SUPERCONDUCTIVE THIN LAYER

Abstract: Superconductive thin layer of YBa₂Cu₃O 7-δ in which reactions with the substrate are prevented in that at least the surface of the substrate consists of a compound having such a composition that in the Y₂O₃-BaO-­ CuO phase diagram it is situated on a segregation line with YBa₂Cu₃O 7-δ .

Description: The invention relates to a superconductive thin layer of YBa₂Cu₃O 7-δ . This material, in which δ = 0.1-0.5, is superconductive at temperatures lower than approximately 90 K. Such a superconductive thin layer is described, for example, in "Thin superconducting oxide films" by R.B. Laibowitz a.o., Physical Review B, 35 (16), pp . 8821-8823 (1987). Sapphire (aluminium oxide) or magnesium oxide is used as a substrate. The thin layer is manufactured by means of electron beam evaporation in a vacuum. The manufacture of superconductive thin layers on aluminium oxide is also described in "Superconducting thin films of the perovskite superconductors by electron-beam deposition" by R.H. Hammond a.o., Extended Abstracts for MRS Symposium on High Temperature Superconductors, Anaheim, April 23-24 (1987). In order to suppress diffusion between the substrate and the superconducting thin layer, buffer layers are used, for example, consisting of MgO, Ta₂O₅, LaF₃ or ZrO₂. Other substrate materials which are used for providing superconductive thin layers of YBa₂Cu₃O 7-δ are strontium titanate and noble metals, for example gold and silver. After the deposition of the superconductive thin layer this must still be subjected to a prolonged treatment at high temperature in oxidizing conditions. The problem then occurs that the superconductive compound in many cases reacts with the substrate which is at the expense of the superconductive properties. The transition temperature T c to superconductive behaviour becomes lower and the transition becomes less steep. The incorporation in particular of metals, for example, aluminium and silicon has proved detrimental. Noble metals and strontium titanate are better suitable, but strontium titanate may also react with YBa₂Cu₃O 7-δ . It is the object of the invention to provide a substrate material in which the problem of a reaction with the superconductive thin layer does not occur. According to the invention, this object is achieved in that the thin layer is provided on a substrate of which at least the surface consists of a compound having such a composition that in the Y₂O₃-BaO-CuO phase diagram it is situated on a segregation line with YBa₂Cu₃O 7-δ . Compounds which are to be considered are CuO, BaCuO₂ and Y₂BaCuO₅. The use of CuO may lead to an excess of CuO in YBa₂Cu₃O 7-δ and a reduction of T c . In practice the use of BaCuO₂ may present difficulties because it reacts strongly with water. Particularly good results were obtained using substrates the surface of which consists of Y₂BaCuO₅. The invention will be described in greater detail with reference to an embodiment and a drawing, in which Figure 1 shows the Y₂O₃-BaO-CuO phase diagram, Figure 2 shows the resistance as a function of the temperature of a thin layer according to the invention, and in which Figure 3 shows the resistance as a function of the temperature of a thin layer on a substrate of aluminium oxide, not according to the invention. Example. A layer of YBa₂Cu₃O 7-δ is provided by means of plasma sputtering on a substrate of Y₂BaCuO₅. Alternative modes of preparation for the thin layer are, for example, electron beam deposition, laser deposition and sputtering. The layer is kept at a temperature of 900°C for 24 hours and is then cooled slowly (in 8 hours to 20°C) in oxidizing conditions, according to the example in oxygen. This after-treatment is necessary to obtain a good oxygen contents in the superconductive material, with δ ≅ 0.3. This, as well as the valency of the copper atoms, is of importance for obtaining a good result. The detrimental effects of the presence of metals, for example, aluminium and silicon, may possibly be explained by a disturbance of the quantity of oxygen and of the copper valency. Figure 1 shows the Y₂O₃-BaO-CuO phase diagram, in which both YBa₂Cu₃O 7-δ and Y₂BaCuO₅ are recorded, which compounds are present on a segregation line in the phase diagram. Figure 2 shows the result of a measurement of the electric resistance as a function of the temperature of a layer according to the invention of YBa₂Cu₃O 7-δ on a substrate of Y₂BaCuO₅. The transition, at approximately 90 K, is particularly steep. For comparison Figure 3 shows the result of a measurement of the resistance as a function of the temperature of a layer of YBa₂Cu₃O 7-δ on a substrate of sapphire, not according to the invention. The transition lies at a lower temperature and is much wider so that a resistance R = 0 is reached only at a temperature lower than 40 K.

Claims: A superconductive thin layer of YBa₂Cu₃O 7-δ , characterized in that the thin layer is provided on a substrate at least the surface of which consists of a compound having such a composition that in the Y₂O₃-BaO-CuO phase diagram it is situated on a segregation line with YBa₂Cu₃O 7-δ . A superconductive thin film as claimed in Claim 1, characterized in that the surface of the substrate consists of Y₂BaCuO₅.

Inventors: KEUR, WILHELMUS CORNELIS, MUTSAERS, CORNELIS ADRIANUS HENRICUS ANTONIUS, VAN HAL, HENRICUS ALBERTUS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01L39
IPC codes: C01G3, C23C14, H01L39, H01B12, C01G1