High quality a -Si/Nb and a -SiN/NbN artificial multilayers for Josephson applications
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A high resolution transmission electron microscopy study of multilayer films prepared by magnetron sputtering shows that the morphology of the growing interfaces in a-Si/Nb and a-SiN/NbN multilayers is remarkably uniform and smooth; this is in contrast to the polycrystalline AlN/NbN multilayers grown under similar conditions, which exhibit columnar grain structure with rough interfaces. For proper sputtering parameters, the amorphous layers seem to periodically restore a relatively smooth initial interface condition for the successive Nb (or NbN) layer growth, consequently interrupting the tendency toward increased roughness due to mechanisms such as columnar growth. Artificial multilayers having very flat interfaces could stimulate applications based on multilayer Josephson junctions.
I. INTRODUCTION Artificially synthesized multilayer structures represent a new class of materials having novel optical, electrical, magnetic, mechanical, and superconducting properties for a host of important applications.1-2 Such multilayers have been widely used to study the dimensional crossover phenomena in superconductivity3 and localization/interaction effects.4 Recently, there is growing interest in using multilayers as model systems to understand the underlying physics in high Tc oxides. Most applications require a smooth and sharp surface morphology. In fact, considerable efforts have been devoted to understand the film growth dynamics5'6 and the relation between growth conditions and surface morphology.7"10 In Josephson junction applications involving Nb (or NbN), the commonly encountered columnar growth mode of these materials poses a serious problem in barrier formation since the interface is rough. To circumvent this problem, it is a common practice to coat the Nb surface with a thin layer of Al, 11 since Al wets Nb, resulting in a smooth surface. This surface is then oxidized to form the barrier. In this work we propose an alternate strategy to solve this problem; this is to "break up" the growing columns by depositing a second amorphous constituent, thus forcing the Nb to renucleate with each successive layer, thereby never allowing the faster growing grains to dominate (which will result in a rough surface). In our efforts to fabricate multilayered tunnel junctions as high energy resolution x-ray detectors,12-13 we have prepared and structurally characterized multilayers consisting of various material combinations such as
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http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 7, Jul 1994
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a-Si/Nb, a-SiN/NbN, and polycrystalline AlN/NbN. High resolution transmission electron microscopy (HRTEM) studies show that in contrast to polycrystalline AlN/NbN multilayers, which exhibit columnar growth with rough interfaces, the interfaces in a-Si/Nb and a-SiN/NbN multilayers prepared by magnetron sputtering under similar conditions are very uniform and flat. For proper sputtering parameters, the amorphous layers seem to effectively inte
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