Thesis (M.S.)--Rochester Institute of Technology, 2006. / Typescript. Includes bibliographical references (leaves 145-146).

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 67-72). Also available on the World Wide Web.

Thesis (Master of Engineering in Mechanical Engineering)--Cape Peninsula University of Technology, 2017. / Transparent conductive oxides (TCOs), which are widely used in transparent electronics, possess a spectral selectivity that is suitable for a solar material absorber. TCO materials have a plasma wavelength in the infrared region. Consequently electromagnetic waves shorter than a plasma wavelength are transmitted through the material, while longer electromagnetic waves are reflected on the surface. In contrast to the opaque solar selective absorbers, the plasma wavelength in TCO materials can be easily tuned by controlling the heavy doping process to match the peak shift of thermal radiation at higher temperatures. Furthermore, the use of TCO in conjunction with a solar absorber relaxes the spectral selectivity of the latter and thus widens the selection of the solar absorber; subsequently the only requirement is a thermally stable black body. Aluminum doped Zinc Oxide (AZO) is a class of TCO materials which is cost effective to manufacture due to abundance ZnO, and Aluminum raw materials. This thesis is based on the synthesis of Al doped ZnO thin films nanostructure using radio frequency RF magnetron sputtering process. The influence of the deposition parameters, including argon working pressure and substrate temperature, on the structural and optical properties of the AZO thin films is investigated by means of X-ray diffraction (XRD) and optical spectroscopy (UV-VIS-NIR). The optical constants of AZO films are extracted from transmittance and reflectance spectra using a combination of Drude and Lorentz dielectric function model. A computer simulation is developed to calculate the radiative properties of Al doped ZnO thin films nanostructure. The thermal emittance and solar absorptance is predicted indirectly from optical reflectance and transmittance of AZO films by invoking Kirchhoff’s law. A Special attention has been paid to the parameters that influence the spectral properties of the AZO films including carrier’s mobility, Al doping concentration and film thickness. Carrier’s mobility turned out to have the most significant influence on the spectrally selective performance of AZO films.

White light emitting diodes (LED) have drawn increasing attention due to their low energy consumption, high efficiency and potential to become primary lighting source by replacing conventional light sources. White light emission is usually generated either by coating yellow phosphor on a blue-LED or blending red, green and blue phosphor in an appropriate ratio. Maintaining appropriate proportions of individual components in the blend is difficult and the major demerit of such system is the overall self-absorption, which changes the solution concentration. This results in uncontrolled changes in the whiteness of the emitted light. Zinc Oxide (ZnO), a wide bandgap semiconductor with a large exciton binding energy at room temperature has been recognized as a promising material for ultraviolet LEDs and laser diodes. Tuning of structural, optical and electrical properties of ZnO thin films by different dopants (Lithium, Indium and Gallium) is dealt in this thesis. The achievement of white light emission from a semiconducting material without using phosphors offers an inexpensive fabrication technology, good luminescence, low turn-on voltage and high efficiency. The present work is organized chapter wise, which has 8 chapters including the summary and future work. Chapter 1: Gives a brief discussion on the overview of ZnO as an optoelectronic material, crystal structure of semiconductor ZnO, the effect of doping, optical properties and its possible applications in optoelectronic devices. Chapter 2: Deals with various deposition techniques used in the present study, includes pulsed laser deposition and thermal evaporation. The experimental set up details and the deposition procedures are described in detail. A brief note on the structural characterization equipments, namely X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the optical characterization techniques namely Raman spectroscopy, transmission spectroscopy and photoluminescence (PL) spectroscopy is presented. The electrical properties of the films were studied by current- voltage, capacitance - voltage and Hall Effect measurements and the experimental details are discussed. Chapter 3: High quality ZnO/Si heterojunctions fabricated by growing ZnO thin films on p-type Si (100) substrate by pulsed laser deposition without using buffer layers are discussed in this chapter. The crystallinity of the heterojunction was analyzed by high resolution X-ray diffraction and atomic force microscopy. The optical quality of the film was analyzed by room temperature (RT) photoluminescence measurements. The high intense band to band emission confirmed the high quality of the ZnO thin films on Si. The electrical properties of the junction were studied by temperature dependent resistivity, current- voltage measurements and RT capacitance-voltage (C-V) analysis. ZnO thin film showed the lowest resistivity of 6.4x10-3 Ω.cm, mobility of 7 cm2/V.sec and charge carrier concentration of 1.58x1019cm-3 at RT. The charge carrier concentration and the barrier height (BH) were calculated to be 9.7x1019cm-3 and 0.6 eV respectively from the C-V plot. The BH and ideality factor, calculated by using the thermionic emission (TE) model were found to be highly temperature dependent. We observed a much lower value in Richardson constant, 5.19x10-7 A/cm2K2 than the theoretical value (32 A/cm2K2) for ZnO. This analysis revealed the existence of a Gaussian distribution (GD) with a standard deviation of σ2=0.035 V. By implementing GD to the TE, the values of BH and Richardson constant were obtained as 1.3 eV and 39.97 A/cm2K2 respectively from the modified Richardson plot. The obtained Richardson constant value is close to the theoretical value for n-ZnO. These high quality heterojunctions can be used for solar cell applications. Chapter 4: This chapter describes the structural and optical properties of Li doped ZnO thin films and the properties of ZnO/Li doped ZnO multilayered thin film structures. Thin films of ZnO, Li doped ZnO (ZLO) and multilayer of ZnO and ZLO (ZnO/ZLO) were grown on silicon and Corning glass substrates by pulsed laser deposition technique. Single phase formation and the crystalline qualities of the films were analyzed by X-ray diffraction and Li composition in the film was investigated to be 15 Wt % by X-ray photoelectron spectroscopy. Raman spectrum reveals the hexagonal wurtzite structure of ZnO, ZLO and ZnO/ZLO multilayer, confirms the single phase formation. Films grown on Corning glass show more than 80 % transmittance in the visible region and the optical band gaps were calculated to be 3.245, 3.26 and 3.22 eV for ZnO, ZLO and ZnO/ZLO respectively. An efficient blue emission was observed in all films that were grown on silicon (100) substrate by photoluminescence (PL). PL measurements at different temperatures reveal that the PL emission intensity of ZnO/ZLO multilayer was weakly dependent on temperature as compared to the single layers of ZnO and ZLO and the wavelength of emission was independent of temperature. Our results indicate that ZnO/ZLO multilayer can be used for the fabrication of blue light emitting diodes. Chapter 5: This chapter is divided in to two parts. The fabrication and characterization of In doped ZnO thin films grown on Corning glass substrate is discussed in the first section. Zinc Oxide (ZnO) and indium doped ZnO (IZO) thin films with different indium compositions were grown by pulsed laser deposition technique. The effect of indium concentration on the structural, morphological, optical and electrical properties of the film was studied. The films were oriented along the c-direction with wurtzite structure and are highly transparent with an average transmittance of more than 80 % in the visible wavelength region. The energy band gap was found to be decreasing with increasing indium concentration. High transparency makes the films useful as optical windows while the high band gap values support the idea that the film could be a good candidate for optoelectronic devices. The value of resistivity observed to be decreasing initially with doping concentration and subsequently increasing. The XPS and Raman spectrum confirm the presence of indium in indium doped ZnO thin films. The photoluminescence spectrum showed a tunable red light emission with different In concentrations. Undoped and In doped ZnO (IZO) thin films were grown on Pt coated silicon substrates (Pt/Si) to fabricate Pt/ZnO:Inx Schottky contacts (SC) is discussed in the second section. The SCs were investigated by conventional two probe current-voltage (I-V) measurement and by the I-V spectroscopy of conductive atomic force microscopy (C-AFM). X-ray diffraction technique was used to examine the thin film quality. Changes in various parameters like Schottky barrier height (SBH) and ideality factor (IF) as a function of temperature were presented. The estimated BH was found to be increasing and the IF was found to be decreasing with increase in temperature. The variation of SBH and IF with temperature has been explained by considering the lateral inhomogeneities in nanometer scale lengths at metal–semiconductor (MS) interface. The inhomogeneities of SBH in nanometer scale length were confirmed by C-AFM. The SBH and IF estimated from I-V spectroscopy of C-AFM showed large deviation from the conventional two probe I-V measurements. IZO thin films showed a decrease in SBH, lower turn on voltage and an enhancement in forward current with increase in In concentration. Chapter 6: In this chapter the properties of Ga doped ZnO thin films with different Ga concentrations along with undoped ZnO as a reference is discussed. Undoped and Ga doped ZnO thin films with different Ga concentrations were grown on Corning glass substrates by PLD. The structural, optical and electrical properties of Ga doped ZnO thin films are discussed. The XRD, XPS and Raman spectrum reveal the phase formation and successful doping of Ga on ZnO. All the films show good transmittance in the visible region and the photoluminescence of Ga doped ZnO showed a stable emission in the blue- green region. The resistivity of Ga doped ZnO thin films was found to be first decreasing and then increasing with increase in Ga concentrations. Chapter 7: The effect of co-doping to ZnO on the structural, optical and electrical properties was described in this chapter. Ga and In co-doped ZnO (GIZO) thin films together with ZnO, In doped ZnO (IZO), Ga doped ZnO (GZO), IZO/GZO multilayer for comparison, were grown on Corning glass and boron doped Si substrates by PLD. GIZO showed better structural, optical and electrical properties compared with other thin films. The Photoluminescence spectra of GIZO showed a strong white light emission and the current-voltage characteristics showed relatively lower turn on voltage and larger forward current. The CIE co-ordinates for GIZO were observed to be (0.31, 0.33) with a CCT of 6650 K, indicating a cool white light and established a possibility of white light emitting diodes. Finally the chapter 8 presents the summary derived out of the work and a few suggestions on future work.

Zinc oxide (ZnO) is a versatile environmentally benign II-VI direct wide band gap semiconductor with several technologically plausible applications such as transparent conducting oxide in flat panel and flexible displays. Hence, ZnO thin films have to be processed below the glass transition temperatures of polymeric substrates used in flexible displays. ZnO thin films were synthesized via aqueous polymeric precursor process by different metallic salt routes using ethylene glycol, glycerol, citric acid, and ethylene diamine tetraacetic acid (EDTA) as chelating agents. ZnO thin films, derived from ethylene glycol based polymeric precursor, exhibit flower-like morphology whereas thin films derived of other precursors illustrate crack free nanocrystalline films. ZnO thin films on sapphire substrates show an increase in preferential orientation along the (002) plane with increase in annealing temperature. The polymeric precursors have also been used in fabricating maskless patterned ZnO thin films in a single step using the commercial Maskless Mesoscale Materials Deposition system.

Metal oxide nanolaminate films are potential high temperature solid lubricants due to their ability to exhibit significant plasticity when grain size is reduced to the nanometer scale, and defective growth structure is achieved by condensation of oxygen vacancies to form intrinsic stacking faults. This is in contrast to conventional microcrystalline and single crystal oxides that exhibit brittle fracture during loading in a sliding contact. This study emphasizes the additional effect of growth orientation, in particular crystallographic texture, on determining the sliding friction behavior in nanocolumnar grain zinc oxide films grown by atomic layer deposition. It was determined that zinc oxide low (0002) versus higher (101 ̅3) surface energy crystallographic planes influenced the sliding friction coefficient. Texturing of the (0002) grains resulted in a decreased adhesive component of friction thereby lowering the sliding friction coefficient to ~0.25, while the friction coefficient doubled to ~0.5 with increasing contribution of surface (101 ̅3) grains. In addition, the variation of the x-ray grazing incident angle from 0.5° to 5° was studied to better understand the surface grain orientation as a function of ZnO layer thickness in one versus four bilayer nanolaminates where the under layer (seed layer) was load-bearing Zn(Ti,Zr)O3.

This dissertation focuses on two aspects of integrating near-infrared plasmonics with electronics with the intent of developing the platform for future photonics. The first aspect focuses on fabrication by introducing and developing a simple, single reflective optical element capable of high–throughput, large scale fabrication of micro- and nano-sized structure templates using holographic lithography. This reflective optical element is then utilized to show proof of concept in fabricating three dimensional structures in negative photoresists as well as tuning subwavelength features in two dimensional compound lattices for the fabrication of dimer and trimer antenna templates. The second aspect focuses on the study of aluminum zinc oxide (AZO), which belongs to recently popularized material class of transparent conducting oxides, capable of tunable plasmonic capabilities in the near-IR regime. Holographic lithography is used to pattern an AZO film with a square lattice array that are shown to form standing wave resonances at the interface of the AZO and the substrate. To demonstrate device level integration the final experiment utilizes AZO patterned gratings and measures the variation of diffraction efficiency as a negative bias is applied to change the AZO optical properties. Additionally efforts to understand the behavior of these structures through optical measurements is complemented with finite difference time domain simulations.

Growing demand for clean source of energy in the recent years has increased the manufacture of solar cells for converting sun energy directly into electricity. Research has been carried out around the world to make a cheaper and more efficient solar cell technology by employing new architectural designs and developing new materials to serve as light absorbers and charge carriers. Aluminum doped Zinc Oxide thin film, a Transparent conductive Oxides (TCO) is used as a window material in the solar cell these days. Its increased stability in the reduced ambient, less expensive and more abundance make it popular among the other TCO’s. It is the aim of this work to obtain a significantly low resistive ZnO:Al thin film with good transparency. Detailed electrical and materials studies is carried out on the film in order to expand knowledge and understanding. RF magnetron sputtering has been carried out at various substrate temperatures using argon, oxygen and hydrogen gases with various ratios to deposit this polycrystalline films on thermally grown SiO2 and glass wafer. The composition of the films has been determined by Xray Photoelectron Spectroscopy and the identification of phases present have been made using X-ray diffraction experiment. Surface imaging of the film and roughness calculations are carried out using Scanning Electron Microscopy and Atomic Force Microscopy respectively. Determination of resistivity using 4-Probe technique and transparency using UV spectrophotometer were carried out as a part of electrical and optical characterization on the obtained thin film.The deposited thin films were later annealed in vacuum at various high temperatures and the change in material and electrical properties were analyzed.

No description available.

The atmosphere we live in contains various kinds of chemical species, natural and artificial, some of which are vital to our life, while many others are more or less harmful. The vital gases like oxygen, humidity have to be kept at adequate levels in the living atmosphere, whereas the hazardous and toxic gases like hydrocarbons, H2, volatile organic compounds, CO2, CO, NOx, SO2, NH3, O3 etc should be controlled to be under the designated levels. The measurement technology necessary for monitoring these gases has emerged, particularly as organic fuels and other chemicals have become essential in domestic and industrial life. In addition to other applications, environmental pollution monitoring and control has become a fundamental need in the recent years. Therefore, there has been an extensive effort to develop high-performance chemical sensors of small size, rugged construction, light weight, true portability, and with better sensing characteristics such as high sensitivity, fast response and recovery times, low drift, and high degree of specificity. Among the various types of gas sensors studied, solid state gas sensors based on semiconducting metal oxides are well established, due to their advantages over the other types, and hence cover a wide range of applications. However, the widespread application of these sensors has been hindered by limited sensitivity and selectivity. Various strategies have been employed in order to improved the performance parameters of these sensors. This thesis work has two major investigations, which form two parts of the thesis. The first part of this thesis describes the efforts to improve the sensing behaviour of one of the extensively studied metal oxide gas sensors, namely, ZnO, through a novel, ultrasonic-nebulised spray pyrolyis synthesis method, employing an aqueous combustion mixture (NSPACM). The second part of the thesis deals with the ideal of gas detection by optical means through the reversible phase transformation between V2O5 and V6O13 deposited by metalorganic chemical vapor deposition(MOCVD). The introductory chapter I deals with basics of chemical sensors and the characteristic sensing parameters. Different types of gas sensors based on the phenomena employed for sensing are discussed, with an emphasis on semiconducting metal oxide gas sensors. The importance of material selection for solid state gas sensors, depending on the purpose, location, and conditions of operation are discussed, supporting the assertion that semiconducting metal oxides are better suited to fulfill all the requirements of modern gas sensors. Some of the effective methods to improve performance parameters including the influence of grain size, microstructure, and surface doping are described., followed by the motivation of the present thesis. The part I of the thesis is based on the resistive semiconducting metal oxide, where the system investigated was ZnO. Part one comprises Chapters 2, 3 and 4. In Chapter 2, a brief introduction to the material properties of ZnO, followed by various synthesis techniques are discussed. An overview of spray pyrolysis and combustion synthesis is followed by the details of the method employed in the present study, namely NSPACM, which is based on the above two methods, for the formation of ZnO films. A detailed description of the film deposition system built in house is presented, followed by the deposition procedure and the parameters used. Thermal study of the combustion mixture and non-combustion precursor shows the importance of the fuel, along with oxidizer, in forming the film. The films formed using combustion mixture are found to be polycrystalline, whereas films formed without combustion were found to have preferred crystallographic orientation even on an amorphous substrate, which is explained on the basis of minimization of surface energy. The observed unique microstructure with fine crystallite size and porous morphology is attributed to the combustion method employed, which is interesting from the point of view of gas sensing. Chapter 3 concerns the gas sensing study of these ZnO films. The design of the home made gas sensing system is explained in detail. The study of electrode characteristics is followed by the important steps in gas sensing measurements. ZnO gas sensors were mainly studied for their selectivity between aliphatic and aromatic hydrocarbons. The results show two regions of temperature where the sensitivity peaks for aliphatic hydrocarbons, whereas aromatic hydrocarbons show a single sensitive region. This observation can pave the way for imparting selectivity. Possible reasons for the observed behavior are mentioned. Chapter 4 describes the chemical and physical modifications done to ZnO thin films by doping with catalysts, and through the use of x-y translational stage for large-area deposition.. Homogenous distribution of catalysts achieved by the NSPACM synthesis procedure, determined by the x-ray elemental mapping, is discussed. The addition of catalysts improved the sensing both because of catalytic effects and by promoting preferred crystallographic orientation, with Ni addition showing the better effects. The use of the x-y stage in producing the films with high orientation, which improved the gas sensing behavior, is explained. Part II of the thesis comprises Chapters 5,6 and 7, and describes a detailed study of V2O5 and V6O13 thin films deposited by MOCVD for optical sensing of chemical species. In Chapter 5, a brief introduction to chemical vapor deposition is given, followed by the importance of the characteristics of CVD precursors – in particular, the importance of their thermal behavior in film formation. This is followed by the importance of vapor pressure and partial pressure studies in the MOCVD of oxides of a multivalent metal such as vanadium. Various techniques of measuring vapor pressure are listed, followed by the details of the method used in the present study employing rising temperature thermogravimetry, based on the Langmuir equation. Thermogravimetric analysis performed, both at atmospheric as well as at low pressure, using commercial and home made apparatus, respectively is discussed. A detailed description of the home made setup is also presented. Chapter 6 describes the application of the vapor pressure and partial pressure studies to the deposition of films using MOCVD. Here, a detailed description of the vanadium oxide phase diagram and the stability of various phases is presented, which points the importance of precise parameter control during the deposition to obtain pure phases. The details of the CVD setup, followed by the procedure and parameters of deposition, are presented. The films deposited at various deposition temperatures, analyzed using XRD and SEM, are discussed. The effect of temperature on the growth is explained. The effect of vapor pressure is studied by varying the precursor vaporizer temperature, with a growth temperature maintained invariant. The influence of the amount of precursor on film growth, with a particular crystalline orientation and phase content, is explained followed by the description of the deposition of pure phases of V2O5 and V6O13 through the optimization of CVD parameters. Chapter 7 deals with the optical study of the films deposited by the above method. Here, the importance of two phases of vanadium oxide, V2O5 and V6O13, to the proposed gas sensing action, is presented. Their structural similarity in terms of polyhedral arrangement in the ab plane can be the basis of a reversible phase change. The difference in the optical transmittance in two phases forms the basis for the optical method for chemical sensing. The details of the laser-based optical sensing setup, its, design and the detection method, are explained. Studies on hydrocarbon sensing with vanadium, pentoxide films are also presented. The novelty in using reversible chemical transformation of a material system for detection of reducing and oxidizing gases in the ambient gases is discussed. Chapter 8 provides a summary of the present thesis, together with the main conclusions. The work reported in this thesis has been carried out by the candidate as part of the Ph.d training programme. She hopes that this would constitute a worthwhile contribution towards the understanding and subsequent application of ZnO and oxides of vanadium(V2O5 and V6O13) as novel gas sensors which will be useful for environmental protection, as well as for safety in industrial an domestic sectors.