Strain-Driven Selective Stabilization of Metastable TiO₂ Phases

Stabilizing metastable TiO₂ phases in thin films remains a significant challenge. This paper demonstrates a strain-driven approach for selectively stabilizing the metastable phases of orthorhombic TiO₂-II and rutile using (111)-oriented face-centered cubic (FCC) metal substrates via low-temperature atomic layer deposition. Epitaxial FCC metal substrates, including Ir and Pt, exhibit a strong preferential (111) orientation and promote the formation of TiO₂-II with a preferential (200) orientation through favorable lattice matching. In contrast, TiO₂ grown on (111)-textured polycrystalline FCC metals crystallizes into rutile with a preferential (110) orientation despite identical growth conditions, which is attributed to strain relaxation arising from the random in-plane orientations of FCC metals. Compared to the stable anatase phase, TiO₂-II films exhibit higher density (4.45-4.51 g cm^-3), higher refractive indices, and higher dielectric constants (≈75-77). These findings reveal that in-plane strain and lattice matching can be strategically utilized to engineer metastable TiO₂ phases, offering a new approach for the phase-selective growth of functional oxide films at low temperatures.