Vanadium oxides are being used as the thermal sensing layer because of their applications in infrared detectors. They have high temperature coefficient of resistance, favorable electrical resistance and compatibility with the MEMS technology. Of all oxides of vanadium, only vanadium dioxide (VO2)has been highly investigated as it shows first order transition (semiconducting to metal transition-SMT)at 68 oC. First order transition is understood as the sharp change in the electrical resistance. The change in resistivity in this case is of the order of 105 over a temperature change of 0.1 oC at 68 oC in a single crystal. Doping vanadium oxides with elements like Mo and W reduce the transition temperature. This is very important for room temperature electrical and optical detection. Though most of the research groups subscribe to PLD, cost-effective methods with large area deposition are major focus of this research. Hence for synthesizing VO2 in bulk and thin films, Solution Combustion Synthesis (SCS), Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) Chemical vapour deposition (CVD)and microwave are explored. Synthesis of doped VO2 films in CVD has not been done extensively to yield optical quality thin films.
Chapter I surveys the use of phase transition in oxides system for a variety of practical applications. In particular, Vanadium dioxide (VO2) is chosen as it is found to be very useful for infrared and metamaterials based applications. VO2 is known for its first-order semiconducting to metallic transition (SMT). This chapter attempts to explain the influence of processing, doping, annealing, etc on the SMT characteristics. Important aspects such as the idea of hysteresis in VO2 and similarity to martensitic transformation are discussed. The scope and objectives of the thesis are discussed here.
Chapter II explains in detail the materials and methods used to synthesize VO2 both in bulk and in thin lm form and methods used to study their characteristics. Brief description on the principle and the working of the home-built experimental set up needed for this study is elicited.
In chapter III, attempts were made to understand the phase stability of VO2 and the evolution of crystal structures during the phase transition. VO2 crystallizes in P21/c space group at room temperatures with lattice parameters a=5.752 Ab=4.526 Ac=5.382 Aα=90 β=122.60 γ=90 . Precise control of synthesis parameters is required in stabilizing pure phase in bulk as well as thin lm form. This study focuses on the novel large scale two step synthesis of VO2 using Solution Combustion Synthesis. This involves synthesis of product utilizing redox reaction between metal nitrate and suitable fuel. Generally the products are nanocrystalline in nature due to self-propagation of the exothermic combustion reaction. First step involved the synthesis of V2O5 by combustion reaction between Vanadyl nitrate and urea. In the second step, the as-synthesized V2O5 has been reduced by a novel reduction technique to form monophasic VO2. The presence of competing phases like M1, M2, M3 and R are investigated by XRD, Raman spectroscopy, DSC, Optical and high temperature X-ray diffraction.
Chapter IV deals with the reduction in phase transition temperature by doping the SCS synthesized VO2 with W and Mo. Effect of doping on the transition temperature was studied using differential scanning calorimetry (DSC) in both W and Mo. Electrical characteristics of Mo doped VO2 and Optical characteristics of the W-doped VO2 were also studied using four probe resistivity measurements and UV-VIS Spectroscopy respectively. W addition was found to be more effective in reducing the phase transition temperature. To understand further more on the W addition, X-ray photo-electron spectroscopy measurements were performed. W-addition alters the V4+-V4+ bonding and with W addition it is observed that V was present in V3+state. W was present in W6+ state. The addition of W to VO2 introduces more electrons to the systems and disturbs the V4+-V4+ thus reducing the phase transition temperature of VO2.
Chapter V describes the large scale, large area deposition of thin films of VO2 by a cost effective method. A novel technique to deposit vanadium dioxide thin films namely, UNSPACM is developed. This simple two-step process involves synthesis of a V2O5 lm on an LaAlO3(LAO) substrate followed by a controlled reduction to form single phase VO2. The formation of M1 phase (P21/c) is confirmed by Raman spectroscopic studies. A thermally activated metal{insulator transition (MIT) was observed at 61 oC, where the resistivity changes by four orders of magnitude. The infrared spectra also show a dramatic change in reflectance from 13% to over 90% in the wavelength range of 7-15 m. This indicates the suitability of the films for optical switching applications at infrared frequencies. A trilayer metamaterial absorber, composed of a metal structure/dielectric spacer/vanadium dioxide (VO2) ground plane, is shown to switch reversibly between reflective and absorptive states as a function of temperature. The VO2 lm, which changes its conductivity by four orders of magnitude across an insulator{metal transition, enables the switching by forming a resonant absorptive structure at high temperatures while being inactive at low temperatures. The fabricated metamaterial shows a modulation of the reflectivity levels of 58% at a frequency of 22.5 THz and 57% at a frequency of 34.5 THz.
Chapter VI explains the W doped VO2 thin films synthesized by UN-SPACM. Morphology of the thin films was found to be consisting of globular and porous nanoparticles having size 20 nm. Transition temperature decreased with the addition of W. 1.8 at. %W doping in VO2 transition temperature has reduced upto 25 oC. It is noted that W-doping in the pro-cess of reducing the transition temperature, alters the local structure and also increases room temperature carrier concentration. The presence of W, as was seen in Chapter IV, altered V4+-V4+ bonds and introduced V3+. W was found to be in W6+ state suggesting W addition increased the carrier concentration. Hall Effect measurements suggested the increased carrier con-centration.
The roughness of the synthesized films were very high for them to be of de-vice quality, despite encouraging results obtained by electrical measurements. Hence in order to further improve the smoothness and thereby the optical quality of thin films, Chemical Vapour Deposition (CVD) is employed.
Chapter VII outlines the effect of processing parameters and post pro-cessing annealing on the semiconductor-metallic transition of VO2. Here in this chapter, the influence of substrate temperature on the SMT properties of VO2 is explored. At different substrate temperatures, the percentage of phase fraction of V in V3+, 4+ and V5+ differed. Besides, the morphology also varied with substrate temperatures. Similarly it is observed that with annealing the VO2 film deposited on glass substrates, SMT properties enhanced which was attributed to filling out of oxygen vacancies. Si based substrates and non-Si based substrates were used for depositing VO2 thin films by CVD. Their temperature coefficient of resistance and SMT properties were studied in order to understand their potential in bolometer and thermal to optical valve based applications.
Chapter VIII involves the study of VO2 thin films for thermal to optical valves. ITO coated glass substrates were used for the purpose. Thin films were deposited by both UNSPACM and CVD. It was observed that the reflectivity pro les of the films synthesized by the above said methods were very different. Hence in the process of understanding the huge difference in the reflectivity pro les, classical harmonic oscillator, Lorentzian model was employed to t the experimental data at room temperature whereas Drude-Lorentzian model was used to t the data at higher temperature (at 100 oC- after transition). With this fitting plasma frequencies of the CVD films were calculated. It was observed that defect chemistry of films synthesized by these methods were different.
In order to further improve the smoothness of the films, microwave method was proposed in Chapter IX. The preliminary results showed the presence of uniform spheres and 3 D hierarchical structures of VO2 consisting of nanorods. This was extended to deposit VO2 thin films on ITO. DSC and Infrared reflectance pro le of VO2 nanopowder suggested the phase transition.
Chapter X summarizes the work done for the thesis and provides insights to the applications and to the future work.
The work reported in this thesis has been carried out by the candidate as part of the Ph.D.program. She hopes that this would constitute a worth-while contribution towards development of VO2 thin film technology and its challenges for reliable infrared device applications.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.ncsi.iisc.ernet.in:2005/2966 |
| Date | January 2016 |
| Creators | Bharathi, R |
| Contributors | Umarji, A M |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Relation | G27867 |
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