Electrical And Infrared Modulation Properties Of Vanadium Dioxide Films And Its Applications

Vanadium dioxide(VO₂)is a metal-to-insulator transition(MIT)material.When the temperature reaches around 68?,the first-order phase transition of VO₂ from room temperature monoclinic insulating phase(M1)to high temperature rutile metal phase(R)will occur.The MIT of VO₂ is accompanied by the sudden changes of physical properties such as electrical and optical properties,its applications in switching modulation,photoelectric devices,intelligent windows,sensors,drivers and so on have been widely studied due to the excellent MIT characteristics of VO₂.However,for practical applications,there are still many problems to be solved,such as how to use simple and efficient methods to modify VO₂ films and optimize their MIT properties,and how to prepare high-quality VO₂ films with low cost and high efficiency.Therefore,in this dissertation,VO₂ thin films are prepared by DC magnetron sputtering,and their MIT properties are regulated by doping,co-doping and wet etching,so that VO₂thin films can be used in the fields of electrical modulation and smart window.At the same time,through various characterization of the prepared VO₂ films,the effects of their microstructure,such as morphology and crystallinity,on the MIT properties of VO₂films were systematically studied and discussed.The main contents of this dissertation are summarized into the following three sections.1.Narrow loop width(?H),small phase transition slope(?T),low phase transition temperature(T_C)and appropriate phase transition amplitude(?A)are required for VO₂electrical performance applications,in order to meet the above requirements,Fe³⁺is doping into VO₂ films on sapphire substrate,it is found that the VO₂ films change from M1 phase to M2 phase,which is confirmed by laser Raman spectroscopy.The MIT properties of Fe-doped VO₂ films are characterized and the results show that the electrical MIT properties are all optimized and the maximum?A is up to four orders of magnitude,mainly due to the transformation of VO₂ film into M2 phase and the concentration of defect nucleation site introduced by doping.The VO₂films on sapphire substrate are doped with Ta⁵⁺,and the electrical MIT properties are characterized to find that the decrease degree of?H is very large and the minimum?H is 3.0?,but at the same time,?A also drops sharply to about dozens of times.After doping VO₂ thin films with Ta⁵⁺on Si₃N₄ substrate,it is found that the minimum?H is shortened to 0.9?,which is mainly due to larger ionic radius of Ta⁵⁺and the change of the surface morphology of the film.The VO₂ thin films prepared on sapphire substrate are co-doped with Ta⁵⁺and Fe³⁺,after characterizing the electrical MIT properties,VO₂ film which is suitable for electrical modulation or electrical switch with obvious?A,T_C of 49?,?H of 3.6?and?T of 5.1?is obtained.2.The practical application of thermochromic coatings based on VO₂ mainly faces three challenges:(1)the T_C of intrinsic VO₂ is too high;(2)solar modulation rate(?T_sol)is undesirable;(3)visible light transmittance(T_lum)is low.In this dissertation,wet etching is proposed to improve the thermochromic properties of VO₂ films.After etching VO₂films with different thickness for 3 min,it is found that the T_lum of the films is improved,which is mainly due to the change of surface morphology and the decrease of thickness,while?T_sol shows a decreasing trend.Then,after etching the sample with a thickness of145.9 nm for 3,6,9 and 12 min,the XRD and Raman results show that the crystallinity of the films maintains high quality when the etching time is within 9 min,but the crystallinity begins to decrease after 12 min.According to the results of SEM,it is mainly due to the complete etching of VO₂ particles in some regions,resulting in the destruction of their crystal structure.The cracks and holes on the film surface gradually increase with the increase of etching time and the thickness of VO₂ film is further reduced,resulting in the gradual increase of the T_lum and degeneration of?T_sol.After 9 min and 12 min etching,the T_lumof VO₂ film increases from 16.9%to 32.6%and 56.4%,?T_sol decreases from16.3%to 15.5%and 12.2%,respectively.This conclusion provides a new idea for the application of VO₂ film in the field of smart window.3.Since the T_C of the etched film is still high,the MIT property of VO₂ film is modified by W⁶⁺doping,after doping T_C and?H of VO₂ films decrease significantly,mainly due to the large ion radius of W⁶⁺and defect nucleation site concentration introduced by doping.At the same time,SEM results show that pores and cracks on the surface after doping which resulting in the increase of T_lum and decrease of?T_sol.VO₂films doped with W⁶⁺at different concentrations are etched for 9 min,the crystallinity of the etched films decreases when the W doping concentration is 0.31%and 0.75%,while the morphology of W-doped films with large pores and cracks after 9 min etching is similar to that of undoped films after 12 min etching.After characterizing the MIT performance of these VO₂films,it is proved that both T_C and?H have increased,the main reason is the decrease of W element doping concentration in the films.At the same time,due to the decrease of film thickness and the change of surface morphology,T_lum has increased significantly to about 48%,and?T_sol has decreased to 8.7%after 9 min etching.After 8 min etching of VO₂ films with W doping concentration of 0.75%,VO₂ film with T_C of 27.8?,?H of 4.6?,T_lum of 44.4%and?T_sol of 10.5%is obtained,which is suitable for smart window application.