Ultrathin VO2 grown with oxygen plasma molecular beam epitaxy on TiO2 (001) and Al2O3 (0001)

Bulk VO2 undergoes an insulator-to-metal transition (IMT) with up to five orders of magnitude change in the resistance at 340 K. However, when VO2 is deposited as a film on a substrate, the strain from the substrate can alter the IMT temperature, resistivity ratio of IMT, and hysteresis. Here, we present single-phase VO2 ultrathin films (thickness less than 20 nm) grown using oxygen plasma molecular beam epitaxy (MBE) on TiO2 (001) and Al2O3 (0001) substrates. First, we modify existing recipes employing ozone MBE and reproduce the best films from literature on TiO2 (001), maintaining an almost three orders of magnitude transition in a 12 nm thick film with TIMT of 308 K (296 K) upon warming (cooling). We then extend our recipe to Al2O3 (0001) substrates where we stabilize a 12 nm thin single-phase VO2 film and observe two orders of magnitude transition at 337 K (329 K) upon warming (cooling), expanding the possible growth methods for ultrathin VO2 films on Al2O3 (0001).