A Finite Nonequilibrium Annealing Window Governs M₂‑Dominant Phase Stabilization in VO₂ Thin Films

Stabilizing metastable phases in correlated oxides remains challenging because phase selection depends on a coupled lattice-electronic landscape rather than on a single structural parameter. In nondoped VO₂ thin films, the intermediate M₂ polymorph is particularly difficult to stabilize near room temperature. Here, oriented VO₂/GaN films grown by magnetron sputtering were used as a T-rich baseline to examine whether short oxygen annealing can define a finite nonequilibrium phase-selection window under substrate constraint. Raman spectroscopy suggests a narrow optimum within the short-anneal regime of this annealing matrix: at 15 min, the films show the highest M₂ enrichment, and the representative 500 °C, 19% O₂ condition reaches a M₂-dominant state with a Raman-derived M₂ fraction of ∼62%, based on deconvolution across the annealing matrix, dense 100 × 100 μm Raman mapping, and larger-area stitched measurements. Longer anneals do not further increase the M₂ fraction even though the film continues to evolve. Instead, transport broadens from the sharp first-order metal-insulator transition of the pristine T-rich reference into a wider two-step transition; X-ray diffraction shows a corresponding trend within the limited scan window, and atomic force microscopy reveals continued densification through increasing subgrain character. This nonmonotonic response is consistent with the strongest M₂ state being reached within the short-anneal window and not increasing with longer annealing. The combined measurements are not captured by a simple retained-interface-strain picture and are consistent with a nonuniform strain response developing during short oxygen annealing. These results establish a process-defined route for controlling metastable phase balance and transition behavior in correlated oxide electronic materials.