Controlling metal-insulator transition in (010)-VO2/(0001)-Al2O3 epitaxial thin film through surface morphological engineering

Abstract Surface morphological control of the metal-insulator transition behaviors of VO 2 epitaxial thin films is achieved by annealing substrates of (0001)-Al 2 O 3 single crystals. The well-defined terraces of the (0001)-Al 2 O 3 substrates are formed by annealing in air at 1200 °C. Correspondingly, the surface roughness dramatically decreases in the VO 2 epitaxial thin films on the annealed substrates, compared with that on the unannealed substrates. The order of magnitude of the resistivity change ratio (~ 10 2 ) of annealed samples across the metal-insulator transition (MIT) decreases by a factor of one, compared with that (~ 10 3 ) in unannealed samples. This result is ascribed to grain size effect in the VO 2 epitaxial thin films. Moreover, the MIT temperature is reduced in the annealed samples with various thickness, compared with the unannealed ones. A reduction of 14.4 K of the MIT temperature is observed in the thinnest VO 2 films on the annealed substrates, compared with the unannealed samples. This behavior results from a compressive strain along the V-V atom chains in the annealed samples, which modifies the orbital occupancy of the V 4+ ions. While increasing the film thickness, the MIT change ratio keeps on the order of magnitude 10 2 , and the MIT temperatures of the VO 2 films on the annealed substrates becomes closer and closer to those of the unannealed samples due to the weakened substrate effect. This work suggests a promising approach to decrease the MIT temperature and still maintain a moderate change ratio for the MIT, potentially enabling room-temperature electronic devices based on VO 2 thin films.