JP2013245388A - Manufacturing method of superconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of matching an oxygen composition with a stoichiometric composition without performing annealing treatment under a reducing atmosphere after manufacturing a sample, in manufacture of a copper oxide superconductor having an infinite layer structure or a thin film thereof or a thin film element thereof.SOLUTION: An oxide thin film deposition device for an MBE method includes a deposition chamber 201, an element supply source 202, an electron gun 203, a heating device 204, a radical oxygen generating device 205, a gas supply piping 206, an EIES 207, a QCM 208, a vapor deposition controller 209, a shutter controller 210, an RHEED electron gun 213, a main shutter 214, an RHEED screen 215, and a source, etc. By controlling high frequency power applied to the radical oxygen generating device, generation of Oions is suppressed which causes introduction of excessive oxygen of nonstoichiometric proportion composition into an oxide thin film, and only atomic oxygen Ois generated and supplied.

Description

  The present invention relates to a method for manufacturing a superconductor, and specifically to a method for manufacturing a copper oxide superconductor by MBE.

Infinite layer structure copper oxide represented by the general formula (AE, RE) CuO ₂ (AE is one or more alkaline earth metal elements, RE is a rare earth element) is the simplest copper oxide high-temperature superconductor It has a simple structure. Therefore, if a high quality single crystal thin film can be produced, it is a promising material system for device application.

However, in the preparation of bulk samples, (AE, RE) CuO ₂ itself, which has an infinite layer structure, is synthesized under high pressure except for the case of Sr _0.14 Ca _0.86 CuO _{2 which} can only be obtained by solid-phase reaction under normal pressure. Therefore, it is known that the system is difficult to produce a single crystal sample.

  In contrast to this high-pressure synthesis, in the case of thin-film synthesis, a high non-equilibrium molecular beam epitaxy (hereinafter referred to as MBE) method is used to appropriately select the substrate and the substrate temperature during growth. It has been demonstrated that the restriction on pressure that can only be synthesized under pressure is relaxed, and a sample with high crystallinity can be obtained in a vacuum (see Non-Patent Document 1).

However, infinite layer structure samples obtained by the conventional MBE method usually contain non-stoichiometric oxygen incorporated into non-normal sites other than oxygen existing at normal oxygen sites in the crystal. FIG. 1 shows the crystal structure of an infinite layer structure copper oxide superconductor obtained by the conventional MBE method. The crystal structure shown in FIG. 1 is composed of a plurality of CuO ₂ layers 101 stacked in the [001] direction and (Sr, La) 102 between CuO ₂ layers. In the conventional method, oxygen is taken into the non-normal oxygen site 104 that is not the normal oxygen site 103 of the CuO ₂ layer 101.

  Therefore, in order to remove the oxygen taken into the non-regular site and develop good superconducting characteristics, there has been a problem that it is necessary to perform a precise annealing treatment in a reducing atmosphere after film formation.

  Since the process of removing non-stoichiometric oxygen by annealing is a diffusion process, it becomes more difficult as the thickness of the thin film increases and the device structure using the thin film becomes more complicated. This problem becomes a more serious problem when, for example, a sandwich-type junction is formed by using a thin film with high crystallinity, and becomes a large barrier in the production of a copper oxide superconducting device based on a Josephson junction. It was.

JP 2008-78200 A

S.Karimoto and M.Naito, "Electron-doped infinite-layer thin films with Tc over 40K grown on DyScO3 substrates", Appl.Phys.Lett., Vol.84, No.12, pp.2136-2138, (2004 ).

  In view of the above-described problems, an object of the present invention is to produce a copper oxide superconductor having an infinite layer structure or a thin film or thin film element thereof without performing annealing treatment in a reducing atmosphere after sample preparation. It is to provide a method capable of adjusting the composition to the stoichiometric composition. Thereby, a copper oxide superconductor having good superconducting properties, or a thin film or thin film element thereof can be obtained efficiently.

  The present invention relates to a method for producing a copper oxide superconductor by MBE using an oxide thin film growth apparatus equipped with a radical oxygen generator, and the step of controlling the high-frequency power applied to the radical oxygen generator It is characterized by having provided.

In one embodiment of the present invention, the step of controlling the high frequency power applied to the radical oxygen generator is such that only O ^* is supplied from the radical oxygen generator from the value at which O ₂ ⁺ ions are supplied from the radical oxygen generator. The high-frequency power is reduced to a certain value.

  One embodiment of the present invention is characterized in that an annealing process under a reducing atmosphere after film formation is not required.

In one embodiment of the present invention, the copper oxide superconductor is an infinite layer structure copper oxide represented by (AE, RE) CuO ₂ (AE is one or more alkaline earth metal elements, RE is a rare earth element) Element).

  According to the present invention, a single-crystal high-quality superconducting thin film of an infinite layer structure superconductor, which has the simplest structure among copper oxide superconductors but has not yet been produced in bulk, is annealed after deposition. It became possible to produce only one MBE growth without treatment.

  In addition, since a superconducting thin film can be produced only by MBE growth without the need for annealing after film formation, an insulating thin film is stacked on this superconducting thin film, and an opposing superconductor is further stacked. Thus, it is considered possible to produce a copper oxide-based Josephson junction.

It is a figure which shows the crystal structure of the infinite layer structure copper oxide superconductor obtained by the conventional MBE method. It is a block diagram which shows the structural example of the oxide thin film growth apparatus used by this invention. It is a graph showing a high-frequency power P dependencies of changing the high frequency power P and the formed Sr _0.9 La _0.1 CuO ₂ thin film temperature (K) and resistivity to be applied to the radical oxygen generator and (mΩcm). It is a graph which shows the emission spectrum from a radical oxygen generator with respect to each of the high frequency electric power P. Is a graph showing the X-ray diffraction pattern of Sr _0.9 La _0.1 CuO ₂ thin film formed by changing the high frequency power P applied to the radical oxygen generating apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 2 is a configuration diagram showing a configuration example of an oxide thin film growth apparatus used in the present invention. 2 includes a growth chamber 201, an element supply source 202, an electron gun 203, a heating device 204, a radical oxygen generator 205, a gas supply pipe 206, an electron impact emission spectroscopy sensor (hereinafter referred to as EIES). 207, crystal oscillator type film thickness meter (hereinafter referred to as QCM) 208, vapor deposition controller 209, shutter controller 210, feedback loop 211, shutter control line 212, reflection high-energy electron diffraction (hereinafter referred to as RHEED) electron gun 213, main shutter 214, RHEED screen 215, La source 216, Sr source 217, and Cu source 218 are configured.

  The inside of the growth chamber 201 can be exhausted by a vacuum pump (not shown). The element supply source 202 accommodates a La source 216, a Sr source 217, and a Cu source 218. One electron gun 203 is provided for each source, and an accelerated and converged electron beam emitted from the electron gun 203 is deflected so that each source can be irradiated. By irradiation with this electron beam, each source is locally heated and evaporated.

  The substrate to be processed fixed on the heating device 204 is heated by the heating device 204. The heating device 204 includes a heating element composed of, for example, a heating wire made of platinum.

  An oxidizing gas generated by an atomic oxygen (hereinafter also referred to as radical oxygen) generator 205 can be supplied into the growth chamber 201 through a gas supply pipe 206. The supplied oxidizing gas is discharged to the vicinity of the substrate through the gas supply pipe 206. The radical gas is an oxygen gas containing activated oxygen such as oxygen ionized by plasma or atomic oxygen, for example. The radical oxygen generator 205 is composed of, for example, a high-frequency discharge tube or a coil.

The present invention controls the high-frequency power applied to the radical oxygen generator 205 to suppress the generation of O ₂ ⁺ ions that cause the introduction of excess non-stoichiometric oxygen into the oxide thin film. Only produces and supplies atomic oxygen O ^* (* represents radically active species). Thereby, an infinite layer structure copper oxide superconductor thin film having good superconducting properties can be obtained without annealing in a reducing atmosphere after film formation.

  In the oxide thin film growth apparatus shown in FIG. 2, in order to efficiently oxidize the metal deposited on the substrate, active oxygen having a higher oxidizing power is used, and an oxidizing gas is discharged in the vicinity of the substrate. An oxygen generator 205 and a gas supply pipe 206 are arranged.

  In the growth chamber 201, an EIES 207 is disposed in the vicinity of the substrate. Thereby, the supply rate of the atomic flux inside the film forming chamber 201 can be measured. In EIES207, the intensity | strength of the light emitted when it collides with an atom until the electron discharge | released from the electron emission source is captured by an anode electrode is measured, and the supply rate of an atomic flux is measured.

  The vapor deposition controller 209 controls the output of the electron gun 203 via the feedback loop 211 in accordance with the partial pressure of each material atom measured by the EIES 207 and the film thickness measured by the QCM 208, and the La source 216, the Sr source 217, The amount of evaporation from the Cu source 218 is controlled.

  The shutter controller 210 controls the main shutter 214 via the shutter control line 212 according to the supply rate of the atomic flux of each material measured by the EIES 207 and the film thickness measured by the QCM 208, and the deposition amount of La, Sr, Cu And control the film thickness.

  The oxide thin film growth apparatus shown in FIG. 2 includes an RHEED electron gun 213, and a diffraction image by electrons emitted from the RHEED electron gun 213 can be observed by an RHEED screen 215. An electron diffraction image observed by the RHEED screen 215 makes it possible to grasp the surface structure and composition state of the thin film grown on the substrate. For example, when the composition ratio of each element deposited on the substrate deviates from the stoichiometric composition, precipitates are formed on the thin film surface. Since this state is reflected in the electron diffraction image, a composition shift can be confirmed.

[Example]
FIG. 3 shows the dependency of the relationship between the temperature (K) and the resistivity (mΩcm) of the Sr _0.9 La _0.1 CuO ₂ thin film formed by changing the high frequency power P applied to the radical oxygen generator on the high frequency power P. . In this example, oxygen was supplied to the radical oxygen generator at a rate of 1.5 sccm. The grown thin film is not annealed in a reducing atmosphere.

  As shown in FIG. 3, when P = 300 W, the thin film does not show a superconducting transition. Therefore, an additional annealing process is required in order to develop superconductivity in a sample manufactured at a setting of P = 300 W. On the other hand, when P = 150 W, a superconducting transition is observed at 37K. Thus, it was confirmed that a thin film exhibiting good superconductivity can be obtained without annealing.

  In FIG. 4, the emission spectrum from a radical oxygen generator with respect to each of the high frequency electric power P is shown. From this emission spectrum, the oxygen species generated in the radical oxygen generator can be identified.

As shown in FIG. 4, a peak seen near the wavelength of 605 nm or the like when P = 300 W is not seen when P = 150 W. From this, it was found that the amount of molecular oxygen ions (O ₂ ⁺ ions) was reduced by lowering the power supplied to the radical oxygen generator, and only radical oxygen (O ^* ) was supplied. By supplying only radical oxygen (O ^* ), non-stoichiometric oxygen uptake is suppressed during film formation, and a thin film having good superconducting characteristics can be formed without annealing.

FIG. 5 shows an X-ray diffraction pattern of a Sr _0.9 La _0.1 CuO ₂ thin film formed by changing the high frequency power P applied to the radical oxygen generator. The lattice constant c ₀ in the (001) direction calculated from the X-ray diffraction pattern shown in FIG. 5 is 0.3426 nm when the film is formed at P = 300 W, and 0.3412 nm when the film is formed at P = 150 W. Met. Therefore, the lattice constant c ₀ in the (001) direction is longer when the film is formed at P = 300 W than when the film is formed at P = 150 W. This indicates that oxygen enters the non-regular site when the film is formed at P = 300 W.

  An example of improving the quality of a nitride semiconductor thin film grown by MBE by changing the chemical species generated by changing the power applied to the radical supply source has been reported (see, for example, Patent Document 1). However, there has been no such report in the MBE growth of oxides, and the present invention has the effect that an infinite layer structure copper oxide superconducting thin film can be formed without annealing treatment which has been indispensable in the past. It is considered large.

101 CuO ₂ layer 102 (Sr, La)
103 Regular oxygen site 104 Non-regular oxygen site 201 Growth chamber 202 Element supply source 203 Electron gun 204 Heating device 205 Radical oxygen generator 206 Gas supply piping 207 EIES
208 QCM
209 Deposition controller 210 Shutter controller 211 Feedback loop 212 Shutter control line 213 RHEED electron gun 214 Main shutter 215 RHEED screen 216 La source 217 Sr source 218 Cu source

Claims (4)

A method of producing a copper oxide superconductor by MBE using an oxide thin film growth apparatus equipped with a radical oxygen generator,
A method for producing a copper oxide superconductor, comprising a step of controlling high-frequency power applied to the radical oxygen generator. The step of controlling the high-frequency power applied to the radical oxygen generator is performed by changing the high-frequency power from a value at which O ₂ ⁺ ions are supplied from the radical oxygen generator to a value at which only O ^* is supplied from the radical oxygen generator. The method for producing a copper oxide superconductor according to claim 1, wherein   The method for producing a copper oxide superconductor according to claim 1 or 2, wherein an annealing treatment in a reducing atmosphere after film formation is not required. The copper oxide superconductor is an infinite layer structure copper oxide represented by (AE, RE) CuO ₂ (AE is one or more alkaline earth metal elements and RE is a rare earth element). A method for producing a copper oxide superconductor according to any one of claims 1 to 3.

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