Near bulk semiconductor to metal transition in epitaxial VO2 thin films

Abstract

We have been able to achieve semiconductor-to-metal transition (SMT) temperature in VO2 thin films close to the values reported for bulk VO2 single crystals. This was achieved by complete relaxation of misfit strain, which leads to a negligible tension/compression along VO2 [001], upon introduction of NiO buffer layer on c-plane sapphire substrate. In this paper, we discuss the mechanism behind complete relaxation of misfit strain which occurs under the paradigm of domain-matching epitaxy, where integral multiples of planes match across the interface. NiO buffer layers were grown in situ, prior to the VO2 deposition, using pulsed-laser deposition technique. X-ray θ-2θ, φ, and pole figure scans were performed for structural characterization of the VO2/NiO/Al2O3 (0001) heterostructure. All the constituent layers of the heterostructure were found to be epitaxial with orientation relationship: (020)VO2∥(111)NiO∥(0001)Al2O3 and ⟨100⟩VO2∥⟨110⟩NiO∥⟨101¯0⟩Al2O3. Parameters related to SMT, such as hysteresis and transition width were extracted from the Gaussian fit of temperature dependence of electrical resistance. These parameters have also been discussed in correlation with the strain along c-axis of VO2, in-plane orientation, and microstructure. A comparison has been made between the VO2 thin films deposited with and without NiO buffer layer on c-sapphire substrates to delineate the importance of strain relaxation in attaining near bulk SMT temperature values.