were irradiated with 172 nm vacuum ultraviolet (VUV) light from a xenon excimer (Xe₂*) lamp without external heating. This study investigates the structural transformations within these epitaxial CoO thin films induced by VUV-light irradiation. X-ray and electron beam analyses were used to elucidate the changes in the chemical state and physical properties associated with the phase transition. VUV-light irradiation induced a topotactic phase transformation from rock-salt CoO (111) to spinel Co₃O₄ (111), while preserving the epitaxial relationship with the substrate. This transformation resulted in a sharp heterointerface located approximately 5 nm below the surface. The crystal phase transition was also confirmed through the analysis of changes in chemical state and optical properties. Furthermore, VUV-light irradiation reduced the film resistivity by over than 3 orders of magnitude, resulting in a minimum resistivity of 1.2 × 10⁰ Ω·cm in a 3 nm thick film.