Combined Role of Substrate and Doping on the Semiconductor-to-Metal Transition of VO₂ Thin Films

Abstract

Vanadium dioxide exhibits a sharp temperature-induced structural change (monoclinic to rutile) that induces a semiconductor-to-metal transition together with a major change in its electrical properties. Even though VO2 thin films are a promising candidate for numerous applications to electronic and energy devices, one of the greatest challenges toward integration of this material is to precisely control the transition temperature. Substrate-induced interfacial effects and dopant-induced isostatic stress combined with excess electron injection are the main paths used to reduce the temperature of transition. In this work, we combine metallic doping with tungsten atoms and strain engineering in VO2 thin films in order to lower the transition temperature while maintaining a high resistivity contrast. Epitaxial undoped films are shown to respond to substrate-induced strain by relaxing with increasing thickness but with a limited reduction of the transition temperature. In contrast, epitaxial doped films do not show any dependence on substrate-induced strain but yield a great reduction of the transition temperature as compared to polycrystalline relaxed doped films. This temperature shifts down to room temperature or even close to 0 °C depending on the doping level. This shift is accompanied by a different morphology of the film as compared to undoped films. In contrast to undoped VO2 where the substrate-induced effect is limited to very thin films, combining tungsten doping and substrate-induced strain allows one to take advantage of the interfacial effect at any film thickness. Moreover, activation energies below 100 meV and greatly reduced transition temperatures combined with an electrical contrast still above 2 orders of magnitude make tungsten-doped VO2 films of strong interest for many devices.

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