Structural Properties of Yttria-stabilized Zirconia Thin Films Grown by Pulsed Laser Deposition
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Structural properties of yttria-stabilized zirconia thin films grown by pulsed laser deposition J. Y. Dai,a) H. C. Ong, and R. P. H. Chang Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 1 December 1997; accepted 18 September 1998)
Yttria-stabilized zirconia (YSZ) thin films grown by the pulsed laser deposition method on (0001) sapphire substrates have been studied by x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM). It was found that the crystal orientation of the YSZ films changes as a function of oxygen pressure during deposition. At low oxygen pressure (50 mTorr), well-defined (111) oriented YSZ films are grown. High oxygen pressure favors the nucleation of (001) oriented YSZ grains. A model to explain the preferred growth direction of (001) YSZ is presented. Utilizing the experimental data, we have developed a two-step process to epitaxially grow high-quality (001) oriented YSZ on (0001) sapphire substrate.
I. INTRODUCTION
Yttria-stabilized zirconia (YSZ) is an interesting material for a number of electrical and optical applications. For example, thin films of YSZ have been used as solid electrolytes in gas sensors1 and fuel cells,2 and as insulators in microelectronic devices.3 YSZ has also been extensively used as a buffer layer for the growth of high temperature superconducting films of YBa2 Cu3 Ox (YBCO).4–9 It is well known10,11 that grain boundary misorientation significantly affects the value of critical current density Jc . A large decrease in Jc occurs when the misorientation becomes greater than ,10±. It is desirable to reduce the in-plane and out-ofplane misalignment in order to produce high Jc values (.106 Aycm2 , 77 K). This requires the growth of oriented, crystalline buffer layers for subsequent growth of epitaxial superconducting YBCO films. Growth of biaxially aligned (aligned both normal to and within the film plane), k001l oriented YSZ films on several different substrate materials has been reported,12–15 and heteroepitaxial growth of YSZ films on the basal plane of sapphire has been demonstrated by pulsed laser deposition.16 Our preliminary results reveal that the orientation of YSZ films on (0001) Al2 O3 substrates can be controlled by manipulating the oxygen pressure during the growth process.16 For instance, (001) YSZ films can be grown epitaxially on sapphire by predepositing a thin buffer layer of YSZ which has been grown under a high oxygen pressure condition. This thin layer is speculated to function as a template for a)
On leave from the Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China. J. Mater. Res., Vol. 14, No. 4, Apr 1999
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the subsequent growth of (001) YSZ films. However, the growth mechanism and the evolution of the (001) crystal orientation are still far from being understood.
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