Research on Effect of Point Defects on Structure and Properties of Vanadium Oxide
Author:Huang Tai Xing
Vanadium dioxide(VO2)thin film is a kind of functional thin film with phase change characteristics.It has potential applications in infrared detection,uncooled infrared imaging,optical control devices,intelligent windows,adjustable band stealth equipment,super surface and other fields.The VO2 phase transition process is affected by temperature excitation.The first-order phase transition from monoclinic phase to tetragonal phase occurs during the heating and cooling process.At the same time,the resistivity of VO2 single crystal films changes by 4-5 orders and the optical transmittance changes obviously.Especially in the infrared band,the transmittance changes from high to low,so VO2 single crystal films can be used in intelligent optical devices.However,due to the latent heat of phase change,there is a thermal hysteresis loop in VO2 phase change process,which results in the structure and performance of VO2 cannot corresponding to the temperature one by one,causing the application difficulties of infrared camouflage and tunable optical waveguide devices.On the other hand,the phase transition temperature of VO2 is about 60℃,which is much higher than room temperature,which brings inconvenience to the wide application of VO2 thin films.The above-mentioned problems have become the key difficulties restricting the application of VO2 materials.Point defects such as vacancies,substitution atoms and interstitial atoms have a great influence on the structure and properties of materials,and can have a significant impact on the crystalline phase,electrical and optical properties of materials.In recent years,the effect of point defects on the phase transformation process and structural properties of vanadium dioxide materials has become the focus of research,especially the effect of point defects on the optical and electrical properties of vanadium dioxide can enable us to obtain ideal phase change materials.If the defects are reasonably utilized,the problems of small phase transition amplitude,high hysteresis loop and high phase transition temperature can be solved.In this paper,focusing on the effect of point defects on the structure and properties of VO2 thin films,the laser pulse deposition was used to prepare and control the content of point defects in VO2 thin films.Confocal Raman and other characterization methods were used to test the phase transition process of materials.Based on the equivalent medium method and Drude-Lorentz model,the effects of point defects on the structure and properties of VO2 thin films were studied.The main results are as follows:(1)Pure phase VO2 thin films were prepared by pulsed laser deposition.The effects of annealing at different oxygen partial pressures on the photoelectric properties of the samples were studied.It was found that the phase composition of VO2 films changed from V2O3 to VO2 and then to V2O5 with the increase of oxygen partial pressure during annealing.A new Raman scattering peak at 166 cm-1 was observed by confocal Raman microscopy,which was the mixed valence region of V4+/V5+ in VO2 and V2O5 phases.These regions did not participate in the metal-insulator phase transition process,just as the continuous and relatively stable Raman scattering peak strength reaches the high temperature region.The n and k values of materials in different phase transition states were fitted by temperature-varying ellipsometer.The optical properties of VO2 thin films were fitted by Droud-Lorenz model and equivalent medium theory.The corresponding relationship between oxygen defect and electrical and optical properties of VO2 thin films was obtained,especially the quantitative relationship with optical constants of materials.(2)Hf4+ doped VO2 thin films were prepared by pulsed laser deposition.The effect of Hf4+ defect on the properties of VO2 thin films was studied.It was found that compared with the undoped VO2 films,Hf4+ doped VO2 could significantly reduce the hysteresis width in the metal-insulator phase transition process.After doping,the film resistance exhibited obvious metal-insulator phase transition(MIT)characteristics,with 2-3 orders of resistance change.When the doping concentration was 1 at.%and 3 at.%,the phase transition hysteresis width of the films decreaseed from 8.3℃ to 1.9℃ and 2.7℃ respectively.Raman spectroscopy analysis shows that Hf4+ doping can lead to blue shift of V-0 vibration mode,but has little effect on V-V vibration phonon mode.The mid-infrared thermal radiation imaging results of samples with doping concentration of 1 at.%and 3 at.%show that there is almost no hysteretic behavior during heating and cooling.We further deposited Hf4+ ion-doped VO2 thin films on the surface of silicon microcirculators by pulsed laser deposition technology,and studied the effect of Hf4+ doped VO2 thin films on the performance of silicon microcirculators.It was found that the film could effectively adjust the resonant peak position and quality factor of the silicon micro-ring resonator.(3)Hf4+/W4+ doped vanadium dioxide thin films were prepared by magnetron sputtering.The effect of codoping on the properties of VO2 thin films was studied.The results showthat when concentration is 1%at.%,the diffraction intensity of(011)crystal plane increases gradually with the increase of W4+ concentration,which indicates that the crystallinity of the films improve’s continuously.At the same time,the electrical properties of the prepared samples were tested.The temperature-resistance curves show that the Hf4+/W4+ codoped VO2 film significantly reduces,the phase transition temperature and narrows the hysteresis width.These findings provide insights for understanding the phase evolution,MIT process and optical properties of the vanadium oxide thin film systems with different oxygen stoichiometries,At the same time,Hf4+ ion doping and Hf4+/W4+ ion co-doping can significantly reduce the phase transition temperature and narrow the hysteresis width of VO2 thin films,which indicates that such materials’have good potential in infrared camouflage and thermal radiation control applications.