Substrate having a superconductor layer

Abstract: A substrate having a superconducting thin film of compound oxide thereon. An intermediate layer consists of at least one layer of copper-­ containing oxide is interposed between the substrate and the superconducting thin film.

Description: Field of the invention The present invention relates to a substrate having a superconductor layer thereon. More particularly, the present invention relates to a substrate having an intermediate layer on which a superconductor layer is deposited. Description of the related art The superconductive phenomenon which is explained to be a kind of phase change of electrons was thought to he a phenomenon which can be observed under extremely low temperature. However, in 1986, Bednorz and Müller discovered a new oxide type superconductor [La, Sr] 2 CuO 4 which shows superconductivity at 30 K (Z. Phys. B64, 1988 p 189). C. W. Chin et al. discovered another superconducting material of YBa 2 Cu 3 O 7-x which shows the critical temperature of about 90 K in 1987 (Physical Review letters, Vol. 58, No. 9, p 908). Maeda et al discovered Bi-containing superconductor in 1988 (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). In these newly discovered oxide superconductors, the superconductive phenomenon can be realized with relatively cheaper liquid nitrogen and hence the possibility of an actual utilization of the high Tc superconductors have burst onto the scene. The above-mentioned oxide superconductors were prepared in a bulk form of sintered block by powder sintering technique. The sintered articles, however, shows very poor critical current density. In order to overcome this problem, a variety of deposition techniques have been studied to prepare thin films of these superconductors. The superconductors of compound oxides can be deposited on a substrate such as SrTiO 3 , MgO etc. by physical vapour deposition (PVD) including vacuum-deposition, sputtering, ion-plating, molecular beam epitaxy (MBE) and by chemical vapor deposition (CVD) technique such as MO-CVD. Although remarkable improvement in the critical current density of these compound oxides have been achieved in their thin films, the conventional thin films of superconducting compound oxides are still insufficient to be used for fabricating actual electronics devices because the thin films are not homogeneous microscopically in addition to relatively lower critical current density. Therefore, there is a demand to develop a thin film, preferably a thin film of a single crystal of superconducting compound oxide which is deposited uniform in quality on a large area of a substrate and has an improved critical current density and which can be utilized in fabrication of a variety of electronics devices. It is believed that the thin films of superconducting compound oxide deposited on an oxide substrate of single crystal such as MgO and SrTiO 3 have improved superconducting properties because of the following reasons: Firstly, when a thin film of superconducting compound oxide is deposited directly on a substrate, elements of which the substrate is composed are apt to migrate or diffuse into the thin film of compound oxide, resulting in deterioration of superconducting properties. In the case of single crystal substrates of MgO and SrTiO 3 or the like, such disadvantage can be reduced. Secondly, the thin films deposited on a predetermined plane of single crystal substrate of MgO and SrTiO 3 is easy to become oriented polycrystal or single crystal of superconducting compound oxide. In other words, thin films deposited on the single crystals substrate of MgO and SrTiO 3 can be grown epitaxially, so that the resulting thin film is composed of a single crystal or polycrystals whose crystalline directions are well-ordered. Still more, it is easy to control anisotropy which the superconductors of compound oxide exhibit. However, as stated above, known thin films of compound oxides prepared by the conventional methods were not enough uniform in quality to be utilized in electronics devices, because of bad influence of uneven surface condition of the substrate and mismatch in lattice constant between crystals of superconducting compound oxide and crystals of substrate. The surface uniformity of the superconducting thin film deposited on these substrate may be spoiled because of the following reasons: (1) The surfaces of single crystals of oxides are not smooth in atomic level even their surfaces are polished. In fact, their electron diffraction patters by RHEED are not streak patterns but are rather spotty patters. (2) Unevenness of the thin film can be absorbed by a stress in the thin film at an early stage of film growth. This stress, however, is released when the film becomes thicker, resulting in unevenness of the thin film. Usually, the thin films of superconducting compound oxides prepared by vapour deposition techniques are subjected to after-treatment or post-annealing because their superconducting properties "as deposited" are poor. It is also known that the quality of the thin film is sharply deteriorated in the neighborhood of the substrate owing to diffusion of elements of which the substrate are composed during the post-annealing. U. S. patent No. 4,837,609 proposes to insert a layer of W, Mo or Ta between a superconducting compound oxide material and a silicon single crystal. Japanese patent laid-open No. 63-239,840 proposes to oxidize a copper substrate to form a CuO layer thereon and then a superconducting compound oxide material is deposited on the CuO layer. The present invention solves the problems of the known techniques and to provide a substrate having a superconductor layer which has a smooth smooth surface and which has improved superconducting properties and a process for preparing the same. Summary of the Invention The present invention provides an substrate having a superconducting thin film of compound oxide thereon, having an intermediate layer which is interposed between said substrate and said superconducting thin film, as defined in claim 1 The substrate is preferably a single crystal substrate, particularly an oxide of MgO, SrTiO 3 or yttrium stabilized zirconia (YSZ). Particularly, the present invention is advantageous when the Bi-type superconductor of (1) is used. An essence of the present invention resides in that the intermediate layer comprises at least one copper-containing oxide layer. This copper-containing oxide layer can be layers made of the compound oxides of formula (I) and (II) given hereinabove, having preferably a crystal structure of perovskite type or layered perovskite type. Both of the intermediate layer and the superconducting compound oxide layer can be prepared by any known conventional thin film forming techniques including physical vapour deposition such as vacuum deposition, sputtering, ion-plating, molecular beam epitaxial growth (MBE) and chemical vapour deposition such as MO-CVD. In these techniques, vaporised atom(s) and/or molecular(s) from a vapour source or sources are deposited on a single crystal substrate such as MgO, SrTiO 3 or YSZ. In the present invention, the intermediate layer is made of compound oxide of copper having a crystal structure of perovskite type or layered perovskite type. The present invention is most effectively applicable to bismuth type superconducting compound oxide having a composition represented by the formula: Bi 4 (Sr 1-x ,Ca x ) m Cu n O p+y in which "x", "m", "n", "p" and "y" are numbers each satisfying ranges of 6 ≦ m ≦ 10, 4 ≦ n ≦ 8 0 < x < 1 and -2 ≦ y ≦ + 2, respectively and p = (6 + m + n) . Particularly, following compositions are preferable: (i) 7 ≦ m ≦ 9, 5 ≦ n ≦ 7 0.4 < x < 0.6 (ii) 6 ≦ m ≦ 7, 4 ≦ n ≦ 5 0.2 < x < 0.4 (iii) 9 ≦ m ≦ 10, 7 ≦ n ≦ 8 0.5 < x < 0.7 In this embodiment, the intermediate layer is made of one of following compound oxides (I) and (II): (La 1-x ,α x ) 2 CuO 4-y in which "α" stands for Sr or Ba, and "x" and "y" are numbers each satisfying respective range of 0 ≦ x < 1 and 0 ≦ y < 1, Ln 1 Ba 2 Cu 3 O 7-z in which "Ln" stands for at least one element selected from a group comprising La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb and Lu, and "z" is a number satisfying a range of 0 ≦ z < 1. The substrate is preferably a single crystal substrate of MgO, SrTiO 3 or YSZ. Both of the intermediate layer and the superconducting compound oxide layer can be prepared by any known conventional thin film forming techniques including physical vapour deposition such as vacuum deposition, sputtering or ion-plating and chemical vapour deposition such as MO-CVD. It is also possible to use molecular beam epitaxial growth (MBE) or VPE in which monoatom layers are stratified successively on a substrate so that an interface between the intermediate layer and the superconducting thin film of compound oxide is composed of copper atom and oxygen atom. Following advantages are expected by this embodiment. Namely, in the prior arts, it was very difficult to deposit epitaxially thin films of the above-mentioned superconducting Bi-Sr-Ca-Cu system compound oxides on a {100} plane of MgO single crystal by sputtering, vacuum deposition or MO-CVD, because mismatch in lattice constant between MgO and Bi-Sr-Ca-Cu system compound oxide which have a very high anisotropy. Still more, it was impossible to prevent formation of so-called "lower phase". According to this embodiment of the present invention, a lattice plane of Cu-O composed of copper atoms and oxygen atoms is formed on a surface of the substrate on which the superconducting thin film of compound oxide is to be deposited by depositing an intermediate layer of compound oxide of copper having a crystal structure of perovskite type or layered perovskite type. Since the lattice plane of Cu-O has high crystalline matchability with the bismuth type compound oxide, the thin film of bismuth type compound oxide which is deposited on the intermediate layer can be grown epitaxially, resulting in improvement in the critical current density. The compound oxides (I) and (II) used as the intermediate layer are themselves superconductors. In this embodiment, the compound oxides (I) or (II) grows easily and epitaxially on the substrate of MgO, SrTiO 3 or YSZ to produce a perovskite type or layered perovskite type crystal having Cu-O planes which contribute to superconduction and which are matchable with Cu-O plane of the bismuth type compound oxide. This epitaxial growth may be a reason of improvement in the critical current density. In another embodiment, a typical superconducting thin film of YBa 2 Cu 3 O y (hereinafter, YBCO) is formed on an intermediate layer composed of a compound oxide of La and Cu. It is thought that crystal growth of YBCO thin film is governed inside thereof or depend on its own crystal structure. However, when the YBCO thin film is deposited directly on the substrate, the crystal of YBCO thin film which is effective to superconduction can be maintained after the post-anneal or after-treatment because mismatch in lattice constant between this YBCO thin film and a MgO single crystal which is a typical substrate material is 9.2 % and elements of the substrate migrate or diffuse into the YBCO thin film during the post-anneal. In fact, the film forming-condition which is required to obtain a crystalline YBCO thin film is limited to a very narrow range. According to the example of the present invention, a compound oxide layer La-Cu-O of La and Cu is formed firstly on the substrate, and then a superconducting thin film of compound oxide is formed on this La-Cu-O layer. This compound oxide layer La-Cu-O is a well-crystalline film and mismatch in lattice constant with YBCO thin film is about 2.4 % which is lower than the mismatch with MgO, resulting in that a crystalline film of YBCO is formed easily. Still more, the superconducting property is less influenced by mutual diffusion of La from La-Cu-O and/or Y from YBCO because La and Y belong to same lanthanide elements. Therefore, according to this embodiment, a well-crystalline YBCO thin film can be formed easily and a thickness of a superconducting layer is substantially increased, so that available or effective current density increase. The thickness of the La-Cu-O layer is preferably higher than 100 Å in order to make it possible to produce a desired crystal structure and to prevent diffusion or migration of substrate material. Although special two embodiments according to the present invention are described, it is apparent that the present invention is easily applicable to the other type superconducting thin films of compound oxides. In a summary, the substrate having a thin film of compound oxide superconductor according to the present invention improves in critical current density because the superconducting thin film well-grows epitaxially owing to an intermediate layer interposed between the superconducting thin film and the substrate and which can absorb mismatch in lattice constants. Still more, a well-crystallized superconducting thin film of compound oxide having a uniform wall thickness can be deposited stably and in a reproducible manner owing to reduced influence of migration or diffusion of elements of the substrate. The superconducting thin film according to the present invention can be utilized advantageously in applications of Matisoo switching elements, Annaker memories, Josephson device, a variety of sensors or Superconducting Quantum Interference Device (SQUID) or the like. Brief Description of the drawings FIG. 1 illustrates an apparatus for depositing a thin film on a substrate which is used in an example of the present invention. Fig. 2 is a RHEED pattern of a thin film of Bi 2 Sr 2 Ca n-1 Cu n O x which was grown to a thickness of 500 Å in the example according to the present invention. Now, the present invention will be described with reference to Examples, but the scope of the invention should not be limited to the Examples. Example 1 At first, an intermediate layer of a compound oxide of La-Sr-Cu system was deposited on a {100} plane of a SrTiO 3 single crystal substrate by RF (radio-frequency) sputtering under a film-forming condition shown in Table 1 (1). Then, a thin film of a superconducting compound oxide of Bi-Sr-Ca-Cu system was deposited by RF sputtering on the resulting intermediate layer under a film-forming condition shown in Table 1 (2). Film-forming condition for the intermediate layer Composition of a target used La:Sr:Cu = 2.0 : 0.2 : 1.6 Sputtering gas Ar + O 2 O 2 / Ar + O 2 0.2 (volume ratio) Sputtering gas pressure (Torr) 2 x 10 -2 Substrate temperature (°C) 800 RF-frequency power 50 W 0.64 W/cm 2 Thickness of film (Å) 2,000 Film-forming condition for the superconductor layer Composition of a target used Bi:Sr:Ca:Cu = 2.6 : 2.0 : 2.0 : 3.0 Sputtering gas Ar + O 2 O 2 / Ar + O 2 0.2 (ratio in volume) Sputtering gas pressure (Torr) 2 x 10 -2 Substrate temperature (°C) 750 RF-frequency power 50 W 0.64 W/cm 2 Thickness of film (Å) 4,000 As a comparative example, another superconductor layer was deposited directly on a SrTiO 3 single crystal substrate under the same film-forming condition as is shown in Table 1 (2). The resulting samples of the present invention and of the comparative example were subjected to postannealing which was effected by the steps of heating the samples up to 890 °C at a rate of 3 °C/min in an oxygen stream, maintaining the samples at 890 °C for one hour, and then cooling down to room temperature at a rate of 3 °C/min. The samples had following compositions (atomic ratio): Intermediate layer : La:Sr:Cu = 1.85 : 0.15 : 1 Thin film layer of superconductor : Bi:Sr:Ca:Cu = 2 : 2 : 2 : 3 The samples were evaluated by measuring the critical temperature "Tc" and the critical current density "Jc". The results are shown in Table 2. Critical temperature (K) Invention comparative Tco 115 115 Tci 96 98 Critical current density (A/cm 2 ) 7.5 x 10 5 1.2 x 10 4 Table 2 reveals that higher critical current density is obtained in the sample according to the present invention without spoiling the other properties. The crystal structure of the samples were evaluated by an electron diffraction analyzer to find that the superconducting thin film in the sample of the present invention grow epitaxially. Example 2 In this example, a vacuum deposition technique was used. A plurality of vapour sources were energized simultaneously. At first, an intermediate layer of compound oxide of Y-Ba-Cu system was deposited on a {100} plane of MgO single crystal substrate and then a superconducting compound oxide of Bi-Sr-Ca-Cu system was deposited on the resulting intermediate layer. As the vapour sources, commercially available metallic Y, Bi, Ca, Cu and fluorides of BaF 2 and SrF 2 were used. Metal elements were evaporated by means of K-cells while BaF 2 and SrF 2 were evaporated by electron guns. Table 3 (1) and (2) show film forming conditions. Film-forming condition for the intermediate layer Partial pressure of oxygen (Torr) 2 x 10 -5 Substrate temperature (°C) 700 Film-forming speed (Å/sec) 5 Thickness of film (Å) 2,000 Film-forming condition for the superconductor layer Partial pressure of oxygen (Torr) 2 x 10 -5 Substrate temperature (°C) 750 Film-forming speed (Å/sec) 5 Thickness of film (Å) 4,000 As a comparative example, another superconductor layer was deposited directly on a MgO single crystal substrate under the same film-forming condition as is shown in Table 3 (2). The resulting samples of the present invention and of the comparative example were subjected to post-annealing which was effected by the steps of heating the samples up to 890 °C at a rate of 3 °C/min in an oxygen stream, maintaining the samples at 890 °C for one hour, and then cooling down to room temperature at a rate of 3 °C/min. The samples had following compositions (atomic ratio): Intermediate layer: Y:Ba:Cu = 1 : 2 : 3 Thin film layer of superconductor: Bi:Sr:Ca:Cu = 2 : 2 : 2 : 3 The samples were evaluated by measuring the critical temperature "Tc" and the critical current density "Jc". The results are shown in Table 4. Critical temperature (K) Invention comparative Tco 112 113 Tci 90 95 Critical current density (A/cm 2 ) 5.4 x 10 5 7.7 x 10 3 Table 4 reveals that very higher critical current density is obtained in the sample according to the present invention. The crystal structure of the samples were evaluated by an electron diffraction analyzer to find that the superconducting thin film in the sample of the present invention grow epitaxially. Example 3 At first, a thin film of an oxide of La-Cu was deposited on a {100} plane of a MgO single crystal substrate having a diameter of 15 mm by RF (radio-frequency) sputtering under a film-forming condition shown in Table 4 (1). Then, a thin film of an oxide of Y-Ba-Cu system was deposited by RF sputtering on the La-Cu oxide layer under a film-forming condition shown in Table 4 (2) to prepare a sample 1. A comparative sample 2 was prepared by depositing the thin film of an oxide of Y-Ba-Cu directly on a MgO single crystal substrate under the same film-forming condition as is shown in Table 4 (2). Film-forming condition for a La-Cu oxide layer Composition of a target used La : Cu = 2.0 : 1.6 Sputtering gas Ar + O 2 O 2 / Ar + O 2 0.2 (volume ratio) Sputtering gas pressure (Torr) : 4 x 10 -2 Substrate temperature (°C) 800 RF-frequency power 50 W 0.64 W/cm 2 Thickness of film (Å) 2,000 Film-forming condition for a Y-Ba-Cu oxide layer Composition of a target used Y:Ba:Cu = 1.0 : 2.0 : 4.0 Sputtering gas Ar + O 2 O 2 / Ar + O 2 0.2 (ratio in volume) Sputtering gas pressure (Torr) 2 x 10 -2 Substrate temperature (°C) 650 RF-frequency power 50 W 0.64 W/cm 2 Thickness of film (Å) 4,000 The resulting samples of the present invention and of the comparative example were subjected to post-annealing which was effected by the steps of heating the samples up to 900 °C at a rate of 3 °C/min in an oxygen stream, maintaining the samples at 900 °C for one hour, and then cooling down to room temperature at a rate of 3 °C/min. The samples were evaluated by measuring the critical temperature "Tco and Tci" and the critical current density "Jc". The results are shown in Table 5. Critical temperature (K) Invention (Sample 1) comparative (Sample 2) Tco 90 88 Tci 87 88 Critical current density (A/cm 2 ) 1.4 x 10 6 2.8 x 10 4 Table 5 reveals that higher critical current density is realized in the sample 1 according to the present invention without spoiling the other properties. The crystal structure of the samples were evaluated by an electron diffraction analysis to find that the superconducting thin film in the sample 1 according to the present invention grow epitaxially while the superconducting thin film of the comparative sample 2 did not grow epitaxially. From the examples, it was confirmed that superconducting thin films of compound oxides having high critical current density can be produced in a reproducible manner.

Claims: A substrate having a superconducting thin film of compound oxide theron, an intermediate layer being interposed between said substrate and said superconducting thin film, characterised in that said intermediate layer consists of at least one layer of copper-containing compound oxide represented by the formula (I) or (II) : (La 1-x α x ) 2 CuO 4-y in which 〈〈 α 〉〉 stands for Sr or Ba, and 〈〈 x 〉〉 and 〈〈 y 〉〉 are numbers each satisfying respective range of 0 ≤ x <1 and 0 ≤ y < 1, Ln 1 Ba 2 Cu 3 0 7-z in which 〈〈 Ln 〉〉 stands for at least one element selected from a group comprising La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, and Lu, and 〈〈 z 〉〉 is a number satisfying a range of 0 ≤ z < 1, and said superconducting thin film is formed of a compound oxide of formula : Bi 4 (Sr 1-x Ca x ) m Cu n O p+y in which 〈〈 x 〉〉 〈〈 m 〉〉, 〈〈 n 〉〉, 〈〈 p 〉〉 and 〈〈 y 〉〉 are numbers each satisfying ranges of 6 ≤ m ≤ 10, 4 ≤ n ≤ 8, 0 < x <1, and -2 ≤ y ≤ +2 respectively and p = (6 + m +n) , or of formula : Ln' 1 Ba 2 Cu 3 O 7-z in which 〈〈 Ln' 〉〉 stands for at least one element selected from a group comprising La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb and Lu, and 〈〈 z 〉〉 is a number satisfying a range of 0 ≤ z < 1, with the proviso that the copper-containing compound oxide of the intermediate layer is represented by formula (I) when the compound oxide of the superconducting thin film is represented by formula (IV). The substrate set forth in claim 1 wherein said substrate is a single crystal of an oxide. The substrate set forth in claim 2 wherein said substrate is a single crystal of an oxide selected from a group comprising MgO, SrTiO 3 and YSZ. The substrate set forth in claim 1 wherein said intermediate layer has a crystal structure of perovskite or layered preovskite.

Inventors: HARADA KEIZO, HIGAKI KENJIRO, ITOZAKI HIDEO, MATSUURA TAKASHI, OYAMA HITOSHI, YAZU SHUJI, HARADA, KEIZO, HIGAKI, KENJIRO, ITOZAKI, HIDEO, MATSUURA, TAKASHI, OYAMA, HITOSHI, YAZU, SHUJI, HARADA, KEIZO, C/O ITAMI WORKS OF SUMITOMO, HIGAKI, KENJIRO, C/O ITAMI WORKS OF SUMITOMO, ITOZAKI, HIDEO, C/O ITAMI WORKS OF SUMITOMO, MATSUURA, TAKASHI, C/O ITAMI WORKS OF SUMITOMO, OYAMA, HITOSHI, C/O ITAMI WORKS OF SUMITOMO, YAZU, SHUJI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C04B41
IPC codes: C30B23, C04B41, H01L39