Method of producing an SrTiO.sub.3 /YBa.sub.2 Cu.sub.3 O.sub.7 layer system and a layer system as thus produced as a high-temperature superconductor

Abstract

c-axis oriented YBa.sub.2 Cu.sub.3 O.sub.7 layers are grown with intervening SrTiO.sub.3 layers bridged over steps at which there is a transformation to a-axis crystal-oriented growth. The multilayer superconductor has YBa.sub.2 Cu.sub.3 O.sub.7 layers which are not thicker than 500 nm while the intervening layers of SrTiO.sub.3 have thicknesses of 20 to 30 nm.

Description

FIELD OF THE INVENTION

Our present invention relates to a method of producing a multilayer high-temperature superconductor, i.e. a superconductor effective at the temperature of liquid nitrogen or thereabove, and especially a layer system of SrTiO.sub.3 and YBa.sub.2 Cu.sub.3 O.sub.7. More specifically, the invention relates to a layer system having a substrate with c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layers thereon and to a layer system or stack as made by the new method.

BACKGROUND OF THE INVENTION

High temperature superconductor systems in which a YBa.sub.2 Cu.sub.3 O.sub.7 layer with a c-axis crystal orientation with copper oxide planes parallel to the substrate surface are of interest for current transport at especially high current densities. For this purpose, the YBa.sub.2 Cu.sub.3 O.sub.7 layer can be grown with a c-axis orientation.

There are various processes known for effecting the growth of YBa.sub.2 Cu.sub.3 O.sub.7 layers with a c-axis orientation on an appropriate substrate. For example, high pressure dc sputtering can be used to deposit the YBa.sub.2 Cu.sub.3 O.sub.7 layers with a c-axis orientation. The type of crystal grown depends upon geometric conditions, pressure of the sputtering gas, furnace temperature and substrate temperature, choice of substrate and of sputtering gas and the current density during the sputtering.

However, even with an optimum choice of all of these parameters in prior systems, it has been found that with increasing layer thicknesses, the growth of a-axis crystal oriented regions cannot be avoided.

Once c-axis crystal oriented regions arise, they increasingly prevent uniform a-axis oriented growth. Until now there has been no solution to this problem.

OBJECTS OF THE INVENTION

It is the principal object of the present invention, therefore, to provide a process which enables a controlled c-axis oriented YBa.sub.2 Cu.sub.3 O.sub.7 growth for use of the crystal structure as a high structure superconductor, whereby the c-axis crystal orientation regions predominate and whereby disadvantages of the prior art system can be avoided.

It is also an object of this invention to provide a superconductor stack of layers in which c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layers predominate and wherein the formation of a-axis crystal-oriented regions is largely suppressed.

A further object of this invention is to provide an improved method of making a superconductor crystal stack whereby drawbacks of earlier systems are eliminated.

SUMMARY OF THE INVENTION

Surprisingly, we have found that the drawbacks of earlier systems can be obviated in the growth of YBa.sub.2 Cu.sub.3 O.sub.7 layers with a c-axis crystal orientation when, upon the development of undesirable a-axis oriented regions and thus steps between a-axis and c-axis oriented regions which interrupt the c-axis YBa.sub.2 Cu.sub.3 O.sub.7 layer, a SrTiO.sub.3 layer is deposited over the step and the YBa.sub.2 Cu.sub.3 O.sub.7 until the step is covered by the SrTiO.sub.3 layer and thereafter the growth of the YBa.sub.2 Cu.sub.3 O.sub.7 with the c-axis crystal orientation is continued.

More particularly, the method of the invention for producing a multilayer high-temperature superconductor comprises the steps of:

(a) growing a c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer upon a substrate until formation of an a-axis crystal oriented region occurs; (b) upon formation of the a-axis crystal oriented region and development of steps between a-axis crystal-oriented and c-axis crystal-oriented regions on a YBa.sub.2 Cu.sub.3 O.sub.7 surface of the layer, interrupting growth of the c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer; (c) upon interruption in growth of the c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer in step (b), applying an SrTiO.sub.3 layer to the surface until the steps are covered; and (d) thereafter continuing the growth of the c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer upon the SrTiO.sub.3 layer, thereby forming a stack of layers.

Steps (b) and (d) can be repeated until the stack has a multiplicity of SrTiO.sub.3 layers alternating with the YBa.sub.2 Cu.sub.3 O.sub.7 layers.

The invention is based upon the discovery that the hitherto unavoidable uncontrolled growth of a-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 can be avoided when relatively large layer thickness of YBa.sub.2 Cu.sub.3 O.sub.7 layers by incorporating into them intervening layers which allow further controlled c-axis oriented growth. The SrTiO.sub.3 layers have been found to be ideal for this purpose.

Advantageously, the YBa.sub.2 Cu.sub.3 O.sub.7 is grown only to layer thicknesses of 500 nm before a SrTiO.sub.3 layer is applied and further c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 growth is effected.

It has been found, further, that thicknesses of 20 nm to 30 nm SrTiO.sub.3 layers suffice to allow further c-axis oriented crystal growth of the YBa.sub.2 Cu.sub.3 O.sub.7.

A high-temperature superconductor which has, therefore, far more c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 regions than hitherto has been the case can thus be obtained in the form of a multilayer stack with alternating YBa.sub.2 Cu.sub.3 O.sub.7 and SrTiO.sub.3 layers whereby each SrTiO.sub.3 layer of a thickness of 20 to 30 nm is sandwiched between two YBa.sub.2 Cu.sub.3 O.sub.7 layers.

The YBa.sub.2 Cu.sub.3 O.sub.7 layers preferably have thicknesses between 300 nm and 500 nm.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is an electron microscope micrograph of an SrTiO.sub.3 YBa.sub.2 Cu.sub.3 O.sub.7 --layer system embodying the principles of the invention and showing in section the various layers; and

FIG. 2 is a diagrammatic cross sectional view showing a superconductor stack according to the invention.

SPECIFIC DESCRIPTION

In FIG. 1 of the drawing, in which the substrate itself is not visible, a first YBa.sub.2 Cu.sub.3 O.sub.7 layer 1 of uncontrolled growth is provided and is covered by an SrTiO.sub.3 coating 2 of several nm in thickness. A further uncontrolled growth YBa.sub.2 Cu.sub.3 O.sub.7 layer 3 is applied thereto and is covered by a c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer 7.

c-axis oriented regions appear in the illustration for transverse lines and a-axis oriented regions with longitudinally-extending lines, i.e. lines perpendicular to the layer 5. The SrTiO.sub.3 layer of 20 to 30 nm in thickness can bridge the step 6 and enable uniform growth of the c-axis YBa.sub.2 Cu.sub.3 O.sub.7 layer. The thinner layer 2 of SrTiO.sub.3 is not able to bridge the step 4 and thus from this step there is further a-axis crystal-oriented growth. By way of example, the application of the YBa.sub.2 Cu.sub.3 O.sub.7 layer was effected by high pressure dc sputtering under the following conditions:

single stoichiometric target of a diameter of 35 mm and thus a composition of YBa.sub.2 Cu.sub.3 O.sub.7 ; (100)--oriented SrTiO.sub.3 crystal as substrate; 13 mm distance between target and substrate; oxygen as the sputtering gas with a pressure of 3.6 m bar; furnace temperature of 880.degree. C.; surface temperature of substrate 750.degree. C.; current amplitude during sputtering 200 mA; and oxygen after treatment at 550.degree. C.

The SrTiO.sub.3 layers were applied by high-frequency sputtering using a sinusoidal voltage at 1000 volts, an alternating current frequency of 7 MHz and otherwise the same conditions except for the use of an SrTiO.sub.3 target.

In FIG. 2 we have shown the substrate 10 of SrTiO.sub.3, a layer 11 of YBa.sub.2 Cu.sub.3 O.sub.7 on which an a-axis oriented step 12 has grown, the SrTiO.sub.3 layer 13 another c-axis oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer 14 on which an a-axis step 15 has begun to grow, a further SrTiO.sub.3 layer 16 of 20 to 30 nm in thickness and a successive c-axis YBa.sub.2 Cu.sub.3 O.sub.7 crystal-oriented layer 17 forming the superconductor.

Claims

1. A method of producing a multilayer high temperature superconductor, comprising the steps of:

(a) growing a c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer upon a substrate until formation of an a-axis crystal oriented region occurs;

(b) upon formation of said a-axis crystal oriented region and development of steps between a-axis crystal-oriented and c-axis crystal-oriented regions on a YBa.sub.2 Cu.sub.3 O.sub.7 surface of said layer, and, after the growth of said c-axis crystal oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer to a thickness of at least 300 nm and not more than 500 nm, interrupting growth of the c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer;

(c) upon interruption in growth of the c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer in step (b), applying a SrTiO.sub.3 layer in a thickness of at least 20 nm and at most about 30 nm to said surface until said steps between a-axis and c-axis regions are covered; and

(d) thereafter continuing the growth of said c-axis crystal-oriented YBa.sub.2 Cu.sub.3 O.sub.7 layer upon the SrTiO.sub.3 layer, thereby forming a stack of layers.

2. The method defined in claim 1 wherein steps (b) through (d) are repeated until said stack has a multiplicity of said layers formed therein.

3. The method defined in claim 1 wherein said stack is formed with a multiplicity of alternating SrTiO.sub.3 layers and YBa.sub.2 Cu.sub.3 O.sub.7 layers and each SrTiO.sub.3 layer lying between two YBa.sub.2 Cu.sub.3 O.sub.7 layers has a thickness of 20 to 30 nm.

4. A multilayer high temperature superconductor made according to claim 1, comprising a multiplicity of alternating SrTiO.sub.3 layers and superconductive YBa.sub.2 Cu.sub.3 O.sub.7 layers, each SrTiO.sub.3 layer lying between two YBa.sub.2 Cu.sub.3 O.sub.7 layers having a thickness of 20 to 30 nm, wherein said YBa.sub.2 Cu.sub.3 O.sub.7 layers having thicknesses between 300 nm and 500 nm.