JP3152322B2 - Twinless (Nd, La) GaO3 single crystal and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a twinless (Nd, La) G
aO ₃ single crystal and its manufacturing method, more specifically used as a substrate material for oxide superconducting thin film growth (Nd,
La) It relates to a mixed composition and a production method capable of producing a GaO ₃ single crystal as a twinless single crystal.

[0002]

2. Description of the Related Art In recent years, gallate oxides represented by LaGaO ₃ and NdGaO ₃ have been reported to be effective as lattice matching substrates for oxide superconductor thin films (for example, Sandst).
rom et al., Applied Physics Letters 53 (1988) 1874). When an oxide superconductor is epitaxially grown on these gallate oxide single crystal substrates and formed into a film, the lattice match between the gallate substrate and the oxide superconductor thin film at the film formation temperature is determined by the growth of the thin film during film formation. Influences style and film quality. Ideally, if the lattice mismatch rate at the film formation temperature is 0%,
It is a desirable substrate that satisfies pseudo homoepitaxial growth conditions at the time of film formation and forms a high-quality film.

When the oxide superconductor is made of YBa ₂ Cu ₃ O _x , the lattice matching rates of the above-mentioned LaGaO ₃ and NdGaO ₃ gallate oxides at the film forming temperature are each -0.10.
%, 0.26%, does not result in until satisfying the pseudo homoepitaxial growth conditions (lattice matching ratio was calculated from the formula _{(a f -a s) / {} (a f + a s) / 2} . here, a _f and a _s denotes the lattice constant of the thin film and the substrate, respectively). However, looking at the lattice mismatch rate in detail, the lattice constants of the respective gallate oxides based on YBa ₂ Cu ₃ O _x are slightly larger for LaGaO _{3 and} slightly smaller for NdGaO ₃ . Therefore, if these two gallate oxides are mixed crystals, a lattice mismatch rate of 0
% Of a single crystal substrate can be manufactured. A report on the use of a mixed crystal material of gallate oxide as the oxide superconductor thin film substrate in this way was published in 1989 by Koen et al. In Applied Physics Letters Vol. The La _0.95 Gd _0.05 GaO ₃ single crystal described at page 54, page 1056 is the first. Regarding the mixed crystal of (Nd, La) GaO ₃ gallate oxide, Oishi et al.
First reported in issue 250. However, it is important that the single crystal for the substrate be a single crystal that does not include crystal defects such as cracks and twins that affect the film quality of the formed thin film in addition to the lattice matching described above. Nd,
When (La) GaO ₃ gallate oxide mixed crystal is used as a substrate for an oxide superconductor thin film, the (Nd, La) GaO 3
_It is important to clarify the composition and the probability of the production method for the purpose of growing and producing a _3- gallate mixed crystal as a single crystal containing no cracks or twins and using it as a substrate. (Nd, L
a) LaGaO ₃ which is one component of GaO ₃ mixed crystal single crystal
Has two structural transformation points between room temperature and its melting point (Kobayashi et al., Journal of Material Science 6 (1992).
1) 97), this structural phase transformation causes cracks and twins to occur during the production of single crystals. Therefore, NdGaO ₃ capable of producing a single crystal having no structural phase transformation and containing no cracks or twins and LaGaO ₃ capable of producing only a single crystal containing cracks or twins are constituent elements (Nd, L
a) It can be easily inferred that the composition range in which a GaO ₃ mixed crystal can produce a single crystal without cracks or twins is limited depending on its La / Nd ratio. However, the above invention (Japanese Patent Laid-Open No. 3-279250) does not mention these points at all.

[0004] The method of producing the above-mentioned single crystal of NdGaO ₃ gallate was first attempted in 1956 by using the flux method. However, the quality was deteriorated due to the incorporation of lead oxide used in the flux and the size of the obtained single crystal was reduced to 5 mm. Angle)) (Remeika, Journal of American
Chemical Society 78 (1956) 4259). In order to solve these problems, a pulling method, which is a general single crystal manufacturing method, has been attempted in the manufacture of silicon single crystals and the like since 1975. However, cracks and twins are generated due to thermal stress. With a general pulling apparatus without a heater, only a small twinless single crystal having a diameter of about 10 mm could be obtained (Ruse et al., Journal of Crystal Growth).
29 (1975) 305). LaGaO ₃ gallate single crystals are produced by the pulling method, but no twin-crystal single crystals have been obtained at present.

FIG. 5 shows a drawing of a high frequency induction heating type pulling apparatus generally used in the prior art. In the figure, 1 is an iridium crucible, 3 is a high-frequency heating coil, 4 is an insulated cylinder,
Reference numeral 5 denotes a melt, 6 denotes a quartz tube, and 7 denotes a thermocouple. A sintered body serving as a base material is filled in an iridium crucible 1, melted by a high-frequency heating coil 3, and a seed crystal is immersed in a melt and rotated to grow (Nd, La) GaO ₃ .

[0006]

When (Nd, La) GaO ₃ single crystals having various La / Nd ratios were grown by the method shown in FIG. 5, cracks and twins were formed in the single crystal during the growth. A good single crystal could not be obtained.
Even in a single crystal having few cracks or twins, cracks or twins may be formed in the single crystal during the cooling step after the pulling step or during standing after being removed from the single crystal pulling apparatus.
Therefore, a single crystal grown by the conventional method has a diameter of 20 mm.
It was impossible to obtain a single crystal substrate having a diameter as large as about mm. Further, there is a problem that the yield is very low even with a single crystal substrate having a small diameter of about 10 mm.

Accordingly, the present invention provides a composition (La / Nd ratio) that enables production of a (Nd, La) GaO ₃ single crystal free of cracks and twins and provides a high-quality lattice-matched substrate. Aim. Also, (Nd, La) G
It is an object of the present invention to provide a method for producing a single crystal in which cracks and twins do not enter during growth or during cooling after growth in a growth technique based on the aO ₃ single crystal pulling method. further,
Another object of the present invention is to provide a method for preventing cracks and twins from entering a (Nd, La) GaO ₃ single crystal produced by a pulling method during standing.

[0008]

Means for Solving the Problems In order to solve the above problems, the twin-free (Nd, La) GaO ₃ single crystal according to the present invention comprises Nd _{1 -x} La _x GaO ₃ (0 <x <0.3). It is characterized by being.

The twinless (Nd, La) G according to the present invention
An aO ₃ single crystal is produced by bringing a seed crystal into contact with a (Nd, La) GaO ₃ melt in a pulling furnace for producing a single crystal, pulling the crystal, and then cooling the twin crystal without twin (Nd, La) GaO ₃ single crystal. Wherein the (Nd, La) GaO ₃ melt has a composition for forming a Nd _{1 -x} La _x GaO ₃ (0 <x <0.3) single crystal, and Is 45 ° C./cm or less immediately above the liquid surface of the (Nd, La) GaO ₃ melt,
d, La) GaO ₃ single crystal was adjusted so as not to be cooled above (melting point −60) ° C., and twin-free (Nd, L
a) After the pulling step of the GaO ₃ single crystal is completed, the twin-free (Nd, La) GaO ₃ single crystal is cooled to room temperature at a cooling rate of 60 ° C./hr or less.

Further, the twinless (Nd, La) G according to the present invention.
An aO ₃ single crystal is produced by bringing a seed crystal into contact with a (Nd, La) GaO ₃ melt in a pulling furnace for producing a single crystal, pulling the crystal, and then cooling the twin crystal without twin (Nd, La) GaO ₃ single crystal. Wherein the (Nd, La) GaO ₃ melt has a composition for forming a Nd _{1 -x} La _x GaO ₃ (0 <x <0.3) single crystal, and Is 45 ° C./cm or less immediately above the liquid surface of the (Nd, La) GaO ₃ melt,
d, La) GaO ₃ single crystal was adjusted so as not to be cooled above (melting point −60) ° C., and twin-free (Nd, L
a) GaO ₃ after the pulling step the end of the single crystal, matchless crystal (Nd, La) GaO ₃ After cooling to room temperature single crystal 60 ° C. / hr or less in the cooling rate, matchless crystals produced (N
d, La) annealing a GaO ₃ single crystal at a temperature of 1000 ° C. or higher.

Further, the single crystal pulling apparatus according to the present invention comprises:
A crucible for melting the melt of (Nd, La) GaO ₃ , heating means for heating the melt in the crucible, and a taper surrounding the side surface of the single crystal pulled from the opening of the crucible and extending in the pulling direction. Having a hollow body after heater,
A single crystal pulling apparatus comprising: a heat retaining cylinder surrounding the outside of the after-heater, wherein the (Nd, La) G
The temperature gradient of the aO ₃ single crystal in the pulling axis direction is (Nd, L
a) The temperature should be 45 ° C./cm or less immediately above the liquid surface of the GaO ₃ melt, and the (Nd, La) GaO ₃ single crystal during pulling should not be cooled beyond (melting point−60) ° C. Adjustments have been made.

The configuration of the present invention will be described below.

A twinless (Nd, La) GaO according to the present invention
_{The 3} single crystal is a mixed crystal of (Nd, La) GaO ₃
It has a composition of d _1-x La _x GaO ₃ (0 <x <0.3).

LaGaO ₃ , which is one component of the (Nd, La) GaO ₃ mixed crystal single crystal, has two structural phase transformation points between room temperature and its melting point, and this structural phase transformation is a single crystal. It causes cracks and twins that occur during fabrication. As a result of detailed thermal analysis of the (Nd, La) GaO ₃ mixed crystal, the present inventor has found that the above-mentioned structural phase transformation exists in the composition 0 <x <0.3 represented by Nd _{1 -x} La _x GaO _3. And a composition range in which a single crystal free of cracks and twins can be produced. FIG. 1 is a diagram showing the relationship between the La concentration, the melting point, and the phase transformation temperature. In the figure, the solid line shows the melting point, and ○ and × show the literature values and measured values of the phase transformation temperature, respectively. As is clear from this figure, in the composition where x is less than 0.3, no structural phase transformation exists from room temperature to the melting point. That is, the above composition is a composition in which no structural phase transformation is present in a temperature range from room temperature to a melting point, and is a composition range in which a (Nd, La) GaO ₃ single crystal without cracks or twins can be produced.

Next, the twin-free (Nd, La) Ga of the present invention
A method for producing an O ₃ single crystal will be described. In the present invention, a seed crystal is (Nd, La) in a pulling furnace for producing a single crystal.
After being brought into contact with the GaO ₃ melt, pulled up, and then cooled to produce a twin-free (Nd, La) GaO ₃ single crystal, first, a single Nd _{1 -x} La _x GaO ₃ (0 <x <0.3) single crystal The (Nd, La) GaO ₃ melt having a composition to become a crystal is formed.

The raw material of the melt in the method for producing a single crystal of the present invention is:
Powders of Nd ₂ O ₃ , La ₂ O ₃ and Ga ₂ O ₃ can be used. These oxide powders are preferably high purity products of 99.99% or more. These oxides can be mixed and immediately melted with a crucible, but after pressing the mixed powder, using a sintered raw material reduces the evaporation of the raw material during melting. Preferred above. In addition, the sintered material has a desired composition ratio (Nd, L
a) GaO ₃ can be used, and (Nd, L
a) Since GaO ₃ is a mixed crystal oxide, its constituent oxides, LaGaO ₃ and NdGaO ₃ , can be once prepared, and their sintered bodies can be filled in a crucible to have a desired composition. A crucible is filled with a sintered material mixed with a desired composition, and is heated to 1500 to 160 by high-frequency heat or resistance heating.
Melt at 0 ° C. to form a melt.

Next, a pulling rod on which a seed crystal is attached is brought into contact with the melt from above, and the pulling rod is pulled while rotating to grow a (Nd, La) GaO ₃ single crystal.

At this time, it is necessary that the temperature gradient on the melt surface of the pulling furnace satisfy two temperature gradient conditions. That is, first, the temperature gradient in the pulling axis direction in the single crystal pulling furnace is set to 45 ° C./cm or less immediately above the liquid level of the (Nd, La) GaO ₃ melt.

The present inventors measured the temperature gradient in a conventional pulling furnace as shown in FIG. 5, and found that the temperature had a very high temperature gradient of about 70 ° C./cm. This high temperature gradient (Nd, La) in GaO ₃ single crystal growth is responsible for inducing excessive thermal stresses extent to generate cracks and twin on the single crystal, (Nd, La) GaO ₃ melt Cracking and twinning occur when the temperature gradient is set gently just above the liquid surface, and the (Nd, La) GaO ₃ single crystal during pulling is not excessively cooled and a sharp temperature gradient is not formed in the single crystal. It was found that it was effective for prevention. That is, when the temperature gradient directly above the (Nd, La) GaO ₃ melt exceeds 45 ° C./cm, thermal stress that generates cracks and twins immediately above the melt is generated, leading to generation of cracks and twins. A certain temperature gradient of the single crystal during pulling is controlled to be gentler than before. Comparing the temperature gradient between the conventional method and the method of the present invention in the above section, the conventional method shows that the temperature gradient is steep throughout the section.

Next, the second temperature gradient will be described.

Even if the temperature gradient immediately above the (Nd, La) GaO ₃ melt is set at 45 ° C./cm or less, the (Nd, La) GaO ₃ single crystal being pulled still has a melting point of −60.
If the temperature gradient exceeds ℃, a thermal stress that generates cracks and twins is generated in the single crystal, which leads to the generation of cracks and twins.

In order to create a temperature gradient that does not generate cracks and twins, it is necessary to use (Nd, La) Ga in the crucible.
When the heating means of the O ₃ melt is a high-frequency induction coil,
In addition to the components of the conventional pulling apparatus described above, it is necessary to add an after-heater, adjust the position of the after-heater, adjust the position of the high-frequency induction coil, etc., as shown in the single crystal pulling apparatus shown in FIG. . Further, when heating the crucible by resistance heating, a multi-stage heater is provided, and a predetermined temperature gradient can be set by adjusting each heater.

Next, the (Nd, La) GaO ₃ pulled up
The single crystal is cooled to room temperature at a cooling rate of 60 ° C./hr or less.

Further, even when the generation of cracks and twins is prevented by using a method in which two temperature gradients are set, twins may be slightly generated. The present inventor has discovered that the cause is thermal stress generated in the pulled twinless (Nd, La) GaO ₃ single crystal when the single crystal is cooled to room temperature. That is, as is apparent from FIG. 2 illustrating the critical cooling rate corresponding to the critical stress, cooling to room temperature at a cooling rate of 60 ° C./hr or less more stably prevents twins generated at the time of cooling. I found what I could do. In the cooling step after pulling, the cooling rate at which cracks and twins are easily generated in the grown single crystal is moderated, which is effective in preventing cracks and twins.

In the method described above, twinless (Nd, L
a) Although a GaO ₃ single crystal can be produced, in the present invention, in order to prevent cracks and twins from occurring during standing in a grown ingot, the method of the present invention requires 1000 g after pulling and cooling. Annealing can be performed at a temperature of not less than ° C.

Annealing is a means for reducing the thermal stress distortion remaining in the ingot after pulling. This method is effective for preventing cracks and twins that occur during standing after being taken out of the furnace by the conventional method. As a result, the ingot can be uniformly heated over the entire length and cross section, and the thermal stress strain remaining in the ingot after pulling / cooling can be removed to reduce cracks and twins. Annealing may be performed by using an annealing furnace independent of the pulling furnace, or, if good soaking conditions are obtained, annealing may be performed continuously after pulling in the pulling furnace.

Next, a pulling furnace for performing the manufacturing method of the present invention will be described.

That is, as shown in FIG. 3, a pulling furnace for carrying out the present invention heats a crucible 1 for melting a melt of (Nd, La) GaO ₃ and a melt 5 in the crucible 1. Heating means 3, a hollow after-heater 2 surrounding the side surface of the single crystal pulled from the opening of the crucible 1 and having a taper in the pulling direction, and a heat retaining cylinder 4 surrounding the outside of the after-heater 2. A temperature gradient in the direction of the pulling axis of the (Nd, La) GaO ₃ single crystal is set to 45 ° C./cm or less directly above the liquid surface of the (Nd, La) GaO ₃ melt. In addition, the (Nd, La) GaO ₃ single crystal that is being pulled is melted (melting point−
60) Adjustments have been made to satisfy the requirement not to cool the temperature beyond ℃. Reference numeral 6 denotes a quartz tube, and reference numeral 7 denotes a thermocouple.

The present inventor has studied various structures of the pulling furnace, and as a result, the after-heater 2 has at least the above (Nd, L
a) It has been found that cracks and twins can be prevented by adopting a structure existing in a section of 20 mm or more from the liquid surface of the GaO ₃ melt. Also, by adjusting the position of the high-frequency induction coil, the temperature gradient in the after-heater can be reduced, and the generation of cracks and twins in the (Nd, La) GaO ₃ single crystal during pulling can be prevented more stably. I found

Hereinafter, the present invention will be described in more detail with reference to an embodiment with reference to FIG.

[0031]

EXAMPLE 1 Equimolar amounts of Nd ₂ O ₃ , Ga ₂ O ₃ and La
After mixing ₂ O ₃ and Ga ₂ O ₃ , and pressing each
Sintered at ° C., to produce a NdGaO ₃ sintered and LaGaO ₃ sintered body. The obtained sintered body was subjected to composition Nd _0.8 La _0.2 G
It was placed in an iridium crucible 1 having a diameter of 50 mm so as to obtain aO _3, and was melted at 1600 ° C. in an atmosphere in which 97% nitrogen and 3% oxygen were mixed by heating with a high-frequency coil 3.

Above the iridium crucible 1 of the pulling apparatus, an iridium after heater 2 was provided in a section of 20 mm to 80 mm from the liquid surface of the melt 5 with a taper angle of about 70 °. On the outside thereof, an alumina heat retaining cylinder 4 was provided coaxially. Thus, the temperature gradient in the crystal pulling section is moderated.

A seed crystal (4 mm in diameter) attached to the tip of a pulling rod (not shown) was brought into contact with the melt 5 to rotate at a rotation speed of 2 mm.
A single crystal was grown at 0 rpm at a pulling speed of 1.0 mm / hr while controlling the output of the high-frequency coil 3.

FIG. 4 shows the temperature distribution in the direction of the single crystal pulling axis obtained by the above method. The temperature gradient just above the melt is 45
° C / cm, and the temperature in the single crystal pulling section is below the melting point.
It did not cool down above ° C.

The cooling was performed at a cooling rate of 60 ° C./hr or less.

Four ingots were produced by the above method, but no cracks or twins were generated during single crystal growth and single crystal cooling.

The manufactured twinless Nd _0.8 La _0.2 Ga
YBa ₂ Cu at film formation temperature based on lattice constant of O ₃ single crystal
_When the lattice matching ratio with ₃ O _x was estimated, it was 0.002%, which revealed that the lattice matching ratio was the best as compared with the conventional substrate. Therefore, this single crystal has no cracks or twins, and is a single crystal useful as a substrate for growing a YBa ₂ Cu ₃ O _x oxide superconductor thin film that satisfies pseudo homoepitaxial growth conditions during film formation. Can be.

[0038]

COMPARATIVE EXAMPLE The ingot was pulled in the same manner as in Example 1 except that no after-heater was added using the apparatus shown in FIG. 5, and the temperature gradient in the single crystal pulling section was 70 ° C./cm. Cracks occurred during growth of the four ingots, and cracks and twins occurred during the standing of the three ingots.

[0039]

Example 2 An ingot in which cracks did not occur during growth in Example 1 (slowly cooled state after growth) was immediately placed in a muffle furnace having excellent heat uniformity, and heated at 20 ° C./hr in oxygen.
After annealing at 1000 ° C. for 10 hours and cooling at 20 ° C./hr, no increase in the number of cracks or twins during subsequent standing occurred.

[0040]

As described above, according to the present invention, in the production of (Nd, La) GaO ₃ single crystal, the composition is represented by the composition 0 <x <represented by Nd _{1 -x} La _x GaO _3.
Only 0.3, cracks and twins can be prevented from being generated both during growth of the single crystal and during standing, thereby contributing to improvement in yield and production of large substrates.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between La concentration, melting point, and phase transformation temperature.

FIG. 2 is a diagram illustrating a critical cooling rate corresponding to a critical stress.

FIG. 3 is a cross-sectional view showing a structure of a single crystal pulling apparatus for performing the manufacturing method of the present invention.

FIG. 4 is a diagram showing a temperature gradient in the first embodiment.

FIG. 5 is a sectional view of a conventional single crystal pulling apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crucible 2 After-heater 3 High-frequency heating coil 4 Insulation tube 5 Melt 6 Quartz tube 7 Thermocouple

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-144994 (JP, A) JP-A-4-114993 (JP, A) JP-A-2-27585 (JP, A) JP-A-3- 199199 (JP, A) JP-A-4-97989 (JP, A) JP-A-3-199198 (JP, A) JP-A-3-279250 (JP, A) OISHI AKIRA et al. , "New substrate La1-xNdxGa03 for Hi gh-Tc YBa2Cu30x Epi taxial Films," Adva nces in Supercondu ctivity V, Proceedi ngs of the 5th Int ernational Symposi um on Superconduct ivity (ISS-92), Meeti ng Date Nov. 1992, pp. 869-872 (58) Fields investigated (Int. Cl. ⁷ , DB name) C30B 29/24 CA (STN) JICST file (JOIS) REGISTRY (STN)

Claims (3)

(57) [Claims] 1. Nd _1-x La _x GaO ₃ (0 <x <0.3)
Twin (Nd, La) GaO ₃
Single crystal. 2. A seed crystal (N) is pulled in a pulling furnace for producing a single crystal.
d, La) GaO ₃ is brought into contact with the melt, after pulled up, matchless crystal cooling (Nd, La) in the manufacturing method of GaO ₃ single crystal, the (Nd, La) GaO ₃ melt Nd _1-x La
_x GaO ₃ (0 <x <0.3) has a composition to become a single crystal, and the temperature gradient in the pulling axis direction in the single crystal pulling furnace is directly above the liquid level of the (Nd, La) GaO ₃ melt. At 45
° C / cm or less, and (Nd, La) GaO
_{3 The} single crystal is adjusted so as not to be cooled above (melting point −60) ° C., and after the above-mentioned step of pulling the twin-free (Nd, La) GaO ₃ single crystal, the twin-free (Nd, La) Ga
A twin-free (Nd, La) GaO ₃ characterized by cooling an O ₃ single crystal to room temperature at a cooling rate of 60 ° C./hr or less.
Single crystal production method. 3. A seed crystal (N) in a pulling furnace for producing a single crystal.
d, La) GaO ₃ is brought into contact with the melt, after pulled up, matchless crystal cooling (Nd, La) in the manufacturing method of GaO ₃ single crystal, the (Nd, La) GaO ₃ melt Nd _1-x La
_x GaO ₃ (0 <x <0.3) has a composition to become a single crystal, and the temperature gradient in the pulling axis direction in the single crystal pulling furnace is directly above the liquid level of the (Nd, La) GaO ₃ melt. At 45
° C / cm or less, and (Nd, La) GaO
_{3 The} single crystal is adjusted so as not to be cooled above (melting point −60) ° C., and after the above-mentioned step of pulling the twin-free (Nd, La) GaO ₃ single crystal, the twin-free (Nd, La) Ga
After cooling the O ₃ single crystal to room temperature at a cooling rate of 60 ° C./hr or less, the manufactured twinless (Nd, La) GaO ₃
A method for producing a twin-free (Nd, La) GaO ₃ single crystal, comprising annealing a single crystal at a temperature of 1000 ° C. or higher.