Growth mechanism of biaxially textured YSZ films deposited by ion-beam-assisted deposition
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Growth mechanism of biaxially textured YSZ films deposited by ion-beam-assisted deposition J. Wiesmann, J. Dzick, J. Hoffmann, K. Heinemann, and H. C. Freyhardt Institut f u¨ r Metallphysik, Universit¨at G¨ottingen, Hospitalstr. 3/7, D-37073 G¨ottingen, Germany (Received 9 May 1997; accepted 5 February 1998)
Biaxially textured YSZ films have a large technical relevance for power or electronic applications of HTS films. The YSZ serves as a diffusion barrier and as a template for an epitaxial growth of the HTS. On polycrystalline substrates the biaxial alignment is achieved by using an ion-beam-assisted deposition method. The best obtained textures were characterized by a full width at half maximum of 7± in an k111l x-ray diffraction F scan. The FWHM decreases with increasing film thickness. The growth mechanism is investigated with respect to three important effects: nucleation, growth selection, and homoepitaxial growth. It could be shown that during nucleation at the beginning of deposition the angle between the assisting beam and the substrate normal has to be fixed at 55±, whereas during the growth selection this angle can be varied. Especially the homoepitaxial effects allow changes in the deposition conditions without destroying the already achieved texture quality. I. INTRODUCTION
The modification of growth and properties of thin films with assisting ion beams has been well known for many years. Several important properties of thin films can be manipulated when an ion beam with low energy (,1 keV) bombards the growing film during deposition, for example: structure, refractive index, density. This special deposition technique, the ion-beam-assisted deposition (IBAD), was mainly used to densify films and to change a uniaxial texture. In 1986 Yu et al.1 reported that an in-plane texture also can be achieved by an IBAD technique (in niobium films). A similar effect was found in YSZ (yttria-stabilized zirconia) films by Iijima et al. in 1992.2 There the k111l axis of the YSZ aligns parallel to the assisting Ar beam and the k100l axis lies parallel to the substrate normal. These biaxially textured layers have a great technical relevance for the deposition of HTS (high-temperature-superconductor) films with high critical current densities. Possible applications are in the field of power engineering (e.g., resistive fault current limiters or current leads) or in electronic devices (e.g., filters, satellite devices). Except on single crystals, only on polycrystalline substrates with a biaxially textured YSZ buffer layer can YBCO films be prepared with critical current densities jc . 106 Aycm2 (at 77 K, 0 T).3 –5 Although many research groups report the successful deposition of highly textured YSZ films prepared by IBAD, only a few authors try to explain the underlying growth mechanism of these films. Sonnenberg et al. proposed a deposition mechanism that is more growth controlled than nucleation controlled.6,7 Because of a growth rate variation, faster growing grains with
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