Research On Doped Vanadium Oxide Films Phase Structure And Optoelectronic Properties

Vanadium oxide is one of the important thermo-sensitive film materials foruncooled infrared focal plane arrays. Due to the uncooled operation, small volume,portability and low cost, etc, uncooled focal plane and the corresponding systemshave become a development trend. Vanadium oxide is an ideal material for thethermal imaging since it has a low formation temperature and high temperatureresistance coefficient （TCR）. Vanadium oxide compounds can crystallize with avariety of constitutions, with the resistance and TCR obviously varying with theconstituent. The optoelectronic property of vanadium oxide films can be improvedby modifying constituent. Now, the researches of vanadium oxides focus on theobtaining high quality and low phase transition temperature. Therefore, thepreparation of good performance, low cost, high TCR and low resistance vanadiumoxide films is essential. In this thesis, the preparation, structural and optoelectronicproperties of VO_x/p-Si films with and without Al （Mo） doping have been studied topresent experimental and theoretical foundation for further improving theoptoelectronic property of thermo-sensitive film materials.Employing pulse laser deposition （PLD） method with the elementaryvanadium target reacting with O₂, we have prepared VO₂/p-Si films with VO₂asprincipal phase structure. Micro-structural study indicates: all the crystal film showfavorable growth orientation; exorbitant substrate temperature （over800oC）impedes well crystallization, and the over high laser power （over400mJ） willdestroy oriented growth; appropriate distance between target and substratefacilitates the uniform crystallization; with the increasing pressure of oxygen gas,the vanadium oxide film change as following: amorphous structureâ†'VO₂â†'VO₂+V₃O₇â†'VO₂+V₃O₇+V6O13â†'VO₂+V₃O₇+V₆O₁3+V₂O₅. Basedon the prediction of molecular collision model, the critical oxygen gas pressure ofthe crystalline VO₂/p-Si film prepared by this method approximate to0.083Pa,agreeing well with the experimental result （0.08Pa）. Optoelectronic results indicate:all the VO₂/p-Si film prepared by this method represent phase transition character,with high TCR （maximum value reach4.59%）; VO₂/p-Si film shows favorabletransient optical response, with μs order of magnitude response time; the carriers inVO₂/p-Si semiconductors are primarily provided by deep level impurities anddefects.The V₂O₃phase dominated V₂O₃/p-Si films are prepared by PLD methodwith V₂O₅target and vacuum annealing to decrease the resistances of thermo-sensitive film materials, micro-structural study indicates: textures exist in the V2O₃/p-Si films; VO₂（B） phase without phase transitions exists in the filmsprepared at relatively high substrate temperatures, and does not appear in the filmsprepared at relatively low substrate temperatures. Optoelectronic property resultsindicate: annealing facilitates full and uniform growth, and can deduce resistance,but makes against enhancement of TCR; the resistances of V2O₃/p-Si filmsprepared by this method can be controlled in the range from several KΩ to scores ofKΩ; for the film prepared at substrate temperature850oC without annealing, theresistances are scores of KΩ and the TCR attains to-5%, achieving ideal properties;V2O₃/p-Si films have no transient optical response; the carriers in the V2O₃/p-Sisemiconductors are primarily provided by shallow level impurities and defects.Al （Mo） doped V2O₃/Si films are prepared by PLD method with thealternating elementary Al （Mo） and V2O5targets to examine doping effects on thephase transition character of thermo-sensitive film materials. Micro-structural studyresults indicate: the doping of Al or Mo element could only induce trivial latticedistortion, but could not change the crystal structure of V2O₃phase. The studiedoptoelectronic property results are as followed: two kinds of doping elements canconsiderably decrease film resistance; the resistances of Al doped V2O₃/p-Si filmsare several KΩ, and the resistances of Mo doped V2O₃/p-Si films are scores of KΩ;TCR are relatively lower for the films with relatively high Al doping quantity; Al（Mo） doped V2O₃/Si films exhibit well transient optical response with μs order ofmagnitude response time; the carriers in Al （Mo） doped V2O₃/Si semiconductorsare primarily provided by deep level impurities and defects. In resistance-temperature measurement at room temperature, no substantial phase transition hasbeen observed. But by the energy level changes in the charge-deep level transientspectrum （Q-DLTS）, the phase transition can be determined: before and after phasetransition, only one level has been found in spectrum, while in the phase transitionprocess, two energy level have been observed derived from two structures. Thedeep level center contributed primarily from doped elements, due to structuraldistortion induced by Al （Mo） doping, which leads to additional stress apart fromtexture induced stress, and thus results in deep levels.Polyimide （PI） can provide liquid polymerized precursor, form films by tosssmear method and be used as photoresist to fulfill fine structure byPhotolithographic process. This material can combine well with most of metal,semiconductor materials. Especially, the thermal conductivity of polyimide is verylow, equal to only1/250of silicon nitride, and will be a promising mini-bridgestructural material. But now, mini-bridge structure produced with polyimidematerials has no ready technology, and there still be many difficulties in productionof practical polyimide micro-bolometer. Therefore, the investigation of polyimidemicro-bolometer will effectively promote the further development of flexible infrared micro-structural devices.We have firstly prepared VO_x/PI film an infrared sensitive film on PI at lowtemperature by PLD method with V2O5target and annealing in China, attemptingto design a micro-bolometer unit based on the PI as the flexible circuit boardmaterial, suspended mini-bridge support Layer and insulating layer, and produce2×4uncooled infrared detector array. The thermo-sensitive and optical responseproperties of VO_x/PI microbolometer unit have been analyzed and compared withthose of the conventional microbolometers based on silicon nitride structuralmaterials...