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.