SOLIDIONIC

Influence of Hydrogenation on Electrical Conductivity of Vanadium Dioxide Thin Films

Abstract

The influence of hydrogenation on electrical conductivity of vanadium dioxide thin films has been investigated. It has been shown using measurements of the electrical conductivity that the hydrogenation of vanadium dioxide thin films leads to a decrease in the temperature of the phase transition from the tetragonal phase (with “metallic” conductivity) to the semiconducting monoclinic phase. It has been found that, upon doping of vanadium dioxide with hydrogen, the electrical conductivity of the monoclinic phase can increase by several orders of magnitude. Nonetheless, the temperature dependence of the electrical conductivity of hydrogenated films exhibits a typical semiconducting behavior in the temperature range where the monoclinic phase is stable.

INTRODUCTION

Single crystals of pure stoichiometric vanadium dioxide (VO₂) during cooling at temperatures below
T_c = 640 K undergo a structural phase transition from the tetragonal phase to the monoclinic phase. This phase transition is initiated by the formation of chemical bonds between pairs of the neighboring vanadium ions and accompanied by a considerable (to 10⁵) decrease in the electrical conductivity [1, 2]. Owing to this significant increase in the electrical resistivity due to the chemical localization of conduction electrons in the pairing of vanadium ions, the phase transformation occurring in vanadium dioxide can be considered as a metal–semiconductor phase transition. In our previous works [3, 4], we revealed and investigated an interesting phenomenon of hydrogen penetration into polycrystalline vanadium dioxide thin films in the case
where these films were simply immersed in aqueous solutions of alcohols. It was shown that the rate of
hydrogen penetration into the films very strongly depends on the state of vanadium dioxide. For the
hightemperature (“metallic”) phase, the hydrogen penetration rate is one order of magnitude higher than
that observed at the same temperature for the semiconducting monoclinic phase. In [3, 4], we also found
that, under normal conditions, the hydrogenated vanadium dioxide is unstable. Hydrogen begins to
leave the film as soon as it is removed from the solution, and the rate of dehydrogenation of the “metallic” tetragonal phase is substantially higher than the rate of dehydrogenation of the monoclinic phase. Nonetheless, it was noted in [3, 4] that, upon relatively rapid cooling of the film, the results of hydrogenation
are retained to the liquidnitrogen temperature. In this work, we have investigated the temperature
dependence of the electrical conductivity of hydrogenated vanadium dioxide.

To download or view full article in PDF format click here