Sharpness and intensity modulation of the metal-insulator transition in ultrathin VO2 films by interfacial structure manipulation

Vanadium dioxide (VO₂) undergoes a structural transformation from monoclinic (insulator) to tetragonal (metallic) upon heating above 340 K, accompanied by abrupt changes to its electronic, optical, and mechanical properties. Not only is this transition scientifically intriguing, but there are also numerous applications in sensing, memory, and optoelectronics. Here we investigate the effect different substrates and the processing conditions have on the characteristics metal-insulator transition (MIT), and how the properties can be tuned for specific applications. VO₂ thin films were grown on c -plane sapphire (0001) and p-type silicon <100 > by pulsed laser deposition. High-resolution x-ray diffraction along with transmission electron microscopy reveals textured epitaxial growth on sapphire by domain-matching epitaxy, while the presence of a native oxide layer on silicon prevented any preferential growth resulting in a polycrystalline film. An orientation relationship of <001 > (010)_VO2||<11 ¯00 > (0001)_{Al 2O3} was established for VO₂ grown on sapphire, while no such relationship was found for VO₂ grown on silicon. Surface-energy minimization is the driving force behind grain growth, as the lowest energy VO₂ plane grew on silicon, while on sapphire the desire for epitaxial growth was dominant. Polycrystallinity of films grown on silicon caused a weaker and less prominent MIT than observed on sapphire, whose MIT was higher in magnitude and steeper in slope. The position of the MIT was shown to depend on the competing effects of misfit strain and grain growth. Higher deposition temperatures caused an increase in the MIT, while compressive strain resulted in a decreased MIT.