Structure-functional property relationships in rf-sputtered vanadium dioxide thin films

Abstract

The study of metal-insulator transition (MIT) in VO₂ thin films synthesized by means of rf sputtering from a VO₂ target is presented. A comparison with conventional reactive sputtering from a V target is also given. Detailed x-ray diffraction analysis, electrical resistance switching, and infrared optical reflectance measurements confirm that our sputtering technique yields high-quality VO₂ films. We discuss in depth how synthesis conditions affect MIT parameters derived from temperature dependence of electrical resistance. Sharp MIT is observed in films sputtered on technologically important Si substrates. The choice of Si (or sapphire) substrates results in the transition temperature above (below) the values obtained for single VO₂ crystals. The MIT becomes narrower and stronger in thinner films. This is consistent with the assumption that the increased width of the MIT in thin films with respect to single crystals is the result of averaging of the transition parameters over a distribution of crystallites in the film. The measurements of MIT in VO₂ patterned into devices do not reveal a noticeable lateral size effect down to 20 μ m devices, encouraging use of the phase transition in switching electronic devices. The effect of substrate temperature and ambient during the sputtering on MIT is discussed. While VO₂ films are found to be stable in ambient environment with time, the additional exposure to UV radiation near room temperature is shown to enhance the oxidation kinetics and produce changes in film resistance. Using UV radiation as an additional tool to control the oxidation process during VO₂ synthesis may allow the synthesis temperature to be lowered and an improvement in material quality. We anticipate these results may be of relevance to synthesizing functional oxide films with potential applications in electronics and sensors.