SYNTHESIS AND CHARACTERIZATION OF TRANSITION METAL OXIDE NANOCRYSTALLINE THIN FILMS

Abstract

Transition metal oxide (TMO) belongs to a wide class of materials, with insulating, semi-conducting and conducting electronic properties. This field is very rich in physics because of unusual magnetic, optical and electrical properties and has tremendous device applications. Among these oxides, thin films of wide band gap semiconductors have attracted much attention from last decade due to their wide variety of applications such as gas sensors, optical switches, windows for solar cell, thin film batteries and photocatalysis etc. In recent years interest in field of nanotechnology and nanocrystalline materials of these oxides has been growing steadily. This is because of the technological breakthroughs made possible by making smaller and smaller device components, thereby improving the performance of devices without increasing their physical size. Despite numerous scientific works, new routes of fabrication and the fundamental understanding of these materials are much more essential so that they can be integrated into contemporary and emerging technologies. Therefore it is desirable to establish the process for producing high quality single phase nanocrystalline thin films of these oxides. The main aim of the present work was to synthesize nanocrystalline thin films of selected transition metal oxide especially Zinc oxide (ZnO), Titanium dioxide (Ti02) and Vanadium pentoxide (V205) on various substrates by dc magnetron sputtering technique at room temperature and to investigate the effect of optimized process parameters on structural, optical and transport properties of these materials to obtain device quality thin films. In addition we have also prepared ZnO nanocrystalline thin films and nanopowder via low cost ii ultrasonic spray pyrolysis technique and investigated the effect of transition metal doping on magnetic properties of these films. A chapter- wise summary of the thesis is given below. Chapter 1 gives an overview of nanocrystalline thin films and material background of selected semiconductor oxides ZnO, Ti02 and V205. The chapter includes the discussions on the structural, optical and transport properties of these oxides. The effect of transition metal doping in these oxides has also been described. Chapter 2 presents the details of experimental techniques, which we have used for the synthesis and study of the properties of nanocrystalline oxide _thin films and nanopowder. Section 2.1- Most of the synthesis of thin films in present thesis has been carried out by dc magnetron sputtering . technique. Particle size was controlled by adjusting the substrate temperature, inert gas pressure and sputtering power. We have also developed the setup of Ultrasonic Spray Pyrolysis and Ultrasonic Mist-Chemical Vapour Deposition (UMCVD) technique for the growth of ZnO nanocrystalline thin films and nanopowder. Section 2.2- The first measurement that is usually carried out after synthesis is to record the X-ray diffraction pattern of the deposited nanocrystalline 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), Field Emission Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). Section 2.3- Optical properties. of these films were studied using UV-visible spectrometer. We have also measured the temperature dependent resistivity of V205 thin films using four probe resistivity set-up. The magnetic properties of doped ZnO thin films were measured using SQUID magnetometer. The film thickness was measured using surface profilometer and using multiple-beam Fizeau fringes at reflection with monochromatic light. nl Chapter 3 describes the growth and characterization of nanoerystalline ZnO thin films and nanopowder. This chapter is divided into three sections. The first section (section 3.1) mainly describes the growth and characterization of ZnO nanocrystalline thin films via ultrasonic spray pyrolysis. The effect of substrate and substrate temperature on structural and optical properties was investigated. The films were deposited on Si (100) and glass substrates at different deposition temperature in the range 200-500 °C. Both orientation and size of crystallites were found to depend on the substrate and substrate temperature. XRD & FESEM results confirms the a-b axis- orientation in ZnO films on Silicon substrate with hexagonal columnar like grains and c-axis orientation in case of glass substrate with grains perpendicular to the substrate. AFM images of the films deposited on glass shows vertically align nanotips-like structures with narrow shaft and sharp ends with average graiwisize of 70nm and films deposited on Silicon shows nanobowls like structures. Section 3.2- deals with the synthesis of ZnO nanocrystalline powder by ultrasonic mist-chemical vapour deposition. FESEM and TEM studies revealed that the powder consisted of the mixture of nanoparticles with particle size 50-100 nm. The synthesized nanopowder was found to have pure wurtzite structure with lattice parameters a and c of 3.244 and 5.297 nin, and c/a ratio of 1.6 respectively. In situ high temperature XRD studies of ZnO nanopowder in vacuum at increasing temperature from room temperature-to 1000 °C showed an increase in crystallite size with increasing temperature. No phasechange was observed in,XRD pattern even after in situ heating of the powder at 1000 °C. TEM results were in agreement to XRD results. The synthesized powder exhibited the estimated direct band gap (Eg) of 3.43 eV. The excess free volume associated with the grain boundaries was calculated and was found to decrease with increase in crystallite size. The grain growth kinetics of ZnO nanopowder was investigated and the value of apparent activation energy from this study was found to be 54 kJ/mol in the 1v temperature range 700 to 1000°C. Section 3.3- Nanocrystalline ZnO thin films were also deposited by dc magnetron sputtering using Zn metal target under different argon and oxygen ratios at various sputtering pressures. The structural and optical properties were found to be strongly dependent on the processing conditions; such as gas phase composition, plasma conditions etc. The investigations showed that the films deposited at low oxygen partial pressure (10%) contain mixed phase (Zn and ZnO) and were randomly. oriented with while the films deposited at higher oxygen partial pressure (30%) were single phase (ZnO) and highly oriented along the c-axis. Chapter 4 discusses the growth and characterization of nanocrystalline anatase and rutile Ti02 thin films by do magnetron sputtering. This chapter is divided_into three sections. The first section (section 4.1) describes the effect. of substrate nature on structural-properties of TiO2 thin films. The films were found.to• have different phases andorientation. of Ti02 on different substrates. Single anatase phase with intense (101) reflection in case of the films deposited on silicon.. (111.) substrate and , preferred (004) orientation in case, of films grown - over LaAlO3 substrates was observed.: On the other. hand-rutile,.phase. with .preferred (200)-.r. orientation was observed in case of films deposited over the .Sapphire substrate. It was also interesting to note that with change in silicon substrate from Si(111) to Si(100), the film growth direction changes from preferred (101) to (004) .respectively, indicating that the film growth direction is highly affected with nature of substrate and substrate orientation due to different lattice mismatching between substrate and film, which was further confirmed by the TEM. AFM and FESEM- images showed that nanostructured Ti02. films could be grown on all substrates, but their microstructure and surface roughness depends on the substrate type. Section 4.2 & 4.3- The aim. of the study is' (i) to synthesize single phase anatase nanocrystalline Ti02 thin films at room temperature without substrate heating or post u annealing by dc magnetron sputtering of titanium metal in Ar/02 atmosphere on various substrates and (ii) to study the influence of sputtering parameters especially sputtering power and sputtering pressure on the structural, optical and morphological properties of Ti02 thin films. Our results indicate that it is possible to deposit nanocrystalline anatase Ti02 thin films at room temperature without substrate heating with proper control of sputtering parameters. We believe that crystallization of Ti02 films at room temperature is mainly due to energetic particle bombardment (electrons, atoms, ions, molecules and even charged clusters) on the growing films. Chapter 5 describes the work we did on nanocrystalline V205 thin films of various thickness deposited at room temperature by dc magnetron sputtering. The result of present. investigation indicated the growth of stoichiometric, nanocrystalline and high :quality thing film of V205. Another interesting feature observed in this study was the change; in texture,K: electrical and optical properties with film thickness. The temperature dependent resistivity-(R-T) curve showed an insulating to semiconducting transition, change in hysteresis width and change in order of resistance in R-T curve with variation in thickness of~films. The optical measurement revealed the systematic variation in band gap (from 2.50 to.2.20 eV) & transmittance with change in film thickness. Chapter 6 discusses the work on transition metal (Mn & Co) doping in ZnO thin films by ultrasonic spray pyrolysis technique. In order to investigate the magnetism of transition metal doped ZnO, we have prepared the films of Zna_,,Mn,,O and Znl_,,Co,.O compounds with. varying concentration of x (with x <_ 0.10). --The structural and optical properties of these films reflects that the Coe+ and Mn2+ ions have- substituted the Zn2+ ion without changing the wurtzite structure of ZnO. The observed decrease in the band gap with increase in doping concentration was explained in terms of a sp-d exchange interaction. The vi concave nature of M(T) behavior with steep rise of magnetization was observed at low temperature for both Mn & Co doped ZnO films and was explained in terms of polaron-percolation-theory. We have not observed any prominent hysteresis loop in M(H) curve at 300K showing the absence of room temperature ferromagnetism in case of our Mn and Co doped ZnO films. Chapter 7 presents the summary and conclusion of the entire work presented in the thesis and also propose the future directions in which these studies can be extended. Vii