Fundamental Study On Optical-electrical Properties And Applications Of Vanadium Oxide Thin Films

This thesis emphatically presents the theoretical and experimental research on theoptical and electrical properties of vanadium oxide thin films.In experiment,the depositionof vanadium oxide thermal sensitive layers for uncooled infrared detectors,the applicationsof vanadium dioxide thin films with low phase transition temperature for optical swithesand smart windows were studied.The thermal insulator structure was performed usingPro-Silicon for vanadium oxide uncooled infrared detector.In theory,mechanics of phasetransition and the properties of thermal hysteretical curves for VO₂ thin films were studied.The main contents of the dissertation are as follows.(1) The vanadium oxide thermal sensitive layers were studed for the application ofuncooled infrared detectors.By using the methods of reactive ion beam sputtering andreactive de magnetic control sputtering,the vanadium oxide thin films were deposited inoptimized fabrication craft.The test results show that the thin films are well candidatematerials for uncooled IR detectors.To acquire the surface pattern,crystalline phasestructure and infrared absorption spectrum of vanadium oxide thin films,the typicalsamples were revealed by XRD,SEM and infrared spectroscope measurements.The TCRof-2.26%/℃and square resistance of 31 kΩ/â–¡were acquired by reactive ion sputteringand post annealed method.The typical deposition temperature in dc magnetic sputteringmethod is 210℃,which is compatible with the post-CMOS technology.The as-depositedfilm exhibits sheet resistance,temperature coefficient resistant and thickness of 18.4kΩ./â–¡kΩ,-2.05%/℃and 65nm at room temperature,respectively.The characteristics of VO_xthin films approximate the value reported by Honeywell.(2) To study the phase transition properties of VO₂ thin films,random resistor network(RRN) model was employed to simulate the characteristics of temperature dependence ofresistance in VO₂ thin film.The system was modeled as a composite medium consisting ofsemiconducting and metallic regions randomly distributed in the microcrystals.There wassatisfactory agreement between the calculated resistance-temperature trajectories and themeasured major hysteresis loops for temperature covering the whole range (10℃-75℃).Onthe discussion of physical mechanical in our simulation,the phase segregation occurred and the phase transition in VO₂ thin films was due to the competition between thesemiconducting and metallic regions of these two components in the procedure oftemperature changing.(3) The hysteresis behavior of VO₂ thin films was studied by using the Preisach model.The classical Preisach model could solve the hysteresis behaviors with symmetrical curvesdistributed in four quadrants.While the resistance versus temperature characteristics of theVO₂ thin films were asymmetrical and were only distributed in the first quadrant.In thepresent work,adapted preisach model was employed to simulate the hysteresis ofnanocrystalline vanadium dioxide thin film with low phase transition temperature.Thesimulating result showed that there was satisfactory agreement between the calculatedresistance-temperature trajectories and the measured data for major and minor hysteresisloops,indicating that the adapted Preisach model could provide a theoretic support for theworking mode of the thermal hysteresis in VO₂ thin film.(4) The applications optical switches and smart windows were studied based onmetal-semiconductor phase transition properties of VO₂ thin films.In the study of smartwindows,VO₂ thin films with low phase transition temperature were deposited on sapphireand glass substrates.The transmittance of these thin films in infrared and visible lightwavelength was measured.In order to increase the transmittance of two kinds of smartwindows in visible light,the thickness of anti-reflecting film were simulated by software ofTFcale.It is confirmed that SiO₂ anti-refection coating could improve low transmittance ofthe films in visible light wavelength.In the study of optical switch,the optical switch layerswere used by nano-VO₂ thin films and the properties of optical switch were test.The resultshowed that the insertion lose was about-16dB.To measure the response time of VO₂ thinfilm,the thermal effect induced by laser-heating was simulated by soft ware of ANSYS.The proper laser-heating power was chosen according the simulating result.The test showthat the response time of the thin films is about 50.4ms.(5) To improve the thermal response of uncooled infrared detectors based on vanadiumoxide thin films.The Porous Silicon (PS) sacrifice layers were prepared to fabricate themicro-bridge structure of uncooled microbolometers.To apply the PS layers to themicro-bridge of vanadium oxide film IR detector,two bathes was used to prepare the PSand properties of PS were profoundly studied.The surface pattern of PS was studied by varied craft parameters.The thicknesses of PS layer were studied by changing the erosetime and current density.The crack phenomenon induced by residual stress was measuredby Micro-Raman spectroscopy.The test results showed that the stress was tensile stress.The Micro-Raman spectroscopy provided an analysis method for improving the quality ofPS layer.Finally,the sacrifice layer of PS was successfully applied in the thermal insulatorstructure of vanadium oxide IR detectors.