Preparation, Electrical And Optical Properties Of Vanadium Oxide Thin Films

As a high-quality optoelectronic material, vanadium oxide (VO_X) has received muchattention over the past years, and it shows great potential for various applications suchas information storage device, optical modulator, energy-saving, infrared detector, andso on.In this dissertation, nanostructured VO_Xand sandwich structured W doped VO_Xthinfilms were prepared by applying magnetron sputtering technique. The effects ofpreparation conditions on the microstructures, electrical and optical properties of filmshave been investigated by employing many modern characterization methods. Inaddition, the changes of compositions, electrical and optical properties were studiedwhen the nanostructured VO_Xfilms were exposed to air for a long period of time. Therelationship between the microstructures and physical properties of films was discussedbased on the experimental results and related theories. The main contents andconclusions in the dissertation are shown as follows:Nanostructured VO_Xthin films were prepared by reactive direct current magnetronsputtering and in-situ oxidation annealing. The as-deposited films were amorphous. Theprofile of particles at the film surface became clear, the number of V-O bonds decrease,the surface roughness and the average oxidation state as well as the number of V=Obonds increased after the annealing. Dual optical absorptions were found in thenanostructured VO_Xthin film with band gap states associated with the oxygendeficiencies.Spectroscopic ellipsometry was used to study the optical properties ofnanostructured VO_Xthin film with semiconductor-metal transition characteristic. It wasfound that the Brendel-Bormann (BB) oscillator model gave better fits to theexperimental ψ and Δ values than the Lorentz oscillator model, and the film thicknessobtained by using BB oscillator model agreed well with the cross-section resultmeasured by scanning electron microscopy.The average particle size of nanostructured VO_Xthin film with semiconductor-metaltransition was controlled by altering substrate temperature or oxygen partial pressure. The average VO₂grain sizes, electrical and optical properties of the films with differentaverage particle sizes were investigated.Nanostructured V₂O5-Xthin films were directly prepared by sputtering fromvanadium target under the condition of high oxygen partial pressure without postannealing. Results indicated that the composition showed little change, the shapes ofparticles significantly changed, the average particle size increased, and the molecularstructure became compact when the substrate temperature was elevated. It was believedthat the molecular structure was the main factor affecting the electrical and opticalproperties of the nanostructured V₂O5-Xthin film.The changes of physical properties of nanostructured VO_Xfilms were studied whenthey were exposed to air for a long period of time. Results revealed that the reason tocause the increase of resistance was due to the oxidation of oxygen on the low-valencevanadium ions. Meanwhile, however, H2O reacted with V5+ions, and generated V4+–H+and V4+–OH, which avoided all the vanadium ions to be oxidized by oxygen atoms. Asa consequence, the variation of resistance eventually tended to saturation. Because ofthe reactions of O₂and H2O with film, the molecular structure of film changed, andhence leaded to the change of the shape of thermal hysteresis loop. It was found that theoptical transmittance and optical constants showed slightly change because only thefilm surface was affected by O₂and H2O.VO_X/W/VO_Xand VO_X/WOy/VO_X, namely sandwich structures, were used toprepare W doped VO_Xthin films. The diffusion-concentration of W atoms was changedby altering the annealing time or the deposition time of W or WOymiddle layer tochange the microstructures, electrical and optical properties of films. O₂/Ar hadremarkable effects on the electrical and optical properties of VO_X/WOy/VO_Xfilms. Itwas found that the sheet resistance as well as optical band gap increased with increasingO₂/Ar, and the semiconductor-metal transition characteristic disappeared when...