Study On Improving The Phase Transition Properties Of VO_X Thin Films By Pre-annealing And Graphene Buffer Layer

Vanadium dioxide(VO₂)exhibits metal-to-semiconductor phase transition under the stimulation of light or heat,which is accompanied by reversible changes of the electrical and optical properties.These properties give VO₂ broad application prospects in some optical and electrical devices,such as smart windows,optical switches and memories.Magnetron sputtering metal vanadium film coupling with rapid thermal annealing oxidation is a common method for preparing VO₂ film,and this method has the advantages of good controllability and high repeatability.However,the VO₂ film which is prepared by this method generally has poor phase transition properties and is difficult to meet the application requirements.Therefore,how to improve the phase transition properties of VO₂ films which are prepared by magnetron sputtering coupling with rapid thermal oxidation has become a hot research topic.In this paper,the effect of adding pre-annealing before rapid thermal annealing on the phase transition properties of vanadium oxide(VO_X )films was first studied.Two groups of metal vanadium films were prepared by magnetron sputtering with the same parameters.One group was oxidized into VO_X films by direct rapid annealing,and the other group was oxidized into VO_X films by pre-annealing and subsequent rapid thermal annealing.The crystallinity,surface morphologies,electrical and optical properties of the two groups of VO_X films were investigated by X-ray diffraction(XRD),scanning electron microscope(SEM),atomic force microscope(AFM),four-probe test system and spectrophotometer respectively.The experimental results showed that the VO_X film which was prepared by direct rapid annealing had the maxmuim phase transition amplitude of 24.8 and the solar modulation efficiency of3.8%,and the phase transition temperature reached 55°C.The content of VO₂ in the VO_X film was increased when the pre-annealing treatment was added before the rapid annealing process.This led to the phase transition amplitude of the VO_X film increased to the maximum of 160 and the solar modulation efficiency increased to6.1%.At the same time,the roughness of the VO_X thin film after the pre-annealing treatment was lowered,resulting in the phase transition temperature of the VO_X film decreased to 50°C.Based on the excellent electrical properties of graphene,the VO_X /graphene composite film was prepared by adding a graphene buffer layer between the VO_X film and the sapphire substrate.And its phase transition properties were studied.The results showed that,the VO_X film which was not composited with graphene had phase transition amplitude of 20 and a phase transition temperature of 61°C.The surface of the VO_X film formed uniformly distributed particles after the VO_X film was composited with graphene,which caused the phase transition amplitude of the VO_X film increased to 180.At the same time,graphene and VO₂ formed a semimetal-semiconductor contact after adding the graphene buffer layer between the VO_X film and the sapphire substrate.Since the work function of graphene was smaller than that of VO₂,electrons in graphene would flow into VO₂.This made the semiconductor state of VO₂ unstable,reducing the phase transition temperature of VO_X thin film decreased to 58°C.In this paper,the phase transition properties of VO_X films were improved by pre-annealing process and graphene buffer layer.It is helpful to improve the preparation process of VO₂ film.