Growth and Characterization of Single crystalline VO2 Heterostructures

Abstract

Vanadium dioxide is a strongly correlated material with a temperature driven metal-to-insulator transition (MIT) near room temperature (~340 K). This electronic transition is accompanied by a structural transition from high temperature rutile to low temperature monoclinic structure. The phase transition in VO2 can be controlled by external perturbations such as for example atomic doping, strain in thin films, or by applying an external electric field using an electric double-layer transistor (EDLT). However, all these methods lead to changes in the lattice parameters either due to epitaxial strain from the underlying substrate or due to the creation of defects such as oxygen vacancies or local strain due to dopants. In this work, we set out to explore approaches to control the metal-insulator phase transition in VO2 by controlling the electron carrier density in the insulating phase. We explored modulation-doping and ionic liquid gating of modulation-doped heterostructures of VO2 as two potential approaches to realize a carrier-density control of electronic phase transition in VO2. First, we discuss the deposition of atomically smooth single crystalline VO2 films on TiO2 substrates using pulsed laser deposition technique. Using different thin film characterization techniques, we show that these films are atomically smooth and single crystalline with nearly 3 orders of magnitude change in resistivity across the MIT. Next, we discuss the deposition and characterization of the modulation-doped VO2 heterostructures to control the metal-insulator phase transition in VO2 thin films. We used an oxygen deficient TiO2-x as a dopant layer and LaAlO3 (LAO) as a spacer layer in between VO2 and TiO2-x. Due to low oxygen vacancy-diffusivity in LAO, it does not allow the oxygen migration from VO2 to TiO2. Finally, the full structure was capped with LAO layer to prevent the oxidation of the TiO2-x layer. We see a strong correlation between the temperature of MIT and the thickness of VO2 film in the heterostructure. Based on electrical transport measurements, we infer that decreasing the VO2 thickness in the heterostructure increases the effective carrier density in the insulating phase of VO2. We employed temperature and polarization-dependent X-ray absorption spectroscopy (XAS) to probe the changes to the electronic structure and orbital occupancy in the vicinity of the Fermi level of VO2. We discuss the difference in the XAS spectra in metallic and insulating state, which is a result of the structural and electronic changes across the MIT. We further discuss the implications of XAS spectra on the origin of the increase in carrier density with decreasing thickness of VO2 in modulation-doped heterostructures. Finally, we discuss the fabrication of electrical-double layer transistors (EDLT) of VO2 while ensuring that the MIT in fabricated devices is unchanged during fabrication. Next, using measurements on EDLT devices, we discuss the possibility of reversibly controlling the resistivity change of VO2 heterostructures across the metal insulator transition. We further discuss the implications of our research for a pure electronic control of MIT in VO2.