Controlled Growth of High-Temperature Superconducting Thin Films on Polycrystalline Substrates
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MRS BULLETIN/AUGUST 1992
Even though the technology development in this area is still in its infancy, many successful processing approaches have been developed to set the stage for the eventual use of HTS thin films in power device applications. This paper reviews the processing techniques for controlling the growth of HTS thin films on polycrystalline substrates. The microstructure and properties of grain boundaries and their effect on the current carrying capabilities of HTS polycrystalline thin films will be discussed. The role of buffer layers on the microstructure with respect to c-axis orientation and the effect of ion-bombardment texturing on a-b plane alignment will also be reviewed.
Fabrication of Polycrystalline Thin Films Generally, the fabrication techniques for thin films on polycrystalline substrates are identical to those for single crystal substrates, although some of the deposition parameters (e.g., substrate temperature and oxygen pressure) need to be adjusted. Two general approaches have been developed for the fabrication of high-quality HTS thin films: post-annealed and in-situ techniques. In the post-annealing process, the deposited film is baked at a high temperature (800 to 900°C) to allow the formation of HTS crystals. This high-temperature annealing process, however, is detrimental to film quality because it promotes
chemical interaction and interdiffusion of the HTS film with the substrate. To avoid the high-temperature annealing process, in-situ film growth techniques have been universally used for the fabrication of HTS thin films on polycrystalline substrates. In contrast to the post-annealed techniques, in-situ film-growth techniques produce films with very high critical current densities (about 5 X 106 A/cm2 at 77 K and zero field) at comparatively low temperatures (550-750°C). For general discussions on post-annealed and in-situ film growth, see review papers by Laibowitz,1 Beasley,2 and Simon3. A variety of methods have been used to successfully grow in-situ HTS YBa2Cu3Oy (YBCO) thin films. The most widely used deposition techniques at the present time are pulsed laser ablation, sputtering, e-beam evaporation, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD). These in-situ growth techniques share certain features. First, the deposition is carried out with the substrate maintained at temperatures 50-200°C lower than those required by the postannealing process, and under an oxygen or other oxidizing agent (e.g., O3, N2O, etc.) environment. Second, oxygen is allowed to fill the chamber at pressures ranging from a few torr to atmospheric pressure during the cooling period after the film deposition is complete. In Table I,5'22 four in-situ techniques for the growth of thin films are listed, along with the oxygen pressure during deposition, Po2, and substrate temperature, Ts, for many published HTS thin films. Hammond and Bormann4 have correlated the growth conditions of various in-situ techniques over a wide range of Ts and Po, with respect to the ther
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