Solid-phase epitaxy and pressure-induced topotaxy of the VO2 and V2O3 thin films on sapphire using annealing under uniaxial compression

Abstract

The vanadium oxide thin films of both VO2 (V4+) and V2O3 (V3+) are phase-selectively synthesized on atomically stepped α-Al2O3 (0001) substrates by pulsed laser deposition of an amorphous precursor VxOy (V5+) phase at room-temperature, and subsequent uniaxial compressive annealing (UCA) at at 773 K under vacuum conditions. The thin films are epitaxially crystallized under uniaxial pressure, however, the process produces randomly oriented VO2-based polycrystalline when compression is not used. The VO2 (010) epitaxial film can be obtained under an applied pressure of 1 MPa; further, the V2O3 (0001) epitaxial film is formed under a pressure of more than 10 MPa. The epitaxial films indicate a distinct metal-insulator transition in which the resistivity is varied by 103 in accordance with the ρ-T measurement, i.e., the transition temperature (TC) is estimated to be ∼350 K for VO2 thin films crystallized under a pressure of 1 MPa, and ∼150 K and ∼120 K for V2O3 thin films formed under 10 and 30 MPa, respectively. Additional sequential UCA with distinct pressures of 1, 10, and 1 MPa again reveals a corresponding and reversible phase transformation between VO2 and V2O3 with consistent resistivity variation. Therefore, pressure-induced topotaxy can be proved in both the phases. The obtained epitaxial thin films demonstrate a relatively flat surface with a root-mean square roughness of <1.5 nm with a suppressed excessive grain growth, indicating an anisotropic atomic diffusion owing to the uniaxial compression.