Research On The Effects Of Fe And Mg Doping On Properties Of Vanadium Oxide Thin Films

Vanadium dioxide（VO₂）is a kind of reversible metal-insulator transition（MIT）material with the transition temperature at 68°C.Due to the MIT,an abrupt and large change in the electrical and optical properties will occur quickly,especially within 10^-12s（1ps）.These excellent characteristics make VO₂ potential in various applications such as smart windows,Mott transistors and gas sensors.Due to the sharp variation of optical transmittance in near infrared region（NIR）and low Tc,VO₂ has been considered as the most promising oxide to be used as coating for smart windows.However,it is not widely available because there are three main requirements that have to be met simultaneously for practical applications,such as,the high luminous transmittance(T_lum),the outstanding solar modulation ability(ΔT_sol),and the low transition temperature（Tc）.Doping other elements is the most convenient and effective way to improve the thermochromic properties of VO₂ films.In order to realize the application of vanadium dioxide films in smart windows,VO₂films doped with Fe and Mg were prepared by means of direct current（DC）magnetron sputtering.Through X-ray diffraction,Raman shift,X-ray photo-electron spectroscope and SEM tests,we investigate the effects of doping other elements on VO₂ films,specifically in the aspects of the composition,crystal structure,surface morphology and MIT properties of the films.The main conclusions are summarized as follows:（1）The effects of Fe-doping on the thermochromic properties and microstructures of VO₂ films prepared on glass substrates were investigated.The results show that because of the distortion of VO₂ crystalline and amounts of crystal defections resulted from Fe-doping,the Fe³⁺cation is found to be effective in reducing Tc.when Fe doping level is 9.8 at.%.the Tc reduces to 33.5℃.Moreover,Fe-doping can increase the visible light transmittance of the film while maintaining a large amplitude of the solar light modulation.（2）For the purpose of improving the visible light transmittance of VO₂ films,the effects of Mg-doping on the thermochromic properties and microstructures of VO₂ films prepared on ITO and glass substrates were studied.The results show that Mg-doping can effectively increase the visible light transmittance and reduce the Tc.By comparing the samples prepared on ITO and glass substrates,it was found that different substrates also have the influence on the crystalline states and MIT properties of VO₂ films,mainly reflected in the composition,surface morphology and MIT properties of VO₂ films.（3）Fe/Mg co-doped VO₂ films were successfully prepared on the glass substrates by controlling Fe doping level to be 9.2 at.%strictly and then doing the Mg-doping.The experimental results show that the Tc of the co-doped VO₂ films is effectively reduced due to the synergistic effect of Fe and Mg elements,and the visible light transmittance is also greatly increased owing to the Mg-doping.At the same time,Fe/Mg co-doping does not change the structure of the VO₂ crystals,which remain the monoclinic phase.However,the surface morphology of the co-doped films is greatly affected by doping.The larger the doping level,the smaller the grain size of the VO₂ films,and the grain boundary becomes more blurred.