Domain growth of Dy 2 O 3 buffer layers on SrTiO 3
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Dy 2 O 3 layers have been grown on SrTiO3 by molecular beam epitaxy. X-ray and electron diffraction patterns clearly show that Dy 2 O 3 grows epitaxially on SrTiO3 with {100} planes parallel to the substrate surface. Transmission electron microscopy reveals that the Dy 2 O 3 film breaks up into small domains (10-40 nm). This leads to the formation of terraces which limits the structural perfection of thin overgrown DyBa 2 Cu 3 0 7 by introducing steps and small misorientations (within 3°). The resulting surface corrugation does not preclude the growth of epitaxial c-axis DyBa 2 Cu 3 0 7 films with a TcO of 86 K. Crystallographic analysis and image calculations show that the domain growth of Dy 2 O 3 is associated with the formation of 90° rotation twins.
I. INTRODUCTION The properties of thin superconducting layers (a few unit cells thick) are strongly influenced by the chemical and structural properties of the substrate. Thus, there is an effort toward growth of compatible oxide buffer layers on most commonly used substrates such as SrTiO3, MgO, and Si. This interest is partly driven by the development of tunnel junctions and passive or active devices. Successful applications of buffer layers have been reported for materials that are structurally and chemically very similar to the desired superconductor. For the REBa 2 Cu 3 0 7 (REBCO: RE = rare earth) family, PrBa 2 Cu 3 0 7 (Ref. 1) and LaiBa 2 Cu 3 0 7 (Ref. 2) have been used as a buffer layer and a protection layer for single thin films as well as multilayers3 on MgO or SrTiO3. Other superconductor/buffer layer pairs exist such as (La, Sr) 2 CuO 4 /Sm 2 CuO 4 (Ref. 4) and Bi2Sr2Cao.85Yo.i5Cu208/Bi2Sr2Cao.5Yo.5Cu208.5 In general, such buffer layers have the advantage of low chemical reactivity with the superconductor layer and have a similar surface symmetry. Another approach is to use oxides with structures that are identical to that of the commonly used substrate oxides. Examples for this include SrTiO3,6 MgO, 7 and Y-stabilized ZrO 2 . 8 A third group includes the RE oxides (RE 2 O 3 ), 9 in particular Y 2 O 3 and Dy 2 O 3 , with a low dielectric constant and a lattice mismatch with respect to YBCO or DBCO, which is comparable to that provided by SrTiO3. They were used in sandwich structures to demonstrate Josephson supercurrents10 or to measure tunneling characteristics.11"13 For the fabrication of tunnel junctions it is important to grow planar, pinholefree buffer layers with abrupt interfaces. To reach
^Present address: Prospectives et Recherche, EPFL, 1015 Lausanne, Switzerland. J. Mater. Res., Vol. 8, No. 6, Jun 1993 http://journals.cambridge.org
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these goals a better understanding and control of the growth mechanisms is necessary, which in turn requires a detailed structural analysis of both buffer and DBCO layers. In particular, we need to understand the influence of the buffer layer structure on the overgrown superconducting layer and the resulting interfaces. II. EXPERIMENTAL PROCEDURE In this paper we report on the growth
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