JP3558528B2 - Crystal manufacturing method - Google Patents
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- JP3558528B2 JP3558528B2 JP22687398A JP22687398A JP3558528B2 JP 3558528 B2 JP3558528 B2 JP 3558528B2 JP 22687398 A JP22687398 A JP 22687398A JP 22687398 A JP22687398 A JP 22687398A JP 3558528 B2 JP3558528 B2 JP 3558528B2
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Description
【0001】
【産業上の利用分野】
本発明は、引き上げ法による単結晶成長法に係わり、ニオブ酸ストロンチウム・バリウム(SBN)などのように熱伝導率が低いために凝固熱の種結晶側への放散が困難な結晶において、大口径・長尺の単結晶を製造するための方法に関するものである。
【0002】
【従来の技術】
従来、高品質が要求される光学単結晶の成長には引き上げ法が用いられてきた。しかし、ニオブ酸ストロンチウム・バリウム(SBN)結晶などのように潜熱が大きく熱伝導率の低い結晶では引上げ法により大口径で長尺の単結晶が得られないという問題があった。これは、結晶成長時に放出される潜熱が熱伝導によって結晶から引き上げ軸側に放出されないため、成長界面付近に熱が蓄積されて界面近傍の融液温度上昇を引き起こし、結晶成長が停止するためである。この解決方法として径方向の温度勾配が大きく取れる高周波(RF)加熱法を用い、結晶成長開始より融液温度を順次低下させて潜熱を融液側に放出し、成長界面の温度を適正な温度に保つことにより長尺の結晶を得る方法がSBN結晶成長で開発された。
【0003】
しかしながら、この方法ではアフターヒーターを利用しても径方向の温度勾配が20℃/cm以上と大きくしか設定できないので、SBN結晶特有の性質として結晶肩部での直径を拡大することができず、直径が30mm以上の大口径の結晶が得られなかった。
【0004】
一方、径方向の温度勾配が小さく設定出来る方法として抵抗加熱法がある。この方法では径方向温度勾配を20℃/cm程度にすることができるため、肩部での結晶径を拡大することができる。
【0005】
しかし、次のステップとして、一旦径を拡大したことにより発生する大量の潜熱の放出が今度は低温度勾配のために効率的に行われず長尺の結晶が得られないという問題あった。したがって、SBN結晶は電気光学効果、非線形光学効果、記録材料としての光感度などの優れた特性を有するにもかかわらず、大面積SBN結晶が得られなかったため光デバイスへの利用が出来ないというのが現状であった。
【0006】
【発明が解決しようとする課題】
従来の方法で熱伝導率の低い結晶を成長させる場合、RF加熱などの高温度勾配成長法では結晶径が大きくならず、抵抗加熱などの低温度勾配成長法では結晶成長界面の中心部付近に蓄積される熱の排除が効率的に行われないため、いずれの場合も大口径、長尺の得られないという課題を有している。この問題解決の方法としては、結晶成長初期段階では径方向の温度勾配を低温度勾配とし、中期段階以降は高温度勾配にするのが一つの解決策である。
【0007】
そこで本発明では、抵抗加熱を用いた低温度勾配成長法においてルツボ底を冷却することにより温度を調節し、結晶成長過程に合わせて径方向の温度勾配が任意の値に設定できるようにし、最初に結晶径を拡大し、一旦径を拡大した段階では結晶成長面の中心付近で発生する潜熱を効果的に融液内に吸収して結晶成長を継続させることによって、大口径で長尺の結晶の製造ができる方法を提供するのが課題である。
【0008】
【課題を解決するための手段および作用】
上記課題を解決するため、本発明による結晶成長方法は、低温度勾配炉を用いた引き上げ法による単結晶成長において、耐熱性の円筒多重管とガス流方向転換用円筒部とを組み合わせ、冷却ガスを直接ルツボ底に吹き付けて局所的に融液を冷却し、融液の深さを20〜30mmとし、ルツボの直径に対する融液の深さの比を3/10以下として、融液の流れを止め、かつ成長界面とルツボ底との距離を短くすることにより融液内の径方向に周辺部が高く中心部が低くなるように温度勾配を形成し、かつ結晶成長過程に合わせた冷却ガス流量の調整により前記径方向の温度勾配の大きさを調整することを特徴とする。
【0009】
また、本発明による第二の結晶成長方法は、低温度勾配炉を用いた引き上げ法による単結晶成長において、耐熱性の円筒ルツボ支持台の下部雰囲気を冷却し、かつ前記支持台の内側に挿入した耐熱性の上下可動のシリンダーの位置を変化させてルツボ底温度を調節し、融液の深さを20〜30mmとし、ルツボの直径に対する融液の深さの比を3/10以下として、融液の流れを止め、かつ成長界面とルツボ底との距離を短くすることにより、融液内の径方向に周辺一部が高く中心部が低くなるように温度勾配を形成し、かつ結晶成長過程に合わせてその勾配の大きさを調整することを特徴とする。
【0010】
更に本発明による第三の結晶成長方法は、低温度勾配炉を用いた引き上げ法による単結晶成長において、耐熱性の円筒ルツボ支持台の下部雰囲気を冷却し、かつ前記支持台の内側に挿入した小型ヒーターの加熱によりルツボ底温度を調節し、融液の深さを20〜30mmとし、ルツボの直径に対する融液の深さの比を3/10以下として、融液の流れを止め、かつ成長界面とルツボ底との距離を短くすることにより、融液内の径方向に周辺部が高く中心部が低くなるように温度勾配を形成し、かつ結晶成長過程に合わせてその勾配の大きさを調整することを特徴とする。
前記従来例の課題を解決する具体的手段として本発明は、まず結晶成長初期段階では、第1の手段として抵抗加熱などによる低温度勾配炉を用いて融液の径方向の温度勾配を20℃/cm以下と小さくして結晶径の拡大を図る。
【0011】
次の段階では、第2の手段としてルツボ下部を冷却することにより径方向の温度勾配を増大させて20〜150℃/cmとし、結晶成長面の中心付近で発生する潜熱を効果的に融液内に吸収して結晶成長を継続させ長尺化を図る。
【0012】
このように結晶成長過程に合わせて径方向の温度勾配を任意の値に調整することにより、結晶肩部での径拡大と肩部以降の直胴部伸長を行うことができる。 なお、成長界面付近に蓄積された熱を効率的に融液内に排除させるために界面とルツボ底との距離を短くさせる手段も用いるが、融液深さが浅くなることによりルツボに充填する原料が少量になり結晶化できる融液量が制限される。この場合には、引き上げ結晶重量に応じて追加原料を算定し連続供給することにより有効融液深さを一定に保つことによって長尺の結晶を製造することができる。
【0013】
【実施例1】
本発明をSBN単結晶引き上げに適用した実施例1を図1に示す。
【0014】
図1において1および2は上下2ゾーンの縦型加熱炉の抵抗加熱ヒーターであって、上下のヒーター温度を調整して縦方向の温度勾配を設定する。この場合、縦方向の温度勾配を10〜30℃/cmに設定すると、径方向の温度勾配はその0.5〜1倍で20℃/cm以下の勾配となった。3は直径10cmの白金製ルツボであり、4はSBN単結晶の調和溶融組成を持つ融液であって温度1450〜1600℃、深さ20〜30mmであり、5は成長中のSBN単結晶である。6は単結晶引き上げ棒であって、最初にSBN種結晶を結びつけて融液に接触させ融液温度を調節しながら矢印の方向に回転と引き上げを行うと単結晶が引き上がる。回転速度は結晶径に合せて3〜30rpm、引き上げ速度は0.1〜10mm/時とした。7はセラミックス製の耐熱性の円筒2重管であってルツボの支持とガスの流れを制限している。
【0015】
なお、ルツボ底はガス冷却前には加熱されている必要があるため熱伝達の良い材料を用いている。8は冷却ガスの供給管であって、毎分0〜20リットルの空気、アルゴン、ヘリウムガス等の冷却用ガス(供給時は室温)を供給する。9はルツボと一体化した白金製のガス流方向転換の円筒であって、8より冷却ガスをルツボ底に吹き付け、吹き出たガスはガス流方向転換用の円筒9によって方向転換されAの穴を通過して炉内に排出される。10は熱遮断用の耐熱性繊維である。このような構造にすることにより、まず冷却ガスはルツボ底に吹き付けられてルツボ中心部直上の融液を冷却する。
【0016】
しかし、吹き出たガスは9および7を通過する間に急激に加熱され冷却能力を失うので、ルツボ下部のガス流方向転換用の円筒9の外側の融液に対しては冷却作用がない。したがって融液はルツボ直上を中心に局所的に冷却されるので、融液内の径方向に中心部が低く周辺部が高くなるように径方向温度勾配が形成できた。
【0017】
大口径、長尺の結晶を製造するためには、まず結晶成長の初期段階では、冷却ガス流量を0〜5リットル/分と抑えて融液の径方向の温度勾配を20℃/cm以下に保ち結晶径を所望の径まで拡大させる。
【0018】
次の段階では、発生した潜熱分を吸収するように冷却ガス流量を順次5リットル1分から最大20リットル/分まで増加することにより径方向の温度勾配を20〜150℃/cmと増大させて結晶成長を継続させ長尺化した。なお、最適ガス流量は予め結晶径とルツボ底温度の増加との関係等より決定した量で行った。
【0019】
このようにして結晶径30〜50mm、長さ50〜100mmのC軸方位のSBN単結晶を再現性良く製造することができた。
【0020】
【実施例2】
図2は本発明をSBN単結晶引き上げに適用した実施例2であって、図1との相違点と円筒中の温度分布を示す。
【0021】
図2において11は耐熱性の上下可動のシリンダーであり、12は耐熱性の円筒ルツボ支持台である。本実施例では円筒ルツボ支持台の下端を開放することにより下部雰囲気を冷却し、かつ内側に挿入した耐熱性の可動のシリンダーの位置を上下に変化させてルツボ底温度を調節した。原理を図2で説明する。
【0022】
まず、シリンダーがない場合の円筒中の温度分布は下部が冷却されているため、ルツボ下部より開放端に向けて温度が低下している。
【0023】
ここに、シリンダーを入れると上部から下部への熱の流れが遮断されるので、P1の位置にすると右のT1の温度分布となり、P2の位置にするとT2の温度分布となる。このP1およびP2の位置を調整することにより、ルツボ底の温度を実施例1と同様に調節できた。
【0024】
したがって融液はルツボ直上を中心に局所的に冷却されるので、融液内の径方向に中心部が低く周辺部が高くなるように径方向温度勾配が形成できた。このようにして結晶径30〜50mm、長さ50〜100mmのC軸方位のSBN単結晶を再現性良く製造することができた。
【0025】
【実施例3】
図3は本発明をSBN単結晶引き上げに適用した実施例3であって、実施例2の図2との相違を示す。
【0026】
図3において、13は小型ヒーターで12の円筒内に挿入されている。このヒーターの温度を変化させることにより実施例2の可動シリンダーと同じ効果が実現できた。したがって融液はルツボ直上を中心に局所的に冷却されるので、融液内の径方向に中心部が低く周辺部が高くなるように径方向温度勾配が形成できた。このようにして結晶径30〜50mm、長さ50〜100mmのC軸方位のSBN単結晶を再現性良く製造することができた。
【0027】
【実施例4】
図4は本発明をSBN単結晶引き上げに適用した実施例4であって実施例1との相違点のみを示す。14はルツボ下部液側に窪みを設けた変形ルツボである。本実施例ではルツボの直径を200mmにし、窪みを円筒形状にし直径を30mmとした。さらに、融液表面からルツボ窪みの頂上までの距離を5mmとした。
【0028】
このようにルツボ底の一部を融液側に窪ませて結晶成長界面とルツボ底との距離を短くすることによって、ガス吹き出し口と成長界面とが近づくため成長界面付近に蓄積された熱を効率的に吸収することができた。
【0029】
実施例1において融液深さを浅くした場合と本実施例との差異は、単に深さを浅くした場合は少量の融液充填のため熱容量が小さくなり温度変動の影響を受け易くなって良好な結晶成長が困難になるが、本実施例では融液量が多いため安定な温度分布が得られ良好な結晶育成を行えることである。ただし、結晶化できる融液の深さが本実施例では5mmしかないため、ルツボ径を大きくして結晶化できる融液量を多くした。この結果、結晶径30〜50mm、長さ50〜100mmのC軸方位のSBN単結晶を再現性良く製造することができた。
【0030】
【実施例5】
図5は本発明をSBN単結晶引き上げに適用した実施例5であって実施例1との相違点のみを示す。
【0031】
図5の15は金属製の円柱で、融液径方向中心に沈めてヒートシンクとした。本実施例では白金製の直径30mmの円柱を用いた。また、ルツボの直径を大きくすることが必要なため、200mmにした。さらに、融液表面から白金円柱の頂上までの距離を5mmとした。
【0032】
このようにして、実施例4と同様に結晶成長界面とルツボ底との距離を短くすることによって、ガス吹き出し口と成長界面とが近づくため成長界面付近に蓄積された熱を効率的に吸収することができた。実施例1において融液深さを浅くした場合との差異は、単に深さを浅くした場合は少量の融液充填のため熱容量が小さくなり温度変動の影響を受け易くなって良好な結晶成長が困難になるが、本実施例では融液量が多いため安定な温度分布が得られた。ただし、結晶化できる融液の深さが本実施例では5mmしかないため、ルツボ径を大きくして結晶化できる融液量を多くした。この結果、結晶径30〜50mm、長さ50〜100mmのC軸方位のSBN単結晶を再現性良く製造することができた。
【0033】
【実施例6】
図6は本発明をSBN単結晶引き上げに適用した実施例6であって、実施例4の図4の相違点を示す。
【0034】
図6において、16は重量センサーであり、17は原料供給制御装置であり、18は補充されるSBN原料である。本実施例は実施例4のような大口径のルツボが使用できない場合、あるいは、連続原料供給してより長尺な結晶を製造する場合の実施例である。
【0035】
本実施例では窪みを円筒状にし直径を30mmとし、ルツボの直径を100mmにした。さらに、融液表面からルツボ窪みの頂上までの距離を5mmとした。結晶成長に伴う原料融液の減少を16の重量センサーで検出して、その信号を17の原料供給装置に伝え、必要なSBN原料を補充して融液面高さを常に一定にした。この結果、融液表面からルツボ窪みの頂上までの距離が常に5mmに保たれるため、成長界面付近に蓄積された熱を効率的に吸収することができた。この実施例では長尺の結晶を得ることができ、結晶径30〜50mm、長さ200mmのC軸方位SBN単結晶を再現性良く製造することができた。
【0036】
【発明の効果】
以上説明したように、本発明は、まず結晶成長初期段階で、第1の手段として抵抗加熱などによる低温度勾配炉を用いて融液の径方向の温度勾配を小さくして結晶径を拡大し、次の段階では、第2の手段としてルツボ下部を冷却することにより径方向の温度勾配を増大させて結晶成長面の中心付近で発生する潜熱を効果的に融液内に吸収して結晶成長を継続させ長尺化の結晶を得ることができた。 本発明をCeドープのSBN結晶のC軸方位引き上げに用いた。得られた結晶はC軸に平行に2mm厚にスライスして40mm×40mmサイズの板状に加工し、両面研磨したものをデジタルホログラムメモリの記録媒体として使用した。このサイズは従来にないものであり、また、低温度勾配化で製造したため結晶品質およびフォトリフラクティブ特性が良く、高密度の記録装置が実現できた。
【図面の簡単な説明】
【図1】実施例1のルツボ下部冷却による単結晶成長を示す図。
【図2】実施例2のルツボ下部冷却による単結晶成長を示す図。
【図3】実施例3のルツボ下部冷却による単結晶成長を示す図。
【図4】実施例4であって実施例1との相違点のみを示す図。
【図5】実施例5であって実施例1との相違点のみを示す図。
【図6】実施例6の原料連続供給装置を接続した説明図。
【符号の説明】
1,2 抵抗加熱ヒーター
3 白金製ルツボ
4 融液
5 SBN単結晶
6 単結晶引き上げ棒
7 円筒2重管
8 冷却ガスの供給管
9 ガス流方向転換の円筒
10 耐熱性繊維
11 シリンダー
12 円筒ルツボ支持台
13 小型ヒーター
14 変形ルツボ
15 金属製の円柱
16 重量センサー
17 原料供給制御装置
18 SBN原料[0001]
[Industrial applications]
The present invention relates to a single crystal growth method by a pulling method. In a crystal such as strontium-barium niobate (SBN), which has a low thermal conductivity, and thus it is difficult to dissipate heat of solidification to a seed crystal side, a large-diameter crystal is used. The present invention relates to a method for producing a long single crystal.
[0002]
[Prior art]
Conventionally, a pulling method has been used for growing an optical single crystal requiring high quality. However, a crystal having a large latent heat and a low thermal conductivity, such as a strontium-barium niobate (SBN) crystal, has a problem that a long single crystal with a large diameter cannot be obtained by the pulling method. This is because the latent heat released during crystal growth is not released from the crystal by the heat conduction to the pulling shaft side, so heat is accumulated near the growth interface, causing a rise in the melt temperature near the interface and stopping crystal growth. is there. As a solution to this, a high-frequency (RF) heating method that can take a large temperature gradient in the radial direction is used, the melt temperature is gradually reduced from the start of crystal growth to release latent heat to the melt side, and the temperature of the growth interface is adjusted to an appropriate temperature A method for obtaining a long crystal by maintaining the temperature was developed by SBN crystal growth.
[0003]
However, in this method, even if an after heater is used, the temperature gradient in the radial direction can be set only as large as 20 ° C./cm or more, so that the diameter at the crystal shoulder cannot be enlarged as a characteristic characteristic of the SBN crystal, A large-diameter crystal having a diameter of 30 mm or more was not obtained.
[0004]
On the other hand, there is a resistance heating method as a method capable of setting a small temperature gradient in the radial direction. In this method, the temperature gradient in the radial direction can be set to about 20 ° C./cm, so that the crystal diameter at the shoulder can be increased.
[0005]
However, as the next step, there is a problem that a large amount of latent heat generated by once expanding the diameter is not efficiently discharged due to a low temperature gradient, so that a long crystal cannot be obtained. Therefore, despite the fact that the SBN crystal has excellent characteristics such as an electro-optic effect, a non-linear optical effect, and light sensitivity as a recording material, it cannot be used for an optical device because a large-area SBN crystal cannot be obtained. Was the current situation.
[0006]
[Problems to be solved by the invention]
When a crystal with low thermal conductivity is grown by a conventional method, the crystal diameter does not increase in a high-temperature gradient growth method such as RF heating, and in the vicinity of the center of the crystal growth interface in a low-temperature gradient growth method such as resistance heating. Since the accumulated heat is not efficiently removed, there is a problem that a large diameter and a long length cannot be obtained in any case. One solution to this problem is to use a low temperature gradient in the radial direction in the initial stage of crystal growth and a high temperature gradient in the middle stage and thereafter.
[0007]
Therefore, in the present invention, the temperature is adjusted by cooling the crucible bottom in the low temperature gradient growth method using resistance heating, so that the temperature gradient in the radial direction can be set to an arbitrary value in accordance with the crystal growth process. The crystal diameter is enlarged, and once the diameter is enlarged, the latent heat generated near the center of the crystal growth surface is effectively absorbed into the melt and crystal growth is continued, so that a large-diameter long crystal is obtained. The problem is to provide a method that allows the production of
[0008]
Means and action for solving the problem
In order to solve the above-mentioned problems, a crystal growth method according to the present invention combines a heat-resistant cylindrical multi-tube and a gas flow direction changing cylindrical portion in a single crystal growth by a pulling method using a low temperature gradient furnace, and a cooling gas. Is sprayed directly on the bottom of the crucible to locally cool the melt, the depth of the melt is set to 20 to 30 mm, the ratio of the depth of the melt to the diameter of the crucible is set to 3/10 or less, and the flow of the melt is reduced. By stopping and shortening the distance between the growth interface and the crucible bottom, a temperature gradient is formed so that the peripheral part is high in the radial direction and the central part is low in the melt, and the cooling gas flow rate is adjusted to the crystal growth process. The size of the temperature gradient in the radial direction is adjusted by adjusting the temperature gradient.
[0009]
Further, in the second crystal growth method according to the present invention, in the single crystal growth by a pulling method using a low-temperature gradient furnace, the lower atmosphere of the heat-resistant cylindrical crucible support is cooled, and inserted inside the support. The crucible bottom temperature is adjusted by changing the position of the heat-resistant vertically movable cylinder , the melt depth is set to 20 to 30 mm, and the ratio of the melt depth to the crucible diameter is set to 3/10 or less. By stopping the flow of the melt and shortening the distance between the growth interface and the crucible bottom, a temperature gradient is formed so that the peripheral part is high in the radial direction in the melt and the central part is low, and the crystal grows. It is characterized in that the magnitude of the gradient is adjusted according to the process.
[0010]
Further, in the third crystal growth method according to the present invention, in a single crystal growth by a pulling method using a low temperature gradient furnace, the lower atmosphere of the heat-resistant cylindrical crucible support is cooled and inserted inside the support. adjust the Le Tsu ball bottom temperature by heating a small heater, the depth of the melt and 20 to 30 mm, the ratio of the depth of the melt to the diameter of the crucible as 3/10 or less, stop the flow of melt, In addition, by decreasing the distance between the growth interface and the crucible bottom, a temperature gradient is formed so that the peripheral part is high and the central part is low in the radial direction in the melt, and the magnitude of the gradient is adjusted according to the crystal growth process. It is characterized by adjusting the length .
As a specific means for solving the problems of the conventional example, the present invention firstly uses a low temperature gradient furnace such as resistance heating as a first means to reduce the radial temperature gradient of the melt to 20 ° C. in the initial stage of crystal growth. / Cm or less to increase the crystal diameter.
[0011]
In the next stage, as a second means, the temperature gradient in the radial direction is increased to 20 to 150 ° C./cm by cooling the lower part of the crucible, and the latent heat generated near the center of the crystal growth surface is effectively melted. The crystal is absorbed and the crystal growth is continued to increase the length.
[0012]
Thus, by adjusting the temperature gradient in the radial direction to an arbitrary value in accordance with the crystal growth process, it is possible to enlarge the diameter at the crystal shoulder and to extend the straight body after the shoulder. In order to efficiently remove the heat accumulated near the growth interface into the melt, a means for shortening the distance between the interface and the crucible bottom is also used, but the crucible is filled due to the shallow melt depth. The amount of raw material becomes small and the amount of melt that can be crystallized is limited. In this case, a long crystal can be produced by keeping the effective melt depth constant by calculating and continuously supplying additional raw materials according to the weight of the pulled crystal.
[0013]
Embodiment 1
Embodiment 1 FIG. 1 shows Embodiment 1 in which the present invention is applied to pulling an SBN single crystal.
[0014]
In FIG. 1,
[0015]
Since the crucible bottom needs to be heated before gas cooling, a material having good heat transfer is used. A cooling gas supply pipe 8 supplies 0 to 20 liters per minute of a cooling gas such as air, argon, helium gas or the like (when supplied, room temperature). Reference numeral 9 denotes a platinum gas flow direction changing cylinder integrated with the crucible. Cooling gas is blown to the bottom of the crucible from 8 and the blown gas is turned by the gas flow direction changing cylinder 9 to form a hole A. It passes through and is discharged into the furnace. Reference numeral 10 denotes a heat-resistant fiber for heat insulation. With such a structure, first, the cooling gas is blown to the crucible bottom to cool the melt just above the crucible center.
[0016]
However, since the blown gas is rapidly heated while passing through 9 and 7 and loses its cooling ability, there is no cooling effect on the melt outside the gas flow redirecting cylinder 9 below the crucible. Therefore, since the melt is locally cooled centering directly on the crucible, a radial temperature gradient could be formed such that the central portion was low in the radial direction and the peripheral portion was high in the melt.
[0017]
In order to manufacture a large-diameter, long crystal, first, in the initial stage of crystal growth, the cooling gas flow rate is suppressed to 0 to 5 liter / min and the radial temperature gradient of the melt is set to 20 ° C./cm or less. The retained crystal diameter is expanded to a desired diameter.
[0018]
In the next step, the cooling gas flow rate is sequentially increased from 5 liters 1 minute to a maximum of 20 liters / minute so as to absorb the generated latent heat, thereby increasing the radial temperature gradient from 20 to 150 ° C./cm. It continued to grow and became longer. The optimum gas flow rate was determined in advance based on the relationship between the crystal diameter and the increase in the crucible bottom temperature.
[0019]
In this way, a CBN single crystal with a crystal diameter of 30 to 50 mm and a length of 50 to 100 mm was produced with good reproducibility.
[0020]
FIG. 2 shows a second embodiment in which the present invention is applied to pulling an SBN single crystal, and shows a difference from FIG. 1 and a temperature distribution in a cylinder.
[0021]
In FIG. 2, reference numeral 11 denotes a heat-resistant vertically movable cylinder, and
[0022]
First, as for the temperature distribution in the cylinder without the cylinder, the lower part is cooled, so that the temperature decreases from the lower part of the crucible toward the open end.
[0023]
Here, when the cylinder is inserted, the flow of heat from the upper part to the lower part is cut off, so that the temperature distribution at the right is T1 at the position of P1, and the temperature distribution is T2 at the position of P2. By adjusting the positions of P1 and P2, the temperature of the crucible bottom could be adjusted in the same manner as in Example 1.
[0024]
Therefore, since the melt is locally cooled centering directly on the crucible, a radial temperature gradient could be formed such that the central portion was low in the radial direction and the peripheral portion was high in the melt. In this way, a CBN single crystal with a crystal diameter of 30 to 50 mm and a length of 50 to 100 mm was produced with good reproducibility.
[0025]
Embodiment 3
FIG. 3 shows a third embodiment in which the present invention is applied to pulling an SBN single crystal, and shows a difference of the second embodiment from FIG.
[0026]
In FIG. 3, reference numeral 13 denotes a small heater which is inserted into 12 cylinders. By changing the temperature of the heater, the same effect as that of the movable cylinder of Example 2 was realized. Therefore, since the melt is locally cooled centering directly on the crucible, a radial temperature gradient could be formed such that the central portion was low in the radial direction and the peripheral portion was high in the melt. In this way, a CBN single crystal with a crystal diameter of 30 to 50 mm and a length of 50 to 100 mm was produced with good reproducibility.
[0027]
Embodiment 4
FIG. 4 shows a fourth embodiment in which the present invention is applied to SBN single crystal pulling, and shows only differences from the first embodiment.
[0028]
In this way, by reducing the distance between the crystal growth interface and the crucible bottom by depressing a part of the crucible bottom to the melt side, the heat accumulated near the growth interface is reduced because the gas outlet and the growth interface become closer. It could be absorbed efficiently.
[0029]
The difference between the case where the depth of the melt is shallow in the first embodiment and the present embodiment is that, when the depth is simply reduced, the heat capacity becomes small due to the small amount of the melt filling, and the temperature is easily influenced by the temperature fluctuation. Although crystal growth is difficult, in the present embodiment, since the amount of the melt is large, a stable temperature distribution can be obtained and good crystal growth can be performed. However, since the depth of the melt that can be crystallized is only 5 mm in this example, the crucible diameter was increased to increase the amount of the melt that could be crystallized. As a result, an SBN single crystal with a crystal diameter of 30 to 50 mm and a length of 50 to 100 mm having a C-axis orientation was produced with good reproducibility.
[0030]
Embodiment 5
FIG. 5 shows a fifth embodiment in which the present invention is applied to SBN single crystal pulling, and shows only differences from the first embodiment.
[0031]
Reference numeral 15 in FIG. 5 is a metal column, which was submerged at the center in the radial direction of the melt to form a heat sink. In this embodiment, a column made of platinum and having a diameter of 30 mm was used. In addition, the diameter was set to 200 mm because it was necessary to increase the diameter of the crucible. Further, the distance from the melt surface to the top of the platinum cylinder was 5 mm.
[0032]
By shortening the distance between the crystal growth interface and the crucible bottom in the same manner as in Example 4, the gas outlet and the growth interface become closer, so that heat accumulated near the growth interface is efficiently absorbed. I was able to. The difference from the case where the depth of the melt is made shallow in Example 1 is that if the depth is simply made small, the heat capacity becomes small due to the filling of a small amount of the melt, the temperature is easily influenced by temperature fluctuation, and good crystal growth is achieved. Although it becomes difficult, in this example, a stable temperature distribution was obtained because the amount of the melt was large. However, since the depth of the melt that can be crystallized is only 5 mm in this example, the crucible diameter was increased to increase the amount of the melt that could be crystallized. As a result, an SBN single crystal with a crystal diameter of 30 to 50 mm and a length of 50 to 100 mm having a C-axis orientation was produced with good reproducibility.
[0033]
Embodiment 6
FIG. 6 shows a sixth embodiment in which the present invention is applied to pulling an SBN single crystal, and shows a difference of the fourth embodiment from FIG.
[0034]
In FIG. 6, 16 is a weight sensor, 17 is a raw material supply control device, and 18 is an SBN raw material to be replenished. This embodiment is an embodiment in a case where a large-diameter crucible as in the fourth embodiment cannot be used, or in a case where a longer crystal is manufactured by supplying a continuous raw material.
[0035]
In this example, the hollow was made cylindrical, the diameter was 30 mm, and the diameter of the crucible was 100 mm. Further, the distance from the surface of the melt to the top of the crucible depression was 5 mm. The decrease of the raw material melt accompanying the crystal growth was detected by the 16 weight sensors, and the signal was transmitted to the 17 raw material supply device, and the necessary SBN raw material was replenished to keep the melt surface level constant. As a result, the distance from the melt surface to the top of the crucible depression was always kept at 5 mm, so that heat accumulated near the growth interface could be efficiently absorbed. In this example, a long crystal could be obtained, and a C-axis oriented SBN single crystal having a crystal diameter of 30 to 50 mm and a length of 200 mm could be produced with good reproducibility.
[0036]
【The invention's effect】
As described above, in the present invention, in the initial stage of crystal growth, the temperature gradient in the radial direction of the melt is reduced by using a low temperature gradient furnace such as resistance heating as a first means to increase the crystal diameter. In the next stage, as a second means, the lower part of the crucible is cooled to increase the radial temperature gradient, and the latent heat generated near the center of the crystal growth surface is effectively absorbed in the melt to grow the crystal. Was continued to obtain an elongated crystal. The present invention was used for elevating the C-axis orientation of a Ce-doped SBN crystal. The obtained crystal was sliced in a thickness of 2 mm in parallel with the C axis, processed into a plate shape of 40 mm × 40 mm size, and polished on both sides to be used as a recording medium of a digital hologram memory. This size is unprecedented, and since it is manufactured with a low temperature gradient, the crystal quality and photorefractive characteristics are good, and a high-density recording device can be realized.
[Brief description of the drawings]
FIG. 1 is a diagram showing single crystal growth by cooling a crucible underneath in Example 1.
FIG. 2 is a diagram showing single crystal growth by crucible lower cooling in Example 2.
FIG. 3 is a diagram showing a single crystal growth by cooling a crucible underneath in Example 3.
FIG. 4 is a diagram illustrating a fourth embodiment, showing only differences from the first embodiment.
FIG. 5 is a view showing a fifth embodiment and showing only differences from the first embodiment.
FIG. 6 is an explanatory diagram in which a raw material continuous supply device according to a sixth embodiment is connected.
[Explanation of symbols]
1, 2 Resistance heater 3 Platinum crucible 4 Melt 5 SBN single crystal 6 Single crystal pulling rod 7 Cylindrical double tube 8 Cooling gas supply tube 9 Gas flow direction changing cylinder 10 Heat resistant fiber 11
Claims (9)
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