Recently, there has been an increasing interest in the electronics world for those aspects related to semiconducting gas sensor (SGS) materials. In view of the increasingly strict legal limits for pollutant gas emissions, there is a great interest in developing high performance gas sensors for applications such as controlling air pollution and exhaust gases in automotive industry. In this way, semiconductor gas sensors offer good advantages with respect to other gas sensor devices, due to their simple implementation, low cost and good stability and sensitivity. The first part of the thesis is dedicated to the synthesis, film structural and sensitivity study of the Tin Oxide film deposited by RF sputtering, doped with noble metal Palladium (Pd). Effects on the Gas Sensitivity due to the deposition parameters like thickness of the film, Substrate temperature, Ar /O2 ratio of the sputtering environment, annealing temperature and duration and doping metal weight % into the Tin Oxide films are studied and the results are shown in detail. The sensitivity and selectivity of the gas sensing film is decided by the operating temperature i.e. the temperature of the gas sensing film while it is in the target gas ambience, Microheaters happen to be the very important component in the gas sensor especially with wide band gap semiconducting metal oxides films such as tin oxide, gallium oxide or indium oxides. Other than gas sensing microheater also finds applications in many areas like thermal dip pen nanolithography, polymerase chain reaction (PCR), fluid pumping with bubbles, in vitro fertilization etc. So in this report due importance was given for the design and fabrication of the microheater. Microheaters are the most power consuming element of the integrated Gas sensors. This is also an important reason for the extensive microheater work in this research. Six different heater patterns were simulated by considering low power and temperature uniformity as an important goals. Among them the best three patterns named Double spiral, “Fan” Shape and “S” shape were chosen for fabrication and both thermal and electrical characterization results of them were presented in detail in the Microheater section of the thesis. It is believed that the intelligent design and integration of the electronic circuitry (for drive, signal conditioning/compensation, and read-out) with the gas sensing element can mitigate some of the significant issues inherent in solid-state gas sensors, such as strong temperature and humidity dependence, signal drift, aging, poisoning, and weak selectivity. The sensitivity of the gas sensors which has been indicated as the dynamic change of resistance in wide range should be read out properly. Towards this aim a low cast high efficient readout circuit is designed and implemented. Temperature monitoring and controlling is a key phenomenon in the metal Oxide based gas sensors since the selectivity mainly depends on the operating temperature of the sensing film. So focus was also shown on the design and implementation of the temperature monitoring and control unit, which been presented in the last part of this thesis.

Currently conventional sources like coal, petroleum and natural gas meet the energy requirements of developing and undeveloped countries. Over a period of time there is high risk of these energy sources getting depleted. Hence an alternate source of energy i.e. renewable energy is the need of the hour. The advantages of renewable energy like higher sustainability, lesser maintenance, low cost of operation, and minimal impact on the environment make the role of renewable energy sources significant. Out of the various renewable energy sources like solar energy, wind energy, hydropower, biogas, tidal and geothermal, usage of solar energy is gradually increasing. Among various solar energy sources, Photovoltaics has dominated over the past two decades since it is free clean energy and availability of abundant sunlight on earth. Over the past few decades, thin film solar cells (TFSC) have gained considerable interest as an economically feasible alternative to conventional silicon (Si) photovoltaic devices. TFSCs have the potential to be as efficient as Si solar cells both in terms of conversion efficiency as well as cost. The advantages of TFSC are that they are easy to prepare, lesser thickness, requires lesser materials, light weight, low cost and opto-electronic properties can be tuned by varying the process parameters. The present study is focused on the fabrication of AgInS2/ZnS heterojunction thin film solar cell. AgInS2 absorber layer is deposited using both vacuum (sputtering/sulfurization) and non-vacuum (ultrasonic spray pyrolysis) techniques. ZnS window layer is prepared using thermal evaporation technique, detailed experimental investigation has been conducted and the results have been reported in this work. The thesis is divided into 6 chapters. Chapter 1 gives general introduction about solar cells and working principle of solar cell. It also discusses thin film solar cell technology and its advantages. Layers of thin film solar cell structure, Significance of each layers and possible materials to be used are emphasized. A detailed overview of the available literature on both AgInS2 absorber layer and ZnS window layer has been presented. Based on the literature review, objectives of the present work are defined. Chapter 2 explains the theory and experimental details of deposition techniques used for the growth of AgInS2 and ZnS films. Details of characterization techniques to study film properties are described in detail. Chapter 3 presents a systematic study of AgInS2 thin films deposited by sulfurization of sputtered Ag-In metallic precursors. Initially, AgInS2 films are deposited by varying the substrate temperature and properties of as-deposited films are characterized. Structural, morphological, electrical and optical properties of AgInS2 films are explained. From these studies, samples with better properties at particular substrate temperature are optimized. By fixing the substrate temperature, deposition time of silver is varied by keeping other deposition conditions same and the properties of films are discussed. It was observed that deposition time of silver doesn’t have much impact on structural properties of AgInS2 films. However, opto-electric properties of AgInS2 films are enhanced. Based on characterization studies, deposition time of silver is optimized. Deposition time of indium is varied by keeping substrate temperature and silver deposition to optimized value. The properties of as-deposited films are discussed. Based on the above studies, the optimized p type films have a band gap of 1.64 eV, carrier concentration of 1013 ions/cm3 and Resistivity of order 103 Ω-cm. Chapter 4 presents a systematic study of AgInS2 thin films deposited by ultrasonic spray pyrolysis. AgInS2 films are deposited by varying the substrate temperature and properties of as deposited films are characterized. Structural, morphological, electrical and optical properties of AgInS2 films are explained. From these studies, samples with better properties at particular substrate temperature are optimized. By fixing the substrate temperature, concentration of silver molarity in the precursor solution is varied by keeping other deposition conditions same and the properties of films are discussed. Structural, optical and electrical properties of AgInS2 films are enhanced with the increase in silver concentration. Based on characterization studies, concentration of silver is optimized. Similarly concentration of indium molarity in the precursor solution is varied and the properties of as-deposited films are discussed. Finally, sulfur molarity in the precursor solution is varied and properties of films are discussed. It was observed that increasing sulfur after certain limit does not have any effect on the properties of the films. Based on the above studies, this method resulted in the films with resistivity of 103 Ω-cm and band gap of 1.64 eV. These films showed a carrier concentration of 1013 ions/cm3. Chapter 5 describes the growth of ZnS films using thermal evaporation technique. Influence of thickness on the properties of ZnS films is explained. Samples with good crystallinity, high transmission, and wider gap are selected for device fabrication. This p type layer showed a band gap of 3.52 eV. Solar cells have been fabricated using the AgInS2 films developed by both sputtering and ultrasonic spray pyrolysis techniques. A maximum cell efficiency of 0.92 percent has been achieved for the cell with 0.950 µm thick sputtered AgInS2 layer and thermally evaporated 42 nm thick ZnS layer. In comparison, the ultrasonic spray pyrolysis deposited films gave an efficiency of 0.54 percent. These values are comparable to those mentioned in a couple of reports earlier. Chapter 6 summarizes the conclusions drawn from the present investigations and scope of future work is suggested.

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.