Ambiguous Role of Growth-Induced Defects on the Semiconductor-to-Metal Characteristics in Epitaxial VO₂/TiO₂ Thin Films

Abstract

Controlling the semiconductor-to-metal transition temperature in epitaxial VO thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO epitaxial thin films grown on TiO substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V-V bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO/TiO multilayer structures.

The Effect of Substrate Biasing during DC Magnetron Sputtering on the Quality of VO2 Thin Films and Their Insulator–Metal Transition Behavior

“…Usually the variation of the transition temperature has been associated with the stress and microstructure of the samples [28]. In this work, it is likely that the increased defect density may also lead to an increase of T t along the lines in [29]. Another factor is the grain size.…”

Tuning the Metal–Insulator Transition in TiO2/VO2 Superlattices by Modifying the Layer Thickness or Inducing Defects

“…It has been reported that the VO 2 transition is accompanied by structural changes, such as V–V dimerization along the c- axis. The dimerization of the V–V chains contributes to the formation of an insulating band gap . It has been reported that Ti atoms gradually diffuse from the TiO 2 layer to the VO 2 layers, leading to the formation of a V x Ti 1– x O 2 solid solution.…”

Well-Dispersed Monoclinic VO2 Nanoclusters with Uniform Size for Sensitive near-Infrared Detection

“…The wide range of ordered point defects caused by V n O 2 n –1 (Magnéli phases) or V n O 2 n +1 (Wadsley phases) phases also made it different to synthesize VO 2 with good stoichiometry. , Many methods have been attempted to prepare monoclinic VO 2 material. For example, molecular beam epitaxy, − pulsed laser deposition, − and magnetron sputtering , are usually used to prepare monoclinic VO 2 epitaxial films, whereas chemical vapor deposition, physical vapor deposition, , hydrothermal method, − ion beam lithography technology, and atomic layer deposition technology are used for synthesizing different monoclinic VO 2 nanostructures including nanowires/beams or nanoparticles. It is suggested that the MIT of VO 2 induced phase separations or other emergent phenomena are closely associated with the grain size and boundaries.…”