Thin films of the high-T<SUB>c</SUB> superconductor YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> (YBCO) are the key to devices that function at the temperature of liquid nitrogen. The main priority in depositing YBCO films is the ability to deposit smooth epitaxial films with the desired crystal structure and the correct level of oxygenation. This thesis describes the crystal structure, composition and initial growth stages of YBCO films prepared by planar dc high pressure sputtering. X-ray diffraction (XRD), atomic force microscopy (AFM), and electrical measurements have been used to study the films. Prior to a description of these experiments, two preliminary chapters are included in this thesis. The first chapter is an introduction to some basic concepts of superconductivity, and the crystal structure as well as the physical properties of YBCO. The second chapter reviews important issues and compares the different deposition techniques for preparing YBCO thin films. In a preliminary study of YBCO films deposited under different PxD conditions (the product of pressure and target-to-substrate distance), it was found that a PxD of 2000 Pa-mm is necessary to prevent preferential resputtering. A study of the thermalisation of the high energy particles emanating from a sputtering target was carried out on both Nb films and YBCO films. The sputtering atmospheres and the time for which the ceramic YBCO target has been operated under such atmosphere was found to influence the superconducting critical temperatures of sputtered YBCO films. The effect of oxygenation of the target surface on the superconducting properties of films deposited was studied with various amounts of hydrogen gas added to the sputtering gas or residual water vapour in the vacuum system. The initial stages of YBCO films grown on SrTiO<SUB>3</SUB> substrates were investigated with different film thickness. For the 1-nm-thick film flat island nuclei with one-unit-cell height were found using AFM which is consistent with the observation by XRD. Secondary phases were found to appear in the 2-nm-thick film. The effect of secondary phase precipitates on the superconducting properties and surface morphology of the films is discussed. Spiral growths were found to appear in films with thicknesses larger than 24nm, and their origin is discussed. To study the effect of substrate lattice misfit on the initial stages of film growth, NdGaO<SUB>3</SUB> substrates were chosen.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598932 |
| Date | January 1997 |
| Creators | Fan, S. C. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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