Preparation of superconducting oxide films by reactive evaporati

Details Preparation of superconducting oxide films by reactive evaporati Preparation of superconducting oxide films by reactive evaporati

1990-09-05

1991-08-13

Beck, Shrive

Superconductor technology: apparatus, material, process

High temperature , per se

Having tc greater than or equal to 150 k

505732, 505736, 427 62, 4271263, 427314, B05D 512, C23C 1424

Patent

active

050396579

DESCRIPTION:

BRIEF SUMMARY
exposing the substrate to relatively low temperatures. The present method comprises contacting the heated substrate surface with a stream of ozone (O.sub.3) while evaporating the other (metallic) components of the superconductive ceramic oxide onto the surface, so that a layer of a superconductive ceramic oxide is formed thereon. The metallic components are oxidized in situ by active oxygen atoms (O*) formed by thermal decomposition of the O.sub.3 at the heated surface. Preferably, the ozone is produced by the evaporation of an external source of liquid ozone, which is then introduced into a vacuum chamber containing sources of the other components and the substrate.
The use of essentially pure ozone to provide oxygen atoms for the superconductive lattice allows production of superconductive films of the correct crystal structure, chemical composition and superconductive properties without the need to post anneal the ceramic or to subject it to any additional treatment. The present method can be carried out at about 500.degree.-700.degree. C., preferably at about 450.degree.-500.degree. C., temperatures which permit the formation of superconductive layers on substrates, such as plastics, certain metals, or single crystals of silicon or GaAs, which could not heretofore be used. The layers can also be deposited to virtually any desired thickness, e.g., from about 500 .ANG. to 0.5 .mu.m.
With minor modifications, the present method can be employed to prepare any of the known superconductive ceramic oxides. The structures of three of the most widely-investigated classes of these materials is summarized in Table I, below. Other superconductive ceramic oxides which can be used as substrates in the present method are described hereinbelow.


TABLE I ______________________________________ Superconductive Ceramics Abbrevi- Formula X Y ation ______________________________________ La.sub.2-x A.sub.x CuO.sub.4 0.07 - 0.2 -- 2-1-4.sup.1, 4 RZ.sub.2 Cu.sub.3 O.sub.7-x 0 < x < 1 -- 1-2-3.sup.2, 5 Bi.sup.II Ca.sub.1+x Sr.sub.2-x Cu.sub.2 O.sub.8+y 0 < x < 1 0 < Y < 2 2-1-22.sup.3 Tl.sup.II Ca.sub.x-1 Ba.sub.2 Cu.sub.x O.sub.2x+3 X = 1, 2 or 3 -- 1-(X-1)-(2) (X) ______________________________________ .sup.1 J. G. Bednorz et a1., Z. Phys. B., 64, 189 (1986), (A = Sr). .sup.2 C. W. Chu et al., Phys. Rev. Lett., 58, 405 (1987), (R = Y). .sup.3 H. Maeda et a1., Jpn. J. Appl. Phys., 27, L209 (1988); Z. Z. Sheng et al., Nature, 332, 55 (1988). .sup.4 A = Ba, Sr, Ca. .sup.5 R = lanthanide element, i.e., Y, Sm, Eu, Gd, Dy, Ho, Yb when Z = Ba; also YSr.sub.2 Cu.sub.3 O.sub.7-x.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a multisource high vacuum system equipped with two electron guns (e-guns 1 and 2) and a K cell, which can be used to practice the present method.
FIG. 2 is a graphical depiction of the superconductive transition temperature (Tc) of a film of YBa.sub.2 Cu.sub.3 O.sub.7-x prepared in accord with the present method.
FIG. 3 is the x-ray diffraction pattern obtained for a c-axis oriented film of YBa.sub.2 Cu.sub.3 O.sub.7-x prepared in accord with the present method .


DETAILED DESCRIPTION OF THE INVENTION



Ceramic Superconductors

Superconducting oxides have been known since 1964, but until recently, the intermetallic compounds showed higher superconducting temperatures. In 1975, research scientists at E. I. DuPont de Nemours discovered superconductivity in the system BaPb.sub.1-x Bi.sub.x O.sub.3 with a Tc of 13K (A. W. Sleight et al., Solid State Commun., 17, 27 (1975)). The structure for the superconducting composition in this system is only slightly distorted from the ideal cubic perovskite structure. It is generally accepted that a disporportionation of the Bi(IV) occurs, namely, 2Bi(IV)(6s.sup.1).fwdarw.Bi(III)(6s.sup.2)+Bi(V)(6s.sup.0) at approximately 30 percent Bi. Sleight et al. found that the best superconductors were single phase prepared by quenching from a rather restricted single-phase region, and hence these ph

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