Preparation And Characteristics Of VO_X Thin Films With Low Phase Transition Temperature

Vanadium dioxide (VO2) is well-known for its sharp metal-insulator transition (MIT) at68℃which is accompanied by a structural transition from monoclinic (low temperature) to tetragonal (high temperature) phase. This transition also brings with it abrupt changes in the optical and electrical properties. Based on the phase transition theory, the dissertation presents a detail discussion of fabricating-condition influence on the transition performance of VO2thin films, such as type of substrate, substrate’s temperature, Ar and O2gas mixture ratio, and thermal annealing process. Additionally, several kinds of deposition methods are introduced, such as Sol-Gel, magnetron sputtering, ion beam sputtering, plused laser deposition, chemical vapor deposition, and so on. The main purpose of this work is to fabricate VO2thin films with low phase transition temperature by appling magnetron and ion beam sputterings. The main contents including theory analysis, experiments, and results are summarized as follows:First, the changes in VO2lattice and band structures during the phase transition are introduced. Furthermore, impact factors for Tt values of VO2thin films are discussed, such as the deposition condition, lattice strains, surface structure and doping, and the mechanism of the above factors to determine the VO2film Tt is analysed to guide the following experiments.Second, nanostructural vanadium oxides (VOX) thin films with low MIT temperature were fabricated through reactive ion beam sputtering (LD-3) followed by a thermal annealing process. The VOX films were grown on borosilicate glass substrate with a Si3N4buffer layer at varying substrate temperature, Ar and O2gas mixture ratio and the annealing temperature. The electrical resistance tests indicate that the films’Tt rises (from29to35℃) as the growth temperature increases (from250to310℃). Besides, change of Ar and O2gas mixture ratio has significant impact on the films’ transition temperature and switching efficiency. VOX thin films fabricated with Ar and O2gas mixture ratio of60:20,60:30and60:40SCCM (standard-state cubic centimeter per minute) exhibit a phase transition feature at the temperature of36,30and32℃, respectively. Additionally, the VOX thin films show a phase transition character at temperature of35,30and34℃, as the samples annealed at400,430and460℃, respectively. It can be concluded from the above results that the optimum conditions for fabricating VO2thin films are:substrate temperature280℃, Ar and O2mixture ratio60:30SCCM and annealing temperature430℃.Third, VO2films with a low MIT temperature of45℃were fabricated through direct current magnetron sputtering (MSP-3200E) followed by a post-annealing. The process parameters are:substrate temperature300℃, Ar flow40SCCM,O2flow5SCCM, annealing temperature460℃and annealing time60min. Atomic force microscopy measurements show that the VO2grain size is about one hundred of nanometers. The results of electrical and optical tests reveal that the VO2film not only exhibits outstanding change in resistace, but also behaves excellent IR switching property, which make the VO2thin film a proming material for smart window and laser protection application.Fourth, VOx films with MIT temperature around68℃on different types of substrates have been prepared by HDG dual ion source vacuum equipment at room temperature (RT) and a short time thermal process. The IR transmittance measured below/above Tt illustrates the VOx possess excellent switching efficiency. Furthermore, tungsten-doped VO2(V0.98W0.02O2) thin films with low MIT Tt of34±1℃were grown on borosilicate glass substrates with varying annealing temperature. X-ray photoelectron spectroscope and Raman measurements demonstrate that the tungsten atoms have been successfully doped into VO2films and exist as the form of W6+in the films. Although the tungsten-doped VO2films show a MIT near RT, i.e.34±1℃, the amplitude of the transition and the switching efficiency confirmed by the electrical and optical measurements are not good enough compared with the previous studies. Nevertheless, the RT deposition and easy control of annealing process have laid a good foundation for its industrial production. In addition, the short annealing time, i.e.30min, will not only save the costs, but also speed up the re-production process.The VO2thin film prepared by MSP-3200E magnetron sputtering and a post annealing process is selected for application investigation. The smart window experiment shows that the VO2thin film has an amazing ability to self-adjust the device temperature intelligently, and keeps the indoor temperature at a stable value of47℃under a continuous solar radiation. Moreover, laser radiation test indicates that VO2thin film exhibits excellent high power resist property, and threshold power density is up to1.46×103W/mm2. From the above results we can conclude that the obtained VO2thin film can be effectively used for smart window and laser protection application.At the end of the dessertation, the achievements obtained in this work as well as some existing problems have been summarized, and the prospect of future work has been suggested.