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）at68oC which is accompanied by a structural transition from monoclinic （low temperature）to tetragonal （high temperature） phase. This transition also brings with it abrupt changes inthe optical and electrical properties. Based on the phase transition theory, the dissertationpresents a detail discussion of fabricating-condition influence on the transition performanceof VO₂thin films, such as type ofsubstrate, substrate’s temperature, Ar and O2gas mixtureratio, and thermal annealing process. Additionally, several kinds of deposition methods areintroduced, such as Sol-Gel, magnetron sputtering, ion beam sputtering, plused laserdeposition, chemical vapor deposition, and so on. The main purpose of this work is tofabricate VO₂thin films with low phase transition temperature by appling magnetron andion beam sputterings. The main contents including theory analysis, experiments, and resultsare summarized as follows:First, the changes in VO₂lattice and band structures during the phase transition areintroduced. Furthermore, impact factors for Ttvalues of VO₂thin films are discussed, suchas the deposition condition, lattice strains, surface structure and doping, and the mechanismof the above factors to determine the VO₂film Ttis analysed to guide the followingexperiments.Second, nanostructural vanadium oxides （VOx） thin films with low MIT temperaturewere fabricated through reactive ion beam sputtering （LD-3） followed by a thermalannealing process. The VOxfilms were grown on borosilicate glass substrate with a Si3N4buffer layer at varying substrate temperature, Ar and O2gas mixture ratio and the annealingtemperature. The electrical resistance tests indicate that the films’ Ttrises （from29to35oC）as the growth temperature increases （from250to310oC）. Besides, change of Ar and O2gasmixture ratio has significant impact on the films’ transition temperature and switchingefficiency. VOxthin films fabricated with Ar and O2gas mixture ratio of60:20,60:30and60:40SCCM （standard-state cubic centimeter per minute） exhibit a phase transitionfeature at the temperature of36,30and32oC, respectively. Additionally, the VOxthin filmsshow a phase transition character at temperature of35,30and34oC, as the samplesannealed at400,430and460oC, respectively. It can be concluded from the above resultsthat the optimum conditions for fabricating VO₂thin films are: substrate temperature280oC, Ar and O2mixture ratio60:30SCCM and annealing temperature430oC.Third, VO₂films with a low MIT temperature of45oC were fabricated through directcurrent magnetron sputtering （MSP-3200E） followed by a post-annealing. The process parameters are: substrate temperature300oC, Ar flow40SCCM, O2flow5SCCM,annealing temperature460oC and annealing time60min. Atomic force microscopymeasurements show that the VO₂grain size is about one hundred of nanometers. Theresults of electrical and optical tests reveal that the VO₂film not only exhibits outstandingchange 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, VOxfilms with MIT temperature around68oC on different types of substrateshave 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 illustratesthe VOxpossess excellent switching efficiency. Furthermore, tungsten-doped VO₂（V0.98W0.02O2） thin films with low MIT Ttof34±1oC were grown on borosilicate glasssubstrates with varying annealing temperature. X-ray photoelectron spectroscope andRaman measurements demonstrate that the tungsten atoms have been successfully dopedinto VO₂films and exist as the form of W6+in the films. Although the tungsten-doped VO₂films show a MIT near RT, i.e.34±1oC, the amplitude of the transition and the switchingefficiency confirmed by the electrical and optical measurements are not good enoughcompared with the previous studies. Nevertheless, the RT deposition and easy control ofannealing process have laid a good foundation for its industrial production. In addition, theshort annealing time, i.e.30min, will not only save the costs, but also speed up there-production process.The VO₂thin film prepared by MSP-3200E magnetron sputtering and a post annealingprocess is selected for application investigation. The smart window experiment shows thatthe VO₂thin film has an amazing ability to self-adjust the device temperature intelligently,and keeps the indoor temperature at a stable value of47oC under a continuous solarradiation. Moreover, laser radiation test indicates that VO₂thin film exhibits excellent highpower resist property, and threshold power density is up to1.46×103W/mm2. From theabove results we can conclude that the obtained VO₂thin film can be effectively used forsmart window and laser protection application.At the end of the dessertation, the achievements obtained in this work as well as someexisting problems have been summarized, and the prospect of future work has beensuggested.