Transparent And Conductive Oxide Films With The Perovskite-type Structure

Since the discovery of transparent conducting oxide films (TCO) in 1907s, these materials have attracted much attention for their potential application in liquid crystal displays, organic light emitting diodes, and solar cells. Because of their unique properties of optical transparency in the visible range and controllable electrical conductivity from almost insulating to degenerate semiconducting behaviors, these films are also expected to be applied in transparent electronics, e.g.The conventional TCO films, such as In2O₃, ZnO, SnO₂, were widely used for the past one hundred years. As limitations of the existing materials become more critical in view of the resource problem of rare metal In, there has been increasing interest in finding high-quality single-crystalline TCO films with simple structure and lower growth temperatures.Perovskite-type oxides present an important class of materials since they were discovered as having many properties including superconductivity, ferromagnetism, ferroelectricity, multiferroics, and electro-optic effects. However, due to their similar structures, with the advances in thin film growth, a variety of all-perovskite heteoepitaxial structures have been fabricated for devices with improved performance or novel functionalities. Recently, by ionic substitution and carrier doping, TCO films with the perovskite structure were also explored, and a few of them including In, Sb, and La-doped SrTiO₃, Nb-doped CaTiO₃ have been obtained. The perovskite structure gives them advantages in fabrications of all-perovskite heterostructures. Although doped SrTiO₃and SrSnO₃ ceramics have been extensively studied, the La-doped stannates in the thin-films form grown at higher oxygen pressure are rarely reported.Here we grow epitaxial La-doped SrTiO₃ and SrSnO₃ films by the pulsed laser deposition method and demonstrate that the films have not only high conductivity but also good optical transmittance in the visible range. We show that these films can be potentially used in transparent devices especially based on the all-perovskite heterostructures. The whole thesis consists of four chapters.Chapter 1: The general introduction of the traditional TCO films is given. First, we discuss the important characteristic and physical mechanism of the TCO films, and then briefly review the physical properties, researching actuality, and the widely applications of traditional TCO films. Importantly, we bring forward a new type of TCO films with the perovskite structure: La-doped SrTiO₃ and SrSnO₃. In the end of chapter 1 we simply introduce some preparation methods of thin films.Chapter 2: The films preparation method and samples measurement are introduced. Especially, the pulsed laser deposition method and X-ray diffraction are described in details.Chapter 3: Perovskite-type TCO films:La_0.05Sr_0.95TiO₃(LSTO),were grown by the pulsed laser deposition method.It is revealed that the films grown at high oxygen pressure show single crystallinity with a perovskite structure.Remarkably,these films are both electrically conductive and visually transparent in the visible range.The growth parameters are compatible with those for some other perovskite films,indicating a potential application in transparent devices.The LSTO films were employed as electrodes for the fabrication of epitaxial PbZr0.52Ti0.48O₃(PZT)ferroelectric capacitors,which showed square polarization-electric field hysteresis loops and improved fatigue resistance.Chapter 4: La-doped SrSnO₃ (LSSO), were grown on SrTiO₃ substrates at various temperatures by the pulsed laser deposition method. It is revealed that the films can be grown at relatively lower temperatures up to 550℃and show single crystallinity with a perovskite structure. Remarkably,these films are both electrically conductive and visually transparent in the visible range. The effect of growth temperature on structural, electrical, and optical properties of the LSSO films was probed. The resistivity of the films increases as the growth temperature decrease, which is associated with the degradation of their crytical structure.