DE19634463A1 - Layer sequence of superconductive and non-superconductive materials - Google Patents

Abstract

In a layer sequence including a layer which is based on a high temperature superconducting material with CuO2 plane-containing unit cells and which is joined to a non-superconducting layer, the latter comprises a material selected from: (i) BaTbO3; (ii) Ba1-xSrxTbO3 (x = 0 to 1); (iii) LaCu1-xTbxO3 (x = 0 to 1); (iv) RCu1-xTbxO3 (R = Nd, Eu or Sm and x = 0 to 1); (v) R1-yNyCu1-xTbxO3 (R = La, Nd, Eu or Sm, N = Ba or Sr, x = 0-1 and y = 0-1); (vi) R2-yNyCu1-xTbxO4 (R = La, Nd, Eu or Sm, N = Ba or Sr, x = 0-1 and y = 0-2); and (vii) A'1-xA"xB'1-yB"yO3 (A' = Ba or Sr, A" = La, Nd, Eu, Sm or Sr, B' = Tb or Cu, B" = Y, Yb, Tm, Lu, In, Sc, Sn or Cu, x = 0-1 and y = 0-1). Also claimed is a layer sequence as above in which the non-superconducting layer consists of the above material (vi) or (vii). Further claimed are (a) a multilayer system with several of the above layer sequences; (b) a cryogenic component with one or more of the above layer sequences or multilayer systems; and (c) a Josephson contact as the above cryogenic component.

Description

The invention relates to a layer sequence according to the Preamble of claim 1 or 2. Furthermore, be the invention meets a cryogenic device according to the Preamble of claim 7.

Basis for components of superconducting electronics is an epitaxial multilayer system with at least a layer sequence in which the superconducting Ma material interfaces to non-superconducting materials forms. If the component is to a desired If necessary, a substrate can be grown Buffer layer may be required.

The following is known as prior art:

1. Components of superconducting electronics

Epitaxial layer sequences or multilayer systems for such components consist of one or more  epitaxial thin films of a high temperature superpri tendency material and one or more epitaxial Thin films made of non-superconducting material. This Non-superconducting materials can help play the function of an insulating layer, one Barrier material in Josephson contacts, a passi exercise or a diffusion barrier. Because of of the properties of high temperature superconductors the following to these non-superconducting materials demands made:

The high-temperature superconductor and the non-super-conductive material must be chemically compatible. This means that there must be no chemical reaction of the materials. The non-superconducting material should grow epitaxially on both the high-temperature superconductor and the high-temperature superconductor on the non-superconducting material, specifically in the desired crystallographic orientation. The resulting interface should be atomically sharp, and no misaligned areas and foreign phases should form in its environment. Since an interdiffusion of ions cannot be ruled out due to a relatively high production temperature of the layers, it must be ensured that the foreign ions in the materials impair the properties of these as little as possible. It is known, for example, that especially small ions such as Al, Ga, Ti, W, Fe, Zn, or also Ce, Pr, reduce the superconductivity of the high-temperature superconductor REBa₂Cu₃O _7-z .

Among other things, this also affects the oxygen content and the order of the oxygen atoms in the high temperature superconductors in the case of loss of oxygen or Oxygen disorder weakened the superconductivity becomes. A high level of chemical compatibility is required be required if the non-superconducting material for example as a thin barrier in Josephson contact ten should be used. Due to the low cohesion limit length of high-temperature superconductors - more typical as in the range 1 nm to 2 nm - it is with respect to the Barrier material required on a length range of coherence length of the order parameters of the su electrodes in the vicinity of the interface, for example by ion diffusion or lattice distortion tion, is not reduced. So far there is no material Knows which these requirements are sufficient fills.

The material investigations focus on two material classes. One class is based on the structure of REBa₂Cu₃O _7-z . One tries to reduce or even to suppress the superconducting properties of this compound by targeted replacement and doping of one or more types of ions. The second class includes perovskite or perovskite-like compounds. A representative material from each class will be discussed.

The non-superconducting material PrBa₂Cu₃O _7-z differs chemically from the YBa₂Cu₃O _7-z only by replacing the Y with Pr which causes the loss of superconductivity. The lattice mismatch with YBa₂Cu₃O _7-z is only 1%. Compared to most other non-superconducting materials previously investigated, PrBa₂Cu₃O _{7-z shows} the highest degree of chemical and structural compatibility with YBa₂Cu₃O _7-z . For example, a monolayer of YBa₂Cu₃O _7-z , which is located as an intermediate layer in a PrBa₂Cu₃O _7-z matrix, has a transition temperature of 30K (T. Terashima et al., Phys. Rev. Lett. 67, 1362 (1991)) .

Similar experiments with other non-superconducting materials show that no other material has ever achieved this value. Significant disadvantages of PrBa₂Cu₃O _7-z are, on the one hand, its relatively low ge specific resistance, which makes it suitable for isolation purposes only to a limited extent, and, on the other hand, the reduction in the order parameter caused by a diffusion of the Pr ion into the YBa₂Cu₃O _-z the interface. If, for example, only 5% of the Y atoms in YBa₂Cu₃O _{7-z are} replaced by Pr atoms, the transition temperature is already reduced from 92K to 85K (MS Hedge et al., Phys. Rev. B 48, 6465 (1993)).

A typical representative of the second class of perovskite-like compounds is SrTiO₃. This material has a cubic crystal structure, the lattice mismatch to YBa₂Cu₃O _{7-z is} 1.2%. The speci cal resistance is significantly larger with 200MΩcm than for PrBa₂Cu₃O _7-z . It was shown with this material that a heteroepitaxy with YBa₂Cu₃O _{7-z is} possible. However, the chemical compatibility of the material is limited. The diffusion of Ti ions, as well as their installation on the Cu sites of YBa₂Cu₃O _7-z leads to a reduction of the order parameter in the vicinity of the interface. Furthermore, the lattice strain in YBa₂Cu₃O _7-z , created by the interface with SrTiO₃, significantly reduces the ordering parameter.

2. Buffer layers

Application-oriented requirements may make it necessary to grow the high-temperature superconductor thin layer or the component containing such a layer on a substrate which, for. B. is not suitable from the aspect of chemical compatibility. Examples of this are the materials silicon or sapphire. Both react undesirably chemically with the high-temperature superconductor YBa₂Cu₃O _7-z .

To form an epitaxy on these substrates one or more so-called buffer layers inserted sets that between substrate and thin film / thin  Layer system are arranged. Will be buffer layers also used to achieve smoother surfaces the high-temperature superconductor thin film on be agreed substrates.

A so-called compensating effect has been observed for SrTiO₃ buffer layers. That is, SrTiO₃ is grown on an atomic step containing YBa₂Cu₃O _7-z surface, it covers this surface and forms a smooth [100] surface after a few nm thickness. This surface is then suitable for the c-axis-oriented growth of the YBa₂Cu₃O _7-z . A buffer layer thus generally serves the purpose of improving certain properties of the thin layer on a given substrate.

The requirements for the quality of buffer layers are those of the non-superconducting layers for construction comparable elements. It is conceivable certain To accept the disadvantages of a buffer layer. At for example, a local lowering of the order para meters at the interface between substrate and High temperature superconductors to a certain extent acceptable if the layer thickness of the superconductor is greater than, for example, 30nm.

For silicon, for example, yttrium-stabilized zircon (YSZ) is used as the buffer layer. YSZ's lattice mismatch is relatively large at up to 6%. There is only limited chemical compatibility. It has been observed that BaZrO₃ is formed at the interface in this way, which reduces the order parameter as a foreign phase, the YBa₂Cu₃O _7-z . Diffusion of Zr on Cu sites has also been demonstrated, resulting in a reduced order parameter in the superconductor.

If YBa₂Cu₃O _{7-z is} sputtered directly onto a sapphire substrate, BaAl₂O₄ can form at the interface, which greatly affects the epitaxy of the YBa₂Cu₃O _7-z growing on it (K. Dovidenko, S. Oktyabrsky and A. Ivanov, Mater. Sci. Eng. B 15, 25 (1992)). For Sa phir substrates, CeO₂ is used as a buffer, among others. The case may occur that CeO₂ grows in two different orientations (AG Zaitsev, R. Kutzner, R. Wördenweber, Appl. Phys. Lett. 67, 1 (1995)), so that the epitaxy of the YBa₂Cu₃O deposited on CeO₂ increases _7-z layers deteriorated significantly.

3. Substrate materials

The use of a material as a substrate for epitakti high-temperature superconductor thin films adjustability as a macroscopic single crystal ahead. There is also a requirement for the following sufficient quality to that in question Substrate material required:

• - chemical compatibility with the growing Thin film material
• - surface quality
• - purity of the material
• - Homogeneity of the substrate
• - thermodynamic stability
• - slight lattice mismatch to the thin-film material
• - small difference in thermal expansion coefficients.

In this regard, the requirement of chemical compatibility has an increased priority, because most substrate materials contain ions that damage the high-temperature superconductor if they are interdiffused. However, interdiffusion cannot be ruled out due to the relatively high manufacturing temperature of the high-temperature superconductor. So contain known substrate materials such as SrTiO₃, LaAlO₃ and MgO with Ti ⁺⁴ , Al ⁺³ and Mg ⁺² ions, which significantly reduce the crack temperature in the superconductor, especially in the compound YBa₂Cu₃O _-z .

The following problem areas can be summarized know which more or less in each of the three dis cut application areas - superconducting components te, buffer layers, substrates - occur:  

All previously used as a barrier in Josephson contacts set materials, show only a limited extent of chemical compatibility with the high temperature supra conductors, whereby the achieved properties of the Jo sephson contacts behind the theoretically possible Er maintenance is left behind.

Furthermore, no buffer materials are known on which an ultra-thin YBa₂Cu₃O _7-z layer can be formed from superconductors, and the buffer layer also has an atomic step-balancing effect.

For the three areas of application mentioned are common Materials used that have only limited chemical com are compatible with the high temperature superconductors. This adversely affects the growing supra conductive layers, their superconducting properties shafts, especially with thin layers of superconducting layer, degrade.

It is therefore an object of the invention to have a layered film ge or a component with such a sequence of layers to create an improved chemical compa tibility at the interface between the supra conductive and non-superconducting layer achieved becomes. In particular, the order parameter of the su layer by connecting both layers not be adversely affected.  

The task is solved by a layer sequence according to the entirety of the features of claim 1 or 2. Die The object is further achieved by a component according to the entirety of the features according to claim 7. Further expedient or advantageous embodiments or Variants can be found on each of these Claims related subclaims.

The layer sequence according to the invention is to form egg Barrier can be used in Josephson contacts without that the order parameter of the superconducting Electrodes at the interfaces of the Josephson contact is reduced.

The layer sequence according to the invention is also suitable for forming an ultra-thin high-temperature superconductor or REBa₂Cu₃O _7-z layer on a non-superconducting layer without reducing the order parameter.

In addition, the layer sequence according to the invention is bil an epitaxial, superconducting component applicable, advantageous while avoiding the reduction the order parameter of the superconductor near the Boundary layer between this non-superconducting mate rial and the superconducting layer.

Finally, the layer sequence according to the invention contains a non-superconducting layer which exerts an improved te planarizing effect on a high-temperature superconductor or REBa₂Cu₃O _7-z layer growing thereon without damaging the ordering parameter.

For the purposes of the invention, an ion or component then be chemically compatible if it has the following conditions fulfilled: 10% of an element is exposed to high temperatures superconductor in the layer sequence through these ions variety / component replaced, a reduction in Jump temperature of the superconducting material Do not exceed 5K.

The patent proposal extends to the component in the sense of an epitaxial multilayer system, to the buffer layer and to the substrate if epitaxial thin films made of high-temperature superconductor or materials of similar crystallographic structure are used. Representing the class of high-temperature superconductors, the material YBa₂Cu₃O _7-z or REBa₂Cu₃O _7-z with RE = Y, Ca, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, called Lu.

It was recognized that the chemical compatibility of the material is a main criterion for solving the task. Oxidic non-superconducting materials should be used whose atomic types are super-conductor, especially REBa₂Cu₃O _7-z , chemically compatible. In this regard, as chemically compatible elements, Cu, Ba, Sr, Ca, Cu, O, as well as Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu can be considered.

The invention is further based on figures and Embodiment explained in more detail. It shows:

Fig. 1 Röntgendiffraktogram a layer sequence according to the invention;

Fig. 2 rocking curve in the region of the non-supralei tend layer of a layer sequence according to the invention;

Fig. 3 Rutherford Backscattering-spectrum (RBS) in the area of the non-superconducting layer of a multilayer of the invention;

Fig. 4 Random (top) and channeling (bottom) measurement of the RBS analysis in the area of the non-supralei tend layer of a layer sequence according to the invention;

Fig. 5a TEM image of a layer sequence according to the invention;

Fig. 5b TEM image of a layer sequence according to the invention.

Embodiment

In Fig. 1 the X-ray diffractogram of a 120 nm thick BaTbO₃ layer is shown on a (100) -oriented SrTiO₃ substrate. The so-called Bragg-Bretano geometry was used for the measurement. In addition to the substrate reflections marked with an "S", the (100) and (200) reflections of BaTbO₃ can be clearly seen. In the measured angle range from 2Θ = 5 ° to 2Θ = 60 °, no further reflections, which could indicate foreign phases, were observed.

The half-width of the rocking curve in FIG. 2, a so-called ω-scan, is only 0.1 °, measured at the (200) reflex, which suggests a slight angular tilt of the network planes.

The result of the RBS examination is shown in Fig. 3 ge. Due to the overlapping of the measurement curve and the simulation (solid line), the correct cation ratio could be inferred. The channeling measurement, set out in FIG. 4, gives a favorable minimum yield value of only 7%.

In FIG. 5a is a TEM image of the heterostructure to Verdeutli chung shown. For examination by means of transmission electron microscopy (TEM), such a heterostructure thin-layer sequence according to the invention was produced on a SrTiO₃ substrate, consisting of the following layers:

• 1st layer 35nm YBa₂Cu₃O _7-z
• 2nd layer 12nm BaTbO₃
• 3rd layer 35nm YBa₂Cu₃O _7-z .

In the TEM studies on such a fiction according to the sequence of layers, the following were found lungs:

There was an epitaxial growth of all layers, the YBa₂Cu₃O _7-z advantageously only c-axis oriented and the BaTbO₃ only [100] oriented. A he teroepitaxial layer sequence of BaTbO₃ with YBa₂Cu₃O _{7-z was} shown experimentally. The interfaces were free from foreign phases or misaligned areas, in particular there was no a-axis growth.

Typical interfaces of such a structure are shown in Fig. 5b. Similar to SrTiO₃, BaTbO₃ also has the ability to compensate for irregularities in a YBa₂Cu₃O _7-z surface. Ie BaTbO₃ is grown on a YBa₂Cu₃O _7-z surface that contains steps, covers it and forms a smooth [100] surface after a few nm thickness on which the YBa₂Cu₃O _{7-z is} c-axis-oriented grows. In this regard, it is assumed that low-indexed areas of the BaTbO₃ lattice structure have a low surface energy and are therefore preferred interfaces.

Thus, BaTbO₃ in particular has a planarizing effect in relation to rough epitaxial documents and offers the YBa₂Cu₃O _7-x to be deposited on it, which has a smooth surface.

The material for the non-superconducting layer in the layer sequence is in particular the very suitable, perovskite compound Ba _1-x Sr _x TbO₃, where the value x can be chosen in the range from 0 to 1. The rhombohedral crystal structure of this compound can be understood as a pseudo-cubic perovskite structure due to a slight deviation from a cubic structure. This material is known (E. Paletta, R. Hoppe, Naturwissenschaften 53, 611 (1966) and AJ Jacobson, BC Tofield, BEF Fender, Acta, Cryst. B28, 956 (1972)). So far, however, only the production of powder samples has been reported in the literature. However, thin films and macro-scopic single crystals are not known. This connection advantageously has thermodynamically egg ne comparatively high stability.

The chemical compatibility with the high-temperature su praleiters is given from the point of view of the elements contained in the compound, since on the one hand Ba and O are contained in the REBa₂Cu₃O _7-z compounds, on the other hand Tb up to 40% to the REBa₂Cu₃O _7-z compound doping can be doped without the jump temperature dropping noticeably (less than 1K). Finally, Sr can be doped to the REBa₂Cu₃O _7-z without the jump temperature being noticeably reduced; even if 60% of the Ba atoms are replaced by Sr atoms, the T _{c is} only reduced by 8K.

Within the Rare Earth (RE) series, Tb an exceptional position because it is in octahedral sow material environment in the Perovskite also as a tetravalent, relatively small ion can be installed without as in The case of Pr or Ce also has pair-breaking properties or to have a doping effect which the Supra disadvantageously reduce the cable.

Epitaxial BaTbO₃ thin layers on SrTiO₃- and MgO substrates by RF sputtering in pure acid fabric (U. Poppe et al., Solid State Comm. 66, 661 (1988)) at a pressure in the range of 2-4 mbar provides, whose structure by means of X-ray diffractometry,  RBS / channeling with He ions and transmission electro were examined.

In addition, the following properties of the layer sequence according to the invention and of the component according to the invention which are advantageous for applications were observed: lattice stresses are locally reduced at the interface without introducing extensive misfit dislocations. In addition, foreign phases and misoriented areas at the interface are not observed. No Y-axis growth was observed for YBa₂Cu₃O _7-x .

It is known from the literature that polycrystalline mas siv samples from BaTbO₃ are electrically insulating. The Epitaxial BaTbO₃- produced according to the invention Layers also proved to be insulating.

Due to the variety of materials used with BaTbO₃ from the point of view of chemical compatibility are closely related to high temperature superconductors and the variety of materials of high temperature super ladder or materials with a similar crystallography structure, the invention also extends following materials:

• (i) BaTbO₃
• (ii) Ba _1-x Sr _x TbO₃ with 0x1
• (iii) LaCu _1-x Tb _x O₃ with 0x1;
• (iv) RCu _11-x Tb _x O₃
  with R = Nd, Eu, Sm and 01;
• (v) R _1-y N _y Cu _1-x Tb _x O₃
  with R = La, Nd, Eu, Sm;
  N = Ba, Sr and 0x1, 0y1;
• (vi) R _2-y N _y Cu _1-x Tb _x O₄
  with R = La, Nd, Eu, Sm;
  N = Ba, Sr and 0x↔v1, 0y2;
• (vii) A¹ _1-x A² _x B¹ _1-y B² _y O₃ with 0x1, 0y1
  with A¹ = Ba, Sr;
  A² = La, Nd, Eu, Sm, Sr
  B¹ = Tb, Cu; B² = Y, Yb, Tm, Lu, In, Sc, Sn, Cu.

As chemically compatible with the high-temperature superconductors tible materials should in particular meet the stated Ma materials and material classes with perovskite-like Structure to be understood.

With "high temperature superconductors and materials similar cher crystallographic structure "are meant, Sub punch in with at least three different elements  the unit cell, two of which are oxygen and copper are. Furthermore, at least one should be in the unit cell CuO₂ level - the characteristic of high temperature su praleiter- to be present.

The request _for protection should also extend to REBa₂Cu₃O _7-z connections, with RE = Y, Ca, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

Claims (9)

1. Layer sequence, comprising at least one layer based on a high-temperature superconducting material with at least one CuO₂ plane having a unit cell, this layer being connected to a non-superconducting layer, characterized by one of the materials from one of classes (i) to ( vii) with

• (i) BaTbO₃
• (ii) Ba _1-x Sr _x TbO₃ with 0x1
• (iii) LaCu _1-x Tb _x O₃ with 0x1;
• (iv) RCu _11-x Tb _x O₃
  with R = Nd, Eu, Sm and 01;
• (v) R _1-y N _y Cu _1-x Tb _x O₃
  with R = La, Nd, Eu, Sm;
  N = Ba, Sr and 0x1, 0y1;
• (vi) R _2-y N _y Cu _1-x Tb _x O₄
  with R = La, Nd, Eu, Sm;
  N = Ba, Sr and 0x1, 0y2;
• (vii) A¹ _1-x A² _x B¹ _1-y B² _y O₃ with 0x1, 0y1
  with A¹ = Ba, Sr;
  A² = La, Nd, Eu, Sm, Sr
  B¹ = Tb, Cu;
  B² = Y, Yb, Tm, Lu, In, Sc, Sn, Cu
  as material for the non-superconducting layer.

2. Layer sequence containing at least one layer based on a high-temperature superconducting material with at least one CuO₂ plane having a unit cell, this layer being connected to a non-superconducting layer, characterized by one of the materials from one of the classes
R _2-y N _y Cu _1-x Tb _x O₄
with R = La, Nd, Eu, Sm;
N Ba, Sr and 0x1, 0y2;
or
A¹ _1-x A² _x B¹ _1-y B² _y O₃ with 0x1, 0y1
with A¹ = Ba, Sr;
A² = La, Nd, Eu, Sm, Sr
B¹ = Tb, Cu;
B² = Y, Yb, Tm, Lu, In, Sc, Sn, Cu
as material for the non-superconducting layer. 3. Layer sequence according to claim 1 or 2, characterized characterized that the non-su layer a buffer layer between the high temperature superconducting layer and the Forms surface of a substrate. 4. Layer sequence according to claim 1 or 2, characterized characterized that the non-su is formed as a substrate. 5. Layer sequence according to one of the preceding claims, characterized by Ba _1-x Sr _x TbO₃ with values for x in the range 0x1 as material for the non-superconducting layer. 6. Multi-layer system with several layer sequences any of the preceding claims. 7. Cryogenic component with at least one layer sequence according to one of claims 1 to 5.   8. Cryogenic component with at least one Multila gene system according to claim 6. 9. Josephson contact as a cryogenic component according to An say 7 or 8.