The Modulation of Metal-insulator Transition Properties in Vanadium Dioxide Based Film and Its Applications

Author:Ji Chun Zuo

Supervisor:wu zhi ming


Degree Year:2019





As a functional material with phase transition properties,vanadium dioxide(VO2)can undergo the reversible metal-insulator transition from high-temperature rutile tetragonal phase(R)to low-temperature monoclinic phase(M1)at 68℃,accompanied by huge changes in optical,electrical and magnetic properties.This phase transition has great potential in smart window,switching modulation,field effect transistor,information storage,and optoelectronic devices.However,there are still many problems limiting the practical application of VO2 thin films,such as the simple preparation method of high-quality VO2 thin films and the effective modulation of phase transition properties according to specific application fields.In view of the above problems,the MIT performance of reactive magnetron sputtering VO2 film was adjusted by means of dopants and buffer layer,which makes it suitable for smart window or terahertz modulation application.In addition,through the use of various material characterization methods,the relationship between the microstructure of VO2film and its MIT performance was carefully studied.The main contents are summarized as follows:1.To regulate the high intrinsic phase transition temperature,low visible light transmittance and undesirable solar light regulation efficiency,the Al element is doped into VO2 thin films to improve the thermochromic properties for smart window applications.Differ from previous reports,our results show that the phase transition temperature of Al-doped VO2 films can be significantly reduced to 44.9℃,the visible light transmittance can be increased from 47.5%to 48.9%,and the solar light modulation efficiency can be increased from 6.6%to 7.6%.These improvements in thermochromic properties indicate that Al-doped VO2 films have great potential for smart window applications.Obviously,the increase in visible light transmittance can be attributed to the expansion of optical band gap from 1.75 eV to 1.96 eV and the enhancement of the solar light modulation efficiency corresponds to the obvious improvement of transmittance modulation in near infrared region.On the other hand,the decrease of the phase transition temperature is derived from the peculiar surface morphology of Al-doped VO2 film,i.e.nanowire clusters and polygonal grains.Moreover,the Al-doped VO2 film maintains good thermochromic properties afterundergoing multiple heating and cooling cycles,confirming its good environmental stability.Finally,by studying Al-doped VO2 films on different substrates,it is believed that the interfacial interaction between the glass substrate and the VO2 crystal is the key reason for forming special-shaped particles.2.Considering that a narrow hysteresis width and a large THz transmittance change is preferred for THz modulation application,the high-valent metal ion(W6+or Nb5+)is doped into polycrystalline VO2 film on silicon substrate to satisfy the requirements mentioned above.Specifically,in 6.5%Nb doped VO2 film,the hysteresis is largely reduced to 4.8℃,the phase transition temperature is marginally reduced to 31.1℃ and the THz modulation depth maintains 85.9%.In 1.6%W doped VO2 film,the hysteresis is largely reduced to 7.8℃,the phase transition temperature is marginally reduced to 45.7℃ and the THz modulation depth is 83.2%.Such performances are very excellent for practical applications.Further analysis suggests that the larger ionic radius and high valence state(>+4 valence)of Nb and W element are the key factors in the reduction of phase transition temperature.As for hysteresis loop,the increase of defect nucleation sites induced by doping makes the metal-semiconductor phase transition easier to occur and thus reduces the loop width.Also,similar-sized grains bring about a reduction in the hysteresis width.In addition,it is found that the good balance between THz modulation capability and phase transition temperature in Nb and W doped films is due to the high crystallinity of the film.Appropriate annealing can eliminate most point defects in the film,reduce lattice distortion,and improve crystallinity,so that the doped VO2 film can maintain excellent MIT performance.This result provides a new research idea for optimizing doping effect in VO2 system.3.Firstly,VO2 film with stable M2 phase at room temperature has been formed by Cr doping.The shifting of Raman peak and the appearance of XRD twin peak can confirm the presence of the M2 phase structure in Cr-doped VO2 films.For the optical properties in the terahertz region,it is found that the M2 phase VO2 film can significantly enhance the modulation amplitude at the phonon absorption frequency by suppressing the infrared-activated phonon mode of the material compared to the M1phase VO2 films.Specifically,the modulation amplitude is increased from 29.6%to39.0%at 8.25 THz and from 24.1%to 37.1%at 9.33 THz.These improvements are very helpful for the development of VO2-based devices in high THz frequency.In addition,the M2 phase VO2 film exhibits excellent metal-insulator phase transition characteristics,namely a very narrow hysteresis loop width(8℃)and a small phase transition slope(3.4℃),which are due to the increase of effective nucleation density and the transformation of crystal structure from M1 phase to M2 phase.4.High-quality VO2 thin films have successfully been prepared at low temperature condition(substrate temperature 160℃,annealing temperature 320℃)by means of low-valence vanadium buffer layer between the substrate and VO2 thin films,using a simple conventional magnetron sputtering method.The low-cost buffer layer is obtained by sputtering a metal vanadium target in a lower oxygen partial pressure.By comparing the effect of buffer layers with different composition on VO2 composite film,it is revealed that the low-valence vanadium layer can firstly be very useful in reducing the lattice mismatch between VO2 and Al2O3 substrate.Secondly,the high-quality crystal nucleus in the pre-deposited low-valence vanadium layer can act as the core of crystal growth in the subsequent deposition process,making thin film grow directly and then reducing the crystallization temperature.At the same time,the best low-valence vanadium buffer layer has been selected.On this basis,by optimizing the thickness of the low-valence vanadium buffer layer,the VO2 composite film with excellent electrical properties is obtained,in which the phase transition amplitude is close to 4 orders of magnitude and the loop width is less than 5℃.