Improvement in a process for producing thallium type superconducting thin film

Abstract: Improvement in heat-treatment of a superconducting thin film of thallium type compound oxide deposited on a substrate by PVD or CVD. In the invention, the heat-treatment is effected at a temperature between 880 °C and 920 °C for a predetermined time duration under such a condition that the partial pressure of thallium oxide become higher than the saturated vapour pressure of thallium oxide at said temperature.

Description: Background of the Invention Field of the invention The present invention relates to an improvement in a process for producing superconducting films. More particularly, it relates to an improved process for producing thallium type superconducting thin films of thallium containing-compound oxides such as Tl-Ba-Ca-Cu type oxide having higher critical current density (Jc) as well as a high critical temperature (Tc). Description of the related art The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance become zero and the perfect diamagnetism is observed. Under the superconducting condition, electric current of high density flow continuously without any loss of power, so that the power loss of about 7 % which is lost in the electric power transmission today can be saved greatly when the technology of superconductivity can be applied to the electric power transmission. Development of superconductor is also demanded in the field of measurment in order to detect very weak magnetism by SQUID, in the field of medical treatment by π-neutrons and in the field of high-energy physical experiments. Superconductor is also requested in the field of electromagnets for generating a strong magnetic field in order to develop the technologies of fusion power generation, MHD power generation, magnetic levitation trains and magnetically propelling ships. The critical temperature "Tc" of superconductors, however, could not exceed 23.2 K of Nb₃Ge which was the the highest Tc for the past ten years. The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Müller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p 189). The new type compound oxide superconductor discovered by Bednorz and Müller is represented by [La,Sr]₂CuO₄ which is called the K₂NiF₄-type oxide having a crystal structure which is similar to known perovskite type oxides. The K₂NiF₄-type compound oxides show such higher Tc as 30 K which are extremely higher than known superconducting materials. After then, a variety of compound oxides which show much higher critical temperatures were reported and hence the possibility of an actual utilization of the high Tc superconductors have burst onto the scene. C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa₂Cu₃O 7-x having the critical temperature of about 90 K (Physical Review letters, Vol. 58, No. 9, p 908). Maeda et al reported the other type new superconducting compound oxide of Bi-Sr-Ca-Cu-O system (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). Thallium type compound oxides are also high Tc superconductors of more than 100 K. The present inventors disclosed several kinds of thallium type compound oxides superconductors in a US patent application No. 223,634 filed on July 25, 1988 and Hermann et al. reported Tl-Ba-Ca-Cu-O system in Appl. Phys. Lett. 52 (20) p 1738). Thallium type compound oxides are chemically much stable than the abovementioned YBCO type compound oxide and have such a very important merit that high Tc superconductors of higher than 100 K can be realized without using rear earth elements as a material so that the production cost can be reduced. The above-mentioned new types oxide superconducting materials can be prepared in a form of a thin film on a substrate by physical vapour deposition (PVD) technique or chemical vapor deposition (CVD) technique. In the case of production of thallium type oxide superconductors, however, there is a special problem, because thallium (Tl) is a very volatile element and toxic for human. In fact, thallium type superconducting thin films prepared by the conventional physical vapour technique show relatively lower Tc (critical temperature) and Jc (critical current density) than a balk or a block of this compound oxide which is prepared by sintering technique. It is thought that this deference may be caused by shortage of oxygen in the crystal of which the superconducting thin film is made. Heretofore, it is a usual practice to anneal a deposited thin film at 600 to 900 °C in the presence of oxygen gas in order to improve the superconducting properties of oxide type superconducting thin films deposited. This technique, however, is not effective in the case of thallium type superconducting thin films because thallium has a high vapour pressure and hence a majority of thallium atoms in the thin film escape from the thin film, so that the resulting thin film doesn't show desired high Tc and Jc. A heat treatment in which a piece of elemental thallium (Tl) is placed together with the thin film inside the furnace is presented by Shih and Qiv in Applied Physics Letters, Vol. 53, No. 6, pages 523-525. Therefore, an object of the present invention is to overcome the problem of the prior arts and to provide an improved process for producing superconducting thin films of thallium (Tl) type compound oxides. Summary of the Invention The present invention provides a process for producing a superconducting thin film of thallium type compound oxide, comprising depositing a thin film of thallium-containing compound oxide on a substrate by physical vapour deposition method or chemical vapour deposition method and then subjecting the resulting thin film to heat-treatment, characterized in that the heat-treatment is effected at a temperature between 880 °C and 920 °C for a predetermined time under a partial pressure of thallium oxide higher than the saturated vapour pressure of thallium oxide at the temperature. Preferably, the resulting heat-treated thin film is further subjected to secondary heat-treatment which is effected at a temperature between 600 °C and 900 °C for a predetermined time under a partial pressure of thallium oxide lower than the saturated vapour pressure of thallium oxide at said temperature. The present invention is applicable for any superconducting thin film of compound oxide containing thallium (Tl). One of the typical thallium type compound oxide is represented by the general formula: Tl₄(Ca 1-x ,Ba x ) m Cu n O p+y in which m, n, x and y are numbers each satisfying ranges of 6 ≦ m ≦ 16, 4 ≦ n ≦ 12 0.2 < x < 0.8 and -2 ≦ y ≦ +2, respectively and p = (6+m+n) . For example, this type can have following compositions: Tl₄Ca₄Ba₄Cu₆O 20+y or Tl₂Ca₂Ba₂Cu₃O 10+y . As the other type thallium-containing compound oxides to which the present invention is applicable, it can be mentioned the following systems: Tl-Sr-Ca-Cu-O system (75 to 100 K), Tl-Pb-Sr-Ca-Cu-O system (80 to 122 K), Tl-Pb-Ba-Ca-Cu-O system (90 to 122 K), (Tl, La, Pb)-Sr-Ca-Cu-O system (100 K), Tl-Ba-(Y, Ca)-Cu-O system (92 K), Tl-Pb-Ca-Ce-Sr-Cu-O system (95 K), Tl-Ba-Ce-Cu-O system (90 K), (Tl, Ln)-Sr-Ca-Cu-O system (80 to 90 K), (Bi, Tl)-Sr-Cu-O system (90 K), Pb-Tl-Sr-Cu-O system (42 K), La-Tl-Sr-Cu-O system (37 K) and Nd-Tl-Sr-Cu-O system (44 K), Tl-Bi-Sr-Ca-Cu-O system (25 K). (note) Ln : lanthanoid The superconducting thin film according to the present invention can be deposited on a substrate by physical vapour deposition (PVD) technique such as RF sputtering, vacuum deposition, ion-plating, MBE or by chemical vapor deposition (CVD) technique such as thermal CVD, plasma CVD, photo CVD, MOCVD or the like. When RF magnetron sputtering, vacuum deposition or ion-plating method is used as the physical vapour deposition technique, the atomic ratios of metal elements contained in a vapour source or a target is adjusted according to difference in the evaporation rate as well as difference in adsorption rate of metal elements to the substrate. The vapour source is preferably a sintered mass which is prepared by powder-sintering technique of a material power mixture comprising metal elements and/or their oxides or carbonates or a sintered powder which is obtained by pulverizing the sintered mass. The vapour source can be also divided into a plurality of segments. In the case of the molecular beam epitaxy (MBE), elemental metals of Tl, Ba, Ca and Cu or their compounds such as their oxides, carbonates, fluorides or the like are evaporated by means of a K-cell or an ion beam gun. In this case, oxygen is supplied, if necessary, separately or additionally into an evaporation atmosphere. For example, a vapour source or a target for preparing a thin film of Tl-Ba-Ca-Cu-O type compound oxide is prepared as following: At first, a powder mixture is prepared by mixing powders of compounds of Bad Ca and Cu such as their oxides, carbonates, fluorides or the like. The powder mixture is then subjected to preliminary sintering operation and pulverization. The resulting preliminary sintered powder is mixed with a powder of thallium compound such as thallium oxide, thallium carbonate or thallium fluoride and is sintered finally. The resulting sintered mass thus obtained can be used directly or after pulverization as a vapour source, particularly, as a target for sputtering. This preparation method is very effective to adjust the atomic ratios of the vapour source or target because thallium is a vary volatile and toxic element. In either case, the composition of the thin film deposited can be varied in a wide range by `adjusting the composition in the vapour source and/or by selecting a suitable combination of vapour sources. Generally speaking, supplement of oxygen into the evaporation atmosphere is ensured either by evaporating oxygen-containing compounds such as oxides or carbonates of Tl, Be, Ca and Cu or by supplying oxygen gas separately into a vacuum chamber. In any case, the oxygen contents in the atmosphere during deposition of the superconducting thin film must be controlled precisely in order to adjust the oxygen deficiency which is a critical factor to realize a superconducting thin film of high Tc on the substrate to a predetermined value. The substrate on which the thin film is deposited can be a single crystal of MgO, SrTiO₃, LaAlO₃, LaGaO₃ or the like. When a single crystal of silicon is used as a substrate, it is preferable to cover a surface of the silicon substrate with a buffer layer such as MgO or ZrO₂. The superconducting thin film according to the present invention is preferably deposited on a {001} plane or {110} plane of these single crystals. In the course of the vapour deposition, the substrate is heated at a temperature between a room temperature and 500 °C. Essence of the present invention reside in that a thin film of thallium type compound oxide deposited on a substrate by physical vapour deposition method or chemical vapour deposition method is heat-treated at a temperature between 880 °C and 920 °C for a predetermined time under a partial pressure of thallium oxide vapour higher than the saturated vapour pressure of thallium oxide at said temperature. The present inventors confirmed such a fact that the thin film prepared according to the present invention shows a very smooth surface and is very homogeneous in composition which is thought to be one reason why the thin film according to the present invention show very high Tc and Jc. In other words, the heat-treatment according to the present invention is effective to suppress escape of thallium vapour out of the thin film and to promote proper crystallization of the thin film. When the temperature of heat-treatment is not higher than 880 °C, a uniform smooth thin film can not be obtained becuase a mixture of different phases each having a different critical temperature is produced so that the total Tc of the thin film become lower. To the contrary, if the temperature of heat-treatment is not lower than 920 °C, the evaporation of thallium (Tl) increase excessively so that it is difficult to adjust the composition of the thin film to desired atomic ratios and also it increases precipitates which doesn't contribute the superconductivity. This first heat-treatment is effected preferably for a time duration between 1 minute and 10 hours. Advantage of the heat-treatment according to the present invention can't be expected if the heating time is not longer than 1 minute, while longer heat-treatment than 10 hours may not effective to improve the superconducting property. In a preferred embodiment, the resulting heat-treated thin film is further subjected to secondary heat-treatment which is effected at a temperature between 600 °C and 900 °C for a predetermined time under a partial pressure of thalluim oxide vapour lower than the saturated vapour pressure of thallium oxide at the temperature. This secondary heat-treatment is effective to remove or drive off excess thallium atom trapped in the thin film during the first heat-treatment so that the atomic ratio of thallium atom in the final product is adjusted to a desired stoichiometrlc proportion. This secondary heat-treatment is preferably continued for a time duration between 5 minutes and 50 hours. The above-mentioned heat-treatment condition that the partial pressure of thallium oxide become higher or lower than the saturated vapour pressure of thallium oxide at the temperature can be realized easily by carrying out the heat-treatment in the presence of thallium vapor and oxygen gas. In a simplest case, thallium oxide is used as a vapour source for realizing the condition. Another preferable vapour source is a thallium-containing compound such as Th-Ba-Ca-Cu-O type compound oxide. In a variation, thallium gas and oxygen gas are supplied under control into a chamber in which the heat-treatment is effected. In a preferred embodiment of the present invention, the condition is reatized by creating a temperature gradient in a closed space in which the substrate having the thin film and a thallium oxide vapour source are placed in such a manner that the thallium oxide vapour source is heated at a higher temperature than the substrate. In this case the closed spaced is preferably delimited by a pipe made of precious metal such as silver (Ag) and having an opening communicating with an interior of the chamber. In the heat-treatments, oxygen gas is always supplied into the atmosphere in addition to thallium oxide gas so that the oxygen contents in the thin film is also adjusted to the desired stoichiometric proportion. Now, the process according to the present invention is described with reference to Fig. 1 which illustrate one embodiment of how to carry out the present process. Brief Description of the Drawing Fig. 1 is a drawing illustrating how to carry out the process according to the present invention. A substrate (1) on which a thin film of thallium type compound oxide is deposited is placed in a pipe (4) made of silver (Ag) having an opening at an extreme end. In the silver pipe (4), a vapour source of thallium oxide (2) such as powder of thallium oxide is also placed. The vapour source of thallium oxide (2) can be a sintered mass or powder which is used for preparing the thin film deposited on the substrate in the physical vapour deposition stage. The silver pipe (4) is set in a sintering furnace (5) into which oxygen gas (O₂) is supplied so that oxygen gas can penetrate into the silver pipe through its open end. Usually, the oxygen gas pressure can be about at ambient pressure (1 atm) but the heat-treatment can be effected also at a higher oxygen pressure than 1 atm. In operation, a temperature gradient is created in the sintering furnace (5) in such a manner that substrate (1) on which a thin film of thallium type compound oxide is deposited is heated at a temperature T₂, while the vapour source of thallium oxide (2) is heated at a temperature T₁ respectively. The temperature T₁ is higher than the temperature T₂, for example,T₁ = 930 °C and T₂ = 900 °C, so that the heat-treatment of the thin film deposited on the substrate (1) is effected at T₂ = 900 °C in thallium oxide vapour produced from the vapour source (2) which is heated at a relatively higher temperature than the substrate (1). Under the above-mentioned temperature gradient, the vapour pressure of thallium oxide gas in the neighborhood of the thin film become higher than the saturated vapour pressure of thallium oxide (over-saturated vapour pressure). This first heat-treatment is continued for a desired time duration, usually more than one hour. After this first heat-treatment complete, a second heat-treatment is effected in the same apparatus except that the temperatures of T₁ and T₂ are lowered to an equal temperature such as T₁ = T₂ = 850 °C for a desired time duration, usually more than one hour. The process according to the present invention has the following merits: (1) It is possible to adjust the atomic ratio of thallium in the thin film of compound oxide to a desired value, because the heat-treatment is effected in the saturated vapour pressure of volatile thallium so that escape of thallium atom out of the thin film is suppressed. (2) It is also possible to adjust the oxygen contents in the thin film to a desired value because the heat-treatment is carried out in oxygen-rich condition. (3) The thin film is not contaminated because the heat-treatment is is effected in a pipe made of precious metal which is inactive to the thin film. In a conclusion, according to the process of the present invention, it becomes possible to producing high-quality superconducting thin film of thallium-containing compound oxides such as Tl-Ba-Ca-Cu type oxide superconductor improved in superconducting property, particularly in the critical current density Jc in a stable condition. Now, the method according to the present invention is described with reference to Examples but the scope of the present invention should not limited to the following Examples. Example 1 Preparation of thin film of thallium type compound oxide A thin film of thallium type compound oxide is deposited on a substrate of MgO single crystal by RF magnetron sputtering. (1) Preparation of a target Powders of BaCO₃ and CuO are kneaded in a mortar. The resulting powder mixture is sintered preliminarily at 900 °C for 8 hours. The resulting sintered mass is pulverized to a powder to which powders of Tl₂O₃ and CaO are admixed uniformly to prepare a material powder mixture. The atomic ratios of Tl : Ca : Ba : Cu in the material powder mixture are adjusted to 2.4 : 2.3 : 2.0 : 3.0. The material powder mixture is pressed to a compact which is then wrapped by a foil made of gold (Au). The compact wrapped by the gold foil is then sintered is 905 °C for 3 hours in oxygen gas atmosphere in a sintering furnace. (2) Deposition of a thin film The resulting sintered mass is pulverized to obtain a sintered powder which is used as a powder target for RF magnetron sputtering which is carried out under the following conditions: The resulting thin film is an amorphous film having a composition of Tl:Ba:Ca:Cu = 2:2:2:3 and doesn't show superconducting property. Heat-treatment according to the invention The resulting substrate on which the thin film of thallium-containing compound oxide is deposited is heat-treated in a furnace illustrated in Fig. 1. Namely, the substrate (1) is placed in a pipe made of silver (Ag) having an open end. In the silver pipe (4), a compound of Tl-Ba-Ca-Cu-O (2) which was prepared by the same method as is described in the "(1) preparation of a target" is also placed. This sintered mass (2) is prepared by the same process described in the preparation of the target. Then, the silver pipe (4) is set in a sintering furnace (5) into which oxygen gas (O₂) of one atm is supplied. A temperature gradient is created in the sintering furnace (5) in such a manner that substrate (1) is heated at 900 °C (T₂), while the Tl-Ba-Ca-Cu-O compound (2) is heated at 930 °C (T₁) respectively. Under this temperature gradient, the vapour pressure of thallium oxide gas in the neighborhood of the substrate (1) become higher than the saturated vapour pressure of thallium oxide at 900 °C (over-saturated vapour pressure). This first heat-treatment is continued for one hour. After this first heat-treatment complete, second heat-treatment is effected in the same apparatus except that the temperatures of T₁ and T₂ are lowered to an equal temperature of T₁ = T₂ = 850 °C. Under this temperature gradient, the vapour pressure of thallium oxide gas in the neighborhood of the substrate (1) become lower than the saturated vapour pressure of thallium oxide at 850 °C. This secondary heat-treatment is continued for three hours. The resulting superconducting thin film is evaluated by measuring the critical temperature Tc, the critical current density Jc and by observing their surfaces by a scanning electron microscope (SEM). The critical temperatures Tc is determined by usual four probe method. Temperature is measured by a calibrated Au(Fe)-Ag thermocouple. The critical current density (Jc) of the thin film obtained is determined at 77.0 K and is expressed by A/cm². The results is shown in Table 1. Tc (K) 125 K Jc (77K) 3.2 x 10⁶ A/cm². Surface condition smooth and uniform Comparative example 1 The Example 1 is repeated except that the first heat-treatment is omitted. Namely, the heat-treatment is effected in the same condition as the secondary condition which corresponds to the prior art. The result on this case is shown in Table 2. Tc (K) 103 K Jc (77K) 1.2 x 10⁴ A/cm² Surface condition rough and not uniform Comparing Example 1 with Comparative Example 1, it is apparent that the superconducting property, particularly the critical current density improve remarkably. Example 2 The Example 1 is repeated except that the target is changed to prepare a different thin film of compound oxide having the following proportion: Tl 0.7 Bi 0.3 Ca₂Sr₂Cu₃O x In this case, the target is prepared as following: at first, a powder mixture of Ba₂O₃, CaO, SrCO₃ and CuO is sintered preliminarily at 700 °C for 10 hours. The resulting sintered mass is pulverized to a powder to which powder of Tl₂O₃ is admixed uniformly to prepare a material powder mixture which is sintered finally at 910 °C for 3 hours in the same manner as Example 1. The atomic ratios of the sintered mass are Tl : Bi : Ca : Sr : Cu = 1 : 0.3 : 2.1 : 2 : 3. A thin film is prepared and evaluated by the same method as Example 1. The result on this case is shown in Table 3. Tc (K) 115 K Jc (77K) 3.1 x 10⁶ A/cm² Surface condition smooth and uniform Comparative example 2 The Example 2 is repeated except that the first heat-treatment is omitted. Namely, the heat-treatment is effected in the same condition as the secondary condition which corresponds to the prior art. The result on this case is shown in Table 4. Tc (K) 92 K Jc (77K) 1.2 x 10⁴ A/cm² Surface condition rough and not uniform Example 3 The Example 1 is repeated except that the vapour source is changed to a powder of thallium oxide (Tl₂O₃) and that the thallium oxide powder is heated at 910 °C (T₁) and while the substrate is heated at 900 °C (T₂) in the first heat-treatment for 30 minute. The resulting thin film heat-treated is evaluated by the same method as Example 1. The result on this case is shown in Table 5. Tc (K) 124 K Jc (77K) 3.5 x 10⁶ A/cm² Surface condition smooth and uniform

Claims: A process for producing a superconducting thin film of thallium type compound oxide, comprising depositing a thin film of thallium-containing compound oxide on a substrate by physical vapour deposition method or chemical vapour deposition method and then subjecting the resulting thin film to heat-treatment, characterized in that said heat-treatment is effected at a temperature between 880 °C and 920 °C for a predetermined time under a partial pressure of thallium oxide higher than the saturated vapour pressure of thallium oxide at said temperature. The process set forth in Claim 1 wherein the resulting heat-treated thin film is subjected to secondary heat-treatment which is effected at a temperature between 600 °C and 900 °C for a predetermined time under a partial pressure of thallium oxide lower than the saturated vapour pressure of thallium oxide at said temperature. The process set forth in Claim 1 or 2 wherein the first and second heat-treatment is effected in oxygen-containing atmosphere. The process set forth in any one of Claim 1 to 3 wherein the first and/or second heat-treatment is effected in a partially closed space delimited in a furnace. The process set forth in Claim 4 wherein said partially closed space is formed by a pipe made of precious metal having an opening communicating with an interior space of the furnace. The process set forth in any one of Claim 1 to 5 wherein said condition that the partial pressure of thallium oxide become higher than the saturated vapour pressure of thallium oxide at said temperature is created by heating a thallium-containing compound at a temperature which is higher than said temperature at which said thin film is heat-treated. The process set forth in Claim 6 wherein said thallium-containing compound oxide is thallium oxide. The process set forth in Claim 6 wherein said thallium-containing compound oxide is a compound having the same composition as said thin film to be heat-treated. The process set forth in any one of Claim 1 to 8 wherein said first and secondary heat-treatment is effected for more than one minute. The process set forth in any one of Claim 1 to 9 wherein said superconducting thin film of thallium type compound oxide has a composition represented by the general formula: Tl₄(Ca 1-x , Ba x ) m Cu n O p+y in which m, n, x and y are numbers each satisfying ranges of 6 ≦ m ≦ 16, 4 ≦ n ≦ 12 0.2 < x < 0.8 and -2 ≦ y ≦ +2, respectively and p = (6+m+n) . The process set forth in any one of Claim 1 to 10 wherein said substrate is a single crystal of MgO, SrTiO₃, LaAlO₃ or LaGaO₃.

Inventors: HARADA KEIZO, ITOZAKI HIDEO, YAZU SHUJI, HARADA, KEIZO, ITOZAKI, HIDEO, YAZU, SHUJI
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: H01L39
IPC codes: H01B13, C23C16, H01L39, C01G15, C23C14, C01G1, H01B12