V205 COMPOSITE THIN FILMS FABRICATION AND CHARACTERIZATION

Abstract

Transition metal 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 opto electronics. 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 V205-TiO2 nanocomposite thin films by Pulsed Laser Deposition (PLD) technique and to investigate the effect of optimized process parameters on structural and optical properties of these materials to obtain device quality thin films. Chapter 1 gives an overview of functional oxide thin films and material background of selected semiconductor oxides i.e. V205 and Ti02. The chapter includes the discussions on the structural, optical and transport properties of these oxides. 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 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 Vapour 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. 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 were studied using Atomic Force Microscopy (AFM) and Field Emission Electron Microscopy (FESEM). Section 2-3- Optical properties of these films was studied by using UV-Vis-NIR spectrometer. Chapter 3 describes the growth and characterization of Pure V205 and Ti02 thin films. The chapter is divided into two sections. The first section (section 3-1) describes the influence of varying temperature (in the range 300 °C to 600 °C) on structural and optical properties of Pure V205 thin films synthesized by Pulsed Laser Deposition technique. The dominance of the (001) peak at high temperatures suggests that the texture of the V205 thin film is oriented along the c-axis perpendicular to the surface of the substrate with it's a-, b-axis parallel to the surface. The preferred orientation is related to the in plane organization of the V-0—V chains. FESEM micrographs of V205 thin film deposited at 400 °C shows formation of complete nanorods. AFM data revealed that laser ablated V2O5 thin films were homogenous, smooth and uniform with regard to surface topography. The optical measurement reveals a blue shift in absorption edge and shows the trend toward decreasing transmittance. Chapter 4 describes the work carried out on the synthesis and characterization of selected transition metal oxides by pulsed laser deposition technique. The chapter describes the influence of varying Ti02 content on structural, morphological and optical properties of V205 - Ti02 nanocomposite thin films. X-ray diffraction studies reveal the presence of both Anatase and Rutile phase at 20 % of Ti02 content whereas only Rutile phase was present when Ti02 was increased to 50%. Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in Ti02 content. The optical transmittance of the films was found to decrease with the decrease in temperature. Chapter 5 presents the summary and conclusion of the entire work presented in the dissertation and also proposes the future directions in which these studies can be extended.