FUNCTIONAL OXIDE THIN FILMS: GROWTH CHARACTERIZATION AND PROPERTIES

Abstract

Functional oxides are wide class of materials which exhibit a broad range of novel functionalities based on the tuning of their electrical, optical, magnetic and chemical properties. Among these oxides, wide band gap semiconductors have attracted much attention recently due to their potential for improving performance and extended capabilities of products in a number of industrial sectors, including the aerospace, automotive, electric motor and optoelectronics. The unique characteristic of these oxides have motivated strong research efforts as well as significant technological advances in the field of physics and material science. 'This includes the design and fabrication of engineered heterostructures, multilayers, nanostructures, and composites-that exhibit either enhanced properties and/or multiple functionalities. Searching new routes for synthesis and processing of functional oxide thin films and understanding the relationship between the structures and the properties are part of an emerging and rapidly growing field of nanotechnology. It is desirable to establish the process for producing high quality thin films of these oxides for their integration into emerging technologies. The main aim of the present work was to synthesize good quality ZnO-MgO and V02-W03 nanocomposite thin films by Pulsed Laser Deposition (PLD) technique and to investigate the effect of optimized process parameters on structural, electrical and optical properties of these materials to obtain device quality thin films. In addition ZnO/MgO multilayers were fabricated on quartz substrate and the effect of varying thickness of ZnO sublayer on various properties of the multilayer structure was studied. Chapter 1 gives an overview of functional oxide thin films and material background of selected semiconductor oxides i.e. ZnO, MgO, VO2 and W03. The chapter includes the discussions on the structural, optical and transport properties of these oxides. An insight of ZnO-MgO and V02-W03 systems has also been presented. Chapter 2 presents the details of experimental techniques, which have been used for the growth and characterization of oxide thin films and multilayers. This chapter is divided into three sections which deal with synthesis techniques, basic characterization and measurement of optical, electrical and electrochemical properties. Section 2-1- Most of the synthesis of thin films in present thesis has been carried out by Pulsed Laser Deposition technique, which uses pulses of KrF excimer laser energy to ablate material from the surface of a target. Amongst various Physical Vapor Deposition (PVD) techniques such as evaporation, sputtering and molecular beam epitaxy; PLD allows stoichiometric transfer of material from target to substrate. The use of a carousel in PLD system provides housing for a number of target materials and enables multilayer films to be deposited without the need to break vacuum while changing between the targets. In addition, the films were also prepared by Ultrasonic Spray Pyrolysis technique. Among Chemical Vapor deposition (CVD) techniques spray pyrolysis is quite promising and inexpensive route for the fabrication of thin films. Section 2-2- The first measurement that is usually carried out after synthesis is to record the X-ray diffraction pattern of the deposited material. Analysis of the position and width of the Bragg reflections gives an idea of the crystallographic phase, presence of impurities, particle size etc. Further the surface morphology and microstructure we:. studied using Atomic Force Microscopy (AFM) and Field Emission Electrosi Microscopy (FESEM). Section 2-3- Optical properties of these films were studied by using UV-Vis-NIR spectrometer and Fluorescence spectrometer. The electrical properties of films were measured using four probe resistivity set-up in the temperature range from 250 K to 400 K. The electrochemical properties of V02-W03 nanocomposite thin films were measured from cyclic voltammetry using electrochemical cell. The film thickness was measured using surface profilometer and cross-sectional FESEM. Chapter 3 describes the growth and characterization of Zni _Mgt() nanocomposite thin films. The chapter is divided into two sections. The first section (section 3-1) describes the influence of varying Mg composition (in the range 0.0 < x <1.0) on structural, electrical and optical properties of Zni.„MgrO thin films synthesized by Pulsed Laser Deposition technique. Increase in Mg content reflects the structural phase transition from wurtzite via mixed phase to cubic one. The variation of the cation-anion bond length to Mg content calculated by assuming virtual crystal model of Zni_ MgO„r based on Vegard's law shows the c-axis compression of the hexagonal Zni-_Mgt() films with corresponding increase in Mg content, which result in the structure gradually deviating from the wurtzite structure. The optical measurements reveal a blue ii shift in absorption edge and band gap indicating the possibility of the phase transition or phase separation with increase in Mg content. Tuning of the band gap has been obtained from 3.41 to 6.62 eV with corresponding increase in Mg content from x = 0.0 to 1.0, which demonstrates that the films are useful for window layer of solar cells that improve the overall efficiency by decreasing the absorption loss. Section 3-2 describes the growth of Zn3,Mg,0 films by simple and low cost Ultrasonic Spray Pyrolysis technique. The influence of varying Mg content and substrate temperature on structural, electrical and optical properties of Zni,MgrO films was systematically investigated. The structural transition from hexagonal to cubic phase has been observed for Mg content greater than 70 mol%. AFM images of the Zni,Mgx0 films ( x = 0.3) deposited at optimized substrate temperature clearly reveals the formation of nanorods of hexagonal Zni,MgrO. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zni„Mgx0 decreases with corresponding increase in Mg content, which results in structure gradually deviating from wurtzite structure. The tuning of the band gap was obtained from 3.58 to 6.16 eV with corresponding increase in Mg content. The photoluminescence results also revealed the shift in ultraviolet peak position towards the higher energy side. Chapter 4 describes the influence of varying ZnO sublayer thickness (tznb) on structural, optical and electrical properties of ZnO/MgO multilayers synthesized by Pulsed Laser Deposition technique. Decrease in tzno (in the range of 100-23 nm) on the MgO host layer of constant thickness (tmgo = 40 nm), reflects the structural phase transition from wurtzite phase to cubic phase. The optical band gap tuning in ZnO/MgO multilayer thin films has been systematically investigated by computing them in both the transparent as well as in absorption region of the transmission spectra. The optical measurement reveals the variation in optical absorption edges and band gap energies indicating the possibility of the phase transition or phase separation with decrease in tzno- The indices of refraction below the band gap were well described by Sellmeier relation. An increase in band gap also makes contribution to an increase in electrical resistivity in ZnO/MgO multilayer thin films with decrease in ZnO sublayer thickness. The results provide important information for the design and modeling of ZnO/MgO optoelectronic devices due to their adjustable bandgap energies. iii Chapter 5 describes the work carried out on the synthesis and characterization of selected thermochromic and electrochromic materials by pulsed laser deposition technique. The chapter is divided into two sections. Section 5-1 &scribes the influence of varying W03 content in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of V02-W03 nanocomposite thin films. X-ray diffraction studies reveal the single crystalline monoclinic VO2 phase (m-V02) up to 10 % of W03 content whereas both m-V02 as well as h-W03 (hexagonal W03) phases were present at higher W03 content (0.2 < x < 0.4). Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO3 content. The optical transmittance of the films was found to increase with increase in WO3 content. Temperature dependence of resistivity (R-T) measurements indicates significant variation in Metal-Insulator transition temperature, width of the hysteresis, and shape of the hysteresis curve. Cyclic Voltammetry measurements were performed on V02-W03 thin films. A direct correlation between V/W ratio and structure-property relationship was established. Section 5-2 describes the effect of oxygen partial pressure (P02) and vacuum annealing on structural and optical properties of nanocrystalline WO3 thin films. XRD results show the hexagonal phase of deposited WO3 thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO3 thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effect of VO2 content on structural, electrochemical and optical properties of (W03),..x(V02), nanocomposite thin films has also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for V02-W03 thin-film electrode with higher VO2 contenqx > 0.2). Increase of VO2 content in (W03)14V02), films leads to red shift in optical band gap. Chapter 6 presents the summary and conclusion of the entire work presented in the thesis and also proposes the future directions in which these studies can be extended