JPH04164890A - Method for growing crystal with phase transition - Google Patents

Abstract

PURPOSE:To control the formed state of a twin and grow an excellent crystal by controlling the shape of a solid-liquid interface in a crystal growing part so as to be flattened or protruded on the crystal side in growing the crystal from a melt of a crystal system causing phase transition in a cooling process after growing the single crystal. CONSTITUTION:A melt 3 of a crystal system (e.g. LaAlO3) capable of causing phase transition in a cooling process after growing a single crystal from a melt by a melt growing method is prepared. A crystal 2 is then pulled up and grown from the aforementioned melt 3 by the Czochralski method, etc. In the process, the shape of the interface 1 between the solid and the liquid in a crystal growing part is controlled so as to be flattened or protruded on the side of the crystal 2. Thereby, the crystal 2 in which a twin enters in one direction in a plane such as (100) can be obtained. A wafer obtained from the aforementioned crystal 2 is suitably used as a material for superconducting thin film substrates, etc., having the high critical temperature.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for growing crystals with phase transition, and in particular, a material that has attracted some attention as a substrate material for high Tc superconducting thin films. Lantern Aluminate (LaAQOs)
) The present invention relates to a method of growing a crystal by a melt growth method, such as the Czochralski method, by controlling the state of twin crystal formation to grow a good crystal.

[Prior Art] Single crystals of LaAlO3 have recently attracted attention as substrate materials for high Tc superconducting thin films.

By the way, melt growth methods such as the Czochralski method, the Bridgman method, and the floating zone method have been proposed as methods for growing crystals, and the Czochralski method is mainly used for growing LaAl2O crystals in particular. In these melt growth methods, especially the Czochralski method, L, a A 120 , , N d A 120
, , L a G a O, etc., which undergo a phase transition during the cooling process, are extremely difficult to grow as single crystals. In other words, crystals are generally prone to cracks and twins;
Single crystals cannot be obtained. L a a! ! , Os, the degree of phase transition is small and the occurrence of cracks can be almost ignored, but twins occur all over the surface. To explain this point in detail, L
The aAl2O crystal can be grown using, for example, the Czochralski method, but this crystal undergoes a phase transition during the cooling process after crystal growth. Phase transition temperature is 435-530
It is said that the cubic crystal system changes to the rhombohedral system around ℃,
The degree of transition is very small, about 6 degrees, but
At this temperature of phase transition, twins form throughout the system. Therefore, in the past, attempts have been made to remove the crystallization by applying temperature and stress as a post-treatment, and a patent application has also been filed.

[Problems to be Solved by the Invention] However, when used as a substrate material for high Tc superconducting thin films, since the superconducting thin film creation temperature is higher than the phase transition temperature, a phase transition occurs during cooling and twins are generated. As a result, the detwinning (single crystallization) effect is impaired.

By the way, twins are characterized by twin planes and twin boundaries. Its orientation is (100) in LaAffO, and (
100) There are two directions perpendicular to each other within the plane. Now, if we tentatively call this mutually orthogonal twin boundary a "plane boundary," then twins in only one direction can be seen in one subboundary in the (100) plane, and the twins of the adjacent plane boundaries The directions tend to be perpendicular to each other. Therefore, by suppressing the generation of plane boundaries and allowing twins to enter only one direction within a certain plane, the resulting thin film substrate (
This is a meaningful improvement from the perspective of wafer homogeneity.

The present invention has been made in view of the above-mentioned conventional situation, and is aimed at suppressing the generation of plane boundaries in the growth of crystals with phase transitions, and producing crystals with twins in only one direction within the plane. The purpose of the present invention is to provide a method for growing crystals with phase transitions that can obtain .

[Means for Solving the Problem] The method of growing a crystal with a phase transition according to claim (1) is a method of growing a solid phase of a single crystal by pulling it up from the liquid surface of a melt. In a method of growing a crystal from a crystalline melt that undergoes a phase transition during the cooling process after growing the crystal,
The method for growing a crystal with a phase transition according to claim (2), characterized in that the shape of the solid/liquid interface in the crystal growth section is controlled to be flat or convex toward the crystal side. The method (1) is characterized in that the grown crystal is LaAlO3.

The present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a cross-sectional view illustrating one implementation method of the method of growing a crystal with a phase transition according to the present invention.

The method of the present invention is applicable to crystal systems that undergo a phase transition during the cooling process after crystal growth, such as L a A (l Os).
When growing crystals from melt, as shown in Figure 1,
The crystal 2 is grown from the melt 3 by making the shape of the solid-liquid interface l flat or convex toward the crystal 2 side. If this solid-liquid interface 1 is excessively convex toward the crystal 2 side, voids (pores) and shrinkage pores tend to be generated, which is not preferable. Therefore, in the present invention, the height h of the convex portion of the solid-liquid interface 1 toward the crystal 2 side is set to be approximately 0 to 0.2 d relative to the diameter d of the grown crystal 2 shown in FIG. It is preferable to do so. In FIG. 1, R indicates the direction of pulling the crystal, and ω indicates the direction of rotation.

Note that the method of the present invention is applicable not only to L a A 120 s but also to N d A 120 s , L a Ga O-
It is also effective for other crystal systems that undergo a phase transition during the cooling process, such as LaAlO. It is valid for

[Effect] Crystal heterogeneity due to the generation of twins is thought to have a subtle effect on subsequent processing steps, and obtaining crystals with excellent homogeneity is a key to expanding applications and improving product quality. It is extremely important in terms of For this reason, as mentioned above,
It can be said that suppressing the generation of plane boundaries so that the twins enter only in one direction within the (100) plane is an improvement item that can bring about good results.

The present inventors have suppressed the generation of plane boundaries so that the twin crystals (1
We have conducted repeated studies to obtain a highly homogeneous crystal that extends in only one direction within the 001,e plane, and for this purpose, the key point is how to control the strain distribution of the crystal during phase transition. That is, we focused on the fact that the temperature gradient during crystal growth, the shape of the solid/liquid interface, the cooling rate, the crystal size, etc. are considered to be the factors. Among these, we focused on the shape of the solid-liquid interface, and at the beginning of the study, we tried to grow the solid-liquid interface so that it was convex toward the melt side, that is, the solid-liquid interface was concave. , twinning was observed that predicted facet growth of the (100) plane. This was reminiscent of faceted growth of yttrium, aluminum, and garnet. In other words, a surface boundary was generated in the reproducibility<(110) direction. Therefore, when a crystal was grown under conditions in which this interface was convex toward the crystal side, it was found that there were almost no surface boundaries. As a result, it was possible to obtain a wafer with twins in only one direction within the (100) plane.

The details of the effects of making the solid-liquid interface flat or convex toward the crystal side are not clear.

This is thought to be because when the interface is convex, facet growth occurs and a large strain remains at the facet boundary, whereas when the interface is flat or concave, no large strain remains.

[Examples] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples unless it exceeds the gist thereof. Note that for convenience of explanation, a comparative example will be given first.

Comparative Example 1 An iridium (Ir) crucible with a diameter of 50 mm, a height of 50 mm, and a thickness of 1.5 mm was installed in a 110 KHz high-frequency heating furnace, and 50 mol% of lanthanum oxide with a purity of 99.99% and lanthanum oxide with a purity of 99.99% were placed in the crucible. 3 mol% aluminum raw material
20g was added and melted in an N2 gas atmosphere. The temperature of the melt was lowered to near the melting point, a seed crystal (100) of LaA flooded O3 was immersed, and while rotating at 25 rpm, 180 g of crystal was grown at a pulling rate of about 2 mm/hr and a crystal diameter of 25 mm. Note that during this growth period, the solid-liquid interface is approximately 5
It had a convex shape of about mm. Thereafter, the crystals were cut off, cooled for about 2 days, and then collected.

The obtained crystal was cut out to a thickness of 5 mm in the (100) cross-sectional direction, subjected to distortion-free polishing, and then the state of twin formation was examined. Surface boundaries were observed in the (110) direction and in the core. Within each sub-boundary, twins were generated in only one direction, and between adjacent boundaries, twins were generated that were perpendicular to each other.

Example 1 Crystals were grown and post-processed in the same manner as in Comparative Example 1, except that the rotation speed was 5 Orpm. Note that during crystal growth, the solid-liquid interface had a convex shape of about 3 mm toward the crystal side.

The resulting twinning state was significantly different from that of Comparative Example 1, with twins occurring only in one direction over the entire surface, and almost no plane boundaries were observed.

[Effects of the Invention] As detailed above, according to the method of growing a crystal with a phase transition of the present invention, in a crystal system with a phase transition such as L a A Q O *, a plane such as (100) can be grown. It is possible to obtain a crystal in which the twins are aligned in only one direction. Wafers made of such crystals can be used as thin-film substrate materials such as high-Tc superconducting thin-film substrates, greatly expanding their applications.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view illustrating one implementation method of the method of growing a crystal with a phase transition according to the present invention. 1...solid/liquid interface, 2...crystal, 3...melt. Agent Patent Attorney Tsuyoshi Shigeno

Claims (2)

[Claims] (1) A method of growing a solid phase of a single crystal by pulling it up from the liquid surface of the melt, in which a crystal is grown from a crystalline melt that undergoes a phase transition during the cooling process after growing a single crystal from the melt. A method for growing a crystal with a phase transition, characterized in that the shape of the solid-liquid interface in the crystal growth region is controlled to be flat or convex toward the crystal side. (2) The growing method according to claim (1), wherein the grown crystal is LaAlO_3.