Bi-Sr-Ca-Cu-O system superconducting thin film

Abstract: A Bi-Sr-Ca-Cu-O system superconducting thin film formed on a substrate comprising [110] single crystals of an ABO₃ type oxide having a perovskite structure, in which a (119) face is selectively grown relative to a substrate surface. The film is formed on the substrate by chemical vapor deposition process. A method of manufacturing a BiSrCaCuO system superconducting film in which an a-axis is oriented preferentially relative to the surface of a substrate comprising MgO (100) single crystals, wherein the chemical composition ratio (Sr+Ca+Cu)/Bi of the BiSrCaCuO system superconducting film is made not less than 3.5. A Bi-Sr-Ca-Cu-O system superconducting thin film formed on a substrate comprising MgO [110] single crystals, in which a (110) face is selectively grown to the substrate surface. The film is formed on the substrate by a chemical vapor deposition process.

Description: FIELD OF THE INVENTION The present invention concerns a Bi-Sr-Ca-Cu-O system superconducting film and, more specifically, it relates to a Bi-Sr-Ca-Cu-O system superconducting thin film which is extremely effective for a Josephson junction device, three terminal device or the like. DESCRIPTION OF THE RELATED ART Oxide superconducting thin films show high superconducting critical temperature (Tc) and application of them to electronic devices such as transistors and Josephson junction devices has been expected. In a case of utilizing the oxide high temperature superconductors to various kinds of such devices, it requires a structure in which superconductor/insulator/superconductor are successively laminated on a substrate. Generally, in a case of manufacturing a laminate structure device such as a Josephson junction device or a three terminal device, it has been known that the manufacture of the device is facilitated as the coherence length (ζ), which is one of superconducting physical properties, is greater. That is, it is advantageous in view of the characteristic of the resultant device to provide a device of a structure in which electric current can be caused to flow on a crystal face of long coherence length and, for this purpose, it is important that the film is oriented along the a-axis or the b-axis relative to the substrate surface. By the way, as a method of manufacturing oxide system superconducting thin films, there has been known a physical vapor deposition method such as sputtering (PVD) and a chemical vapor deposition process (CVD). Among them, in the PVD process, the lower limit for a deposition rate is comparatively greater than the lower limit in the CVD process and, if the deposition rate is lowered in the PVD process, the deposition rate and the composition of the formed film are not defined and it is difficult to obtain a thin film of high quality which can be considered to be manufactured at a low deposition rate (refer to Applied Physics Letters, 1988, vol. 53, No. 7, p 624). Generally, in the CVD process, gases of starting compounds can be maintained stably within a certain temperature range. Further, the gases and an oxidizing gas are separated from each other before they react on a substrate to form a film, required reaction can be taken place only on the substrate (Journal of Applied Physics, 1990, vol. 67, No. 3, p 1562). In this point, the CVD process is excellent over the PVD process in which control for the reaction between the starting gas and the oxidizing gas is difficult and the steam concentration of the starting material gas itself in the system is instable. Further, the CVD process, as compared with the PVD process, has various merits in view of the cost required for the manufacturing apparatus, easy for the scale up and high through put. In view of the above, it is industrially advantageous to manufacture the oxide system superconducting thin films by means of the CVD process. As has been described above, in a case of utilizing the oxide system superconducting thin films to various kinds of devices, orientation of the film relative to a substrate surface is desirably along the a- or b-axis or orientation similar thereof. However, since the oxide system superconducting thin film is liable to be oriented along the c-axis in view of its large anisotropy of the crystal structure and, in particular, the Bi-Sr-Ca-Cu-O system superconducting thin film tends to be oriented along the c-axis. That is, because of the large anisotropy of the high temperature oxide superconductor, the coherence length of the high temperature oxide superconductor is longer in the direction of a-, b-axis and shorter in the direction of c-axis. This has resulted in a significant problem for the improvement of the characteristics of a device made into a laminate structure by using the c-axis orientation film. Accordingly, it has been desired for the high temperature oxide superconducting thin film oriented such that the superconducting current caused to flow to a device of the laminate structure flows in the direction of a- or b-axis of a large coherence length. Then, in the prior art, there has been an example of forming a Bi-Sr-Ca-Cu-O system superconducting thin film preferentially oriented along the a- or b-axis by sputtering, or an example of forming a Bi-Sr-Ca-Cu-O system superconducting thin film in which a (119) face or a (117) face is preferentially oriented by means of sputtering or vapor deposition, but the c-axis orientation ingredients are incorporated together in both of them, which can not have a complete orientation film (cf. Japanese Journal of Applied Physics, vol. 30, no. 6A, June, 1991, pp. L 1006-1008). Further, also in view of Tc, crystallinity or the like, they can not have characteristics suitable to the device. Further, it has been reported an example of forming a YBaCuO system superconducting thin film which is preferentially grown along the a-axis by the CVD process, but a BiSrCaCuO system superconducting film oriented along the a-axis by the CVD process has not yet been provided. In view of the above, it has been demanded for a thin film of excellent characteristics such as Tc and crystallinity and selectively oriented along the a-axis or b-axis, or a thin film oriented selectively along a (119), (117) or (110) face or along the a-axis by the CVD process, as well as a manufacturing technique therefor. OBJECT AND SUMMARY OF THE INVENTION The present invention as claimed has been accomplished in view of the foregoing situations in the prior art and it is an object thereof to provide a Bi-Sr-Ca-Cu-O system superconducting thin film suitable to the manufacture of a Josephson junction device, 3-terminal device or the like, in which a (119) face is selectively oriented relative to a substrate surface and excellent in the characteristics such as Tc and crystallinity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a XRD spectral diagram for a Bi-Sr-Ca-Cu-O system superconducting thin film obtained in Example 1, not being an embodiment of the present invention ; Fig. 2 is a graph illustrating resistance-temperature characteristics obtained in Example 1; Fig. 3 is an electron microscopic photograph (30,000 x) illustrating the crystal structure of the Bi-Sr-Ca-Cu-O system superconducting thin film obtained in Example 1; Fig. 4 is a XRD spectral diagram for a Bi-Sr-Ca-Cu-O system superconducting thin film obtained in Example 2; Fig. 5 is a graph illustrating resistance-temperature characteristics obtained in Example 2; Fig. 6 is an electron microscopic photograph (30,000 x) illustrating the crystal structure of the Bi-Sr-Ca-Cu-O system superconducting thin film obtained in Example 2; Fig. 7 is a graph illustrating resistance-temperature characteristics obtained in Example 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention as claimed will now be described more in details. As the Bi-Sr-Ca-Cu-O system superconductor constituting the Bi-Sr-Ca-Cu-O system superconducting film according to the present invention, there can be mentioned, for example, those having a chemical composition: Bi 2 Sr 2 Ca 1 Cu 2 O x Bi 2 Sr 2 Ca 2 Cu 3 O y Further, Pb may be incorporated partially to the above-mentioned composition in order to improve Tc (critical temperature). Then, the Bi-Sr-Ca-Cu-O system superconducting film according to the present invention as claimed is formed on a single crystal substrate of a perovskite structure oxide (ABO 3 type) of face indices (110), by means of a chemical vapor phase deposition process (CVD process); the substrate of the perovskite structure used in the present invention as claimed, is selected from the group consisting of SrTiO 3 , NdGaO 3 , LaAlO 3 , YAlO 3 to the manufacture of a thin film at high quality; for obtaining a thin film of higher quality, the crystal face is inclined by from 2° to 10° from [110] to <1 1 0>. Description will now be made to a method of manufacturing the claimed Bi-Sr-Ca-Cu-O system superconducting thin film. The Bi-Sr-Ca-Cu-O system superconducting thin film according to the present invention as claimed is manufactured by using each of starting material gases, a carrier gas and an oxidizing gas in accordance with the chemical composition and by means of a CVD process of depositing a thin film on (110) single crystals of the ABO 3 type oxide having the perovskite structure as described above. As the starting material gas used in the present invention as claimed, there can be mentioned various metalorganic complex compounds containing, for example, Bi, Sr, Ca or Cu. As the organic portion of the metalorganic complex, that is, ligand of a complex, there can be mentioned acetylacetone (hereinafter simply referred to as "acac", dipivaloylmethane (hereinafter simply referred to as "DPM"), cyclopentadiene, as well as those compounds represented by the following structural formula: R-CO-CH 2 -CO-C(CH 3 ) 3 where R represents a fluorinated lower alkyl group with 1 to 4 carbon atoms. In the case of using the ligand described above, it is extremely advantageous for the preparation of a starting material gas since the synthesis and the isolation of the metal complex are easy. Further, since the vapor pressure of the metalorganic complex per se is or relatively high, it is most suitable to the starting material gas for CVD. In addition, as the ligand, there can be also used a phenyl group (hereinafter simply referred to as "ph"), alkyl group such as methyl group (hereinafter simply referred to as "Me"), ethyl group (hereinafter simply referred to as "Et") and an aryl group. Further, the fluorinated lower alkyl group represented by R in the structural formula described above is, specifically, trifluoromethyl group, pentafluoroethyl group (hereinafter simply referred to as "PPM") and heptafluoropropyl group. As specific metalorganic salts used as the starting material gas, there can be mentioned the followings. Bi(ph) 3 , Bi(DPM) 3 . BiMe 3 , BiEt 3 . Bi alcoholate, Sr(DPM) 2 , Sr(PPM) 2 , Ca(DPM) 2 , Ca(PPM) 2 , Cu(DPM) 2 , Cu(PPM) 2 , Cu(acac) 2 As the carrier gas used for supplying the starting material gas into a reactor in the present invention, there can be mentioned, for example, an inert gas such as Ar, He and Ne, as well as N 2 . Further, as the oxidizing gas used in the present invention, there can be mentioned, for example, O 2 , O 3 , Air, N 2 O, NO and NO 2 . Further, for improving the oxidizing power it is possible to use a laser beam, a visible ray or UV-ray from a lamp as a light source in combination or make the oxidizing gas into plasmas. The partial pressure of the oxidizing gas in the total gas is preferably from 0.01 to 760 torr, particularly, from 1 to 100 torr (1 Torr = 133,32 Pa). The film forming temperature is preferably from 300 to 900°C and, more particularly, from 600 to 820°C in view of the improvement for the Tc and the crystallinity. If the film forming temperature is lower than 300°C, decomposition of the starting material gas is difficult. On the other hand, if it exceeds 900°C, it is not preferred since melting of the thin film occurs to result in the deterioration of the quality of the thin film. There is no particular restriction on the deposition rate but it is preferably from 100 to 0.1 nm/min. The film thickness of the Bi-Sr-Ca-Cu-O system superconducting film according to the present invention as claimed thus obtained is preferably from about 10 to 1000 nm in a usual case. The Bi-Sr-Ca-Cu-O system superconducting film according to present invention as claimed is a high performance superconductor usually having a Tc of 50 to 100 K and it is extremely useful to various kinds of devices since the (119) face is selectively grown relative to the substrate surface. In the Bi-Sr-Ca-Cu-O system superconducting film according to the present invention as claimed, as well as a manufacturing method thereof, a Bi-Sr-Ca-Cu-O system superconductor in which (119) face is selectively grown relative to the substrate surface and which is used as a material for various kinds of devices is provided by a CVD process, which is industrially advantageous as a method of forming an oxide system superconducting film. In particular, according to the invention as claimed, a Bi-Sr-Ca-Cu-O system superconducting thin film of higher quality can be obtained. The present invention as claimed will now be described more specifically referring to inventive and non-inventive examples. The gas flow rate is indicated by sccm, which is a value obtained by converting cc/min into that at 1 atm and 25°C (1 sccm = 1.69 · 10 -3 Nm/s). Example 1 (non-inventive, not being an embodiment of the invention) A Bi-Sr-Ca-Cu-O system superconducting film was manufactured by using the starting material gases shown in the following Table 1 and using the substrate and under the film forming conditions shown in Table 2. Starting material gas Temperature and flow rate Bi(ph) 3 87°C, 50 sccm Sr(DPM) 2 215°C, 50 sccm Ca(DPM) 2 183°C, 50 sccm Cu(DPM) 2 102°C, 50 sccm Oxidizing gas O 2 Substrate SrTiO 3 single crystal (face indices (110)) Substrate temperature 775°C Total pressure (starting gas + Ar + O 2 ) 10 torr O 2 partial pressure about 9 torr O 2 flow rate 1200 sccm Deposition rate about 0.5 nm/min (1 Torr = 133,32 Pa) As a result, a film of about 1900 Å (190 nm) could be formed by the film formation for about 6 hours. Physical properties of the resultant Bi-Sr-Ca-Cu-O system superconductingfilm were as shown below. Chemical composition Bi 2 Sr 2 Ca 1 Cu 2 O x Critical temperature (Tc) about 63 K XRD spectral as shown in Fig. 1, only the peak corresponding to the face indices of (119) Fig. 2 shows resistance-temperature characteristics of the resultant Bi-Sr-Ca-Cu-O system superconducting thin film. In the figure, (a) shows a case of flowing an electric current in the <100> direction, while (b) shows a case of flowing a current in the <1 1 0> direction and it can be seen that Tc is about 63 K in either of the cases. In addition, since it is selectively oriented to the (119) face, if the flowing direction of the current ((a) and (b)) changes, a difference of about five times in the electric resistance is recognized in the normal conductive state. Fig. 3 shows a scanning type electron microscopic photograph for the surface of the resultant Bi-Sr-Ca-Cu-O system superconducting thin film. Only plate-like crystal grains inclined by about 48 ° were observed and it can be seen that they are selectively oriented at (119). Example 2 (embodiment of the invention) Film was formed in the same manner as in Example 1 except for using a SrTiO 3 substrate having a surface inclined by about 5° in the direction from [110] to <1 1 0>, and Fig. 4 shows XRD spectra for the resultant Bi-Sr-Ca-Cu-O system superconducting thin film, Fig. 5 shows resistance - temperature characteristics and Fig. 6 shows a scanning type electron microscopic photograph, respectively. From the result, it can be seen the followings. That is, as shown in Fig. 4, in XRD spectra, only the peak corresponding to the face indices of (119) was observed, and the peak intensity was increased by about 7 times as compared with the thin film manufactured in the Example 1. It can be seen, as shown in the scanning electron microscopic photograph of Fig. 6, that the size of each of crystal grains is increased and that the crystallization directions within the plane of the crystal grains are unified. It is considered that the surface of the SrTiO 3 substrate is slanted by about 5° from [110] to <1 1 0> direction, steps at the atom level are formed in the <1 1 0> direction at the surface of the substrate, and this may give a preferred effect on the improvement of the crystallinity. In Fig. 5, (a) shows characteristics in a case of flowing electric current in the <100> direction, while (b) shows the characteristic in a case of flowing the electric current in the <1 1 0> direction. Tc was 63 K in either of the directions. In addition, the anisotropy of the electric resistance in the normally conductive state was increased by 20 times, which was greater as compared with that of the Bi-Sr-Ca-Cu-O system superconducting thin film in Example 1. This is due to the improvement of the crystallinity as compared with the Bi-Sr-Ca-Cu-O system superconducting thin film in Example 1. Example 3 (embodiment of the invention) A film was formed in the same manner as in Example 1 except for preparing the film by using a SrTiO 3 substrate having the surface inclined by about 5° from [110] to <1 1 0> direction at a film forming temperature of 800°C. Fig. 7 shows the resistance - temperature characteristics of the resultant Bi-Sr-Ca-Cu-O system superconducting thin film. Further, XRD spectrum and the surface shape of the thin film were the same as those of the Bi-Sr-Ca-Cu-O system superconducting thin film in Example 2. As shown in Fig. 7, it can be seen that the resistance - temperature characteristics of the Bi-Sr-Ca-Cu-O system superconductingthin film in this example was improved. That is, the critical temperature Tc was about 70 K in a case of flowing electric current in <100> direction ((a)), while it was about 65 K in a case of flowing the electric current <1 1 0> direction ((b)). Further, the anisotropy of the electric resistance in the normally conductive state was about 200 times.

Claims: A Bi-Sr-Ca-Cu-O system superconducting thin film formed on a substrate comprising single crystals of an ABO 3 type oxide having a perovskite structure, and selected from the group consisting of SrTiO 3 , NdGaO 3 , LaAlO 3 and YAlO 3 , comprising a relative to the substrate surface selectively grown (119) face, wherein said substrate surface is inclined relative to (110) such that the direction perpendicular to the substrate surface is inclined from the [110] direction to the <1 1 0> direction by 2° to 10°. A Bi-Sr-Ca-Cu-O system superconducting thin film according to claim 1, wherein said thin film has a thickness of 10 to 1000 nm and a critical temperature of 50 to 100 K.

Inventors: KUBOTA NOBUHIKO, SHIOHARA YUH, SUGAWARA KAZUSHI, SUGIMOTO TSUNEMI, KUBOTA, NOBUHIKO, SHIOHARA, YUH, SUGAWARA, KAZUSHI, SUGIMOTO, TSUNEMI, Kubota, Nobuhiko, c/o Int. Supercond. Techn.Center, Shiohara, Yuh, c/o Int. Supercond. Techn. Center, Sugawara, Kazushi, c/o Int. Supercond.Techn.Center, Sugimoto, Tsunemi, c/o Int. Supercond.Techn.Center
Applicants: INT SUPERCONDUCTIVITY TECH, ISHIKAWAJIMA HARIMA HEAVY IND, SHARP KK, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES CO., LTD., SHARP CORPORATION
Application year: 1992
Main classification: H01L39
IPC codes: H01L39