JP2014205598A - METHOD FOR PRODUCING Si POLYCRYSTALLINE INGOT, AND Si POLYCRYSTALLINE INGOT - Google Patents
METHOD FOR PRODUCING Si POLYCRYSTALLINE INGOT, AND Si POLYCRYSTALLINE INGOT Download PDFInfo
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 14
- 239000007788 liquid Substances 0.000 claims abstract description 57
- 238000007711 solidification Methods 0.000 claims abstract description 15
- 230000008023 solidification Effects 0.000 claims abstract description 15
- 238000001816 cooling Methods 0.000 claims description 41
- 238000000034 method Methods 0.000 claims description 27
- 239000002019 doping agent Substances 0.000 claims description 6
- 229910052796 boron Inorganic materials 0.000 claims description 3
- 229910052733 gallium Inorganic materials 0.000 claims description 3
- 239000012535 impurity Substances 0.000 abstract description 41
- 239000013078 crystal Substances 0.000 abstract description 39
- 238000005204 segregation Methods 0.000 abstract description 14
- 230000000717 retained effect Effects 0.000 abstract 1
- 239000002994 raw material Substances 0.000 description 17
- 239000000155 melt Substances 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 6
- 238000009792 diffusion process Methods 0.000 description 6
- 229910052751 metal Inorganic materials 0.000 description 6
- 239000002184 metal Substances 0.000 description 6
- 238000005266 casting Methods 0.000 description 5
- 238000009826 distribution Methods 0.000 description 5
- 238000002474 experimental method Methods 0.000 description 5
- 239000010453 quartz Substances 0.000 description 4
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 4
- 239000000463 material Substances 0.000 description 3
- 229910021420 polycrystalline silicon Inorganic materials 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 229910052581 Si3N4 Inorganic materials 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 238000007429 general method Methods 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 230000006798 recombination Effects 0.000 description 2
- 238000005215 recombination Methods 0.000 description 2
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 description 2
- 239000000758 substrate Substances 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- GYHNNYVSQQEPJS-UHFFFAOYSA-N Gallium Chemical compound [Ga] GYHNNYVSQQEPJS-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 238000011109 contamination Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000013507 mapping Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000001000 micrograph Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000002210 silicon-based material Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/546—Polycrystalline silicon PV cells
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- Crystals, And After-Treatments Of Crystals (AREA)
- Photovoltaic Devices (AREA)
Abstract
Description
本発明は、不純物分布の少ない高品質なＳｉ多結晶インゴットの製造方法およびＳｉ多結晶インゴットに関するものである。 The present invention relates to a method for producing a high-quality Si polycrystalline ingot having a small impurity distribution and a Si polycrystalline ingot.
実用太陽電池の主流材料は、一方向凝固キャスト法で作製されるＳｉ多結晶インゴットから切り出された厚さ約２００μｍの基板である。このＳｉ多結晶インゴットのほとんどはＢ（ホウ素）またはＧａ（ガリウム）がドープされたｐ型の結晶である。また、Ｐ（リン）がドープされたｎ型の結晶も同様の手法で製造されている。 The mainstream material for practical solar cells is a substrate having a thickness of about 200 μm cut out from a Si polycrystalline ingot produced by a unidirectional solidification cast method. Most of this Si polycrystal ingot is a p-type crystal doped with B (boron) or Ga (gallium). An n-type crystal doped with P (phosphorus) is also manufactured in the same manner.
一般的に、Ｓｉ多結晶インゴットを作製するための原料Ｓｉは、金属不純物を極力排除した高純度の原料が用いられているが、一方向凝固キャスト装置を用いてＳｉ多結晶インゴットを作製する過程で、キャスト装置の部材や、ルツボの内壁に塗布された窒化ケイ素を主成分とする離型剤や、装置内の雰囲気から、Ｓｉ多結晶インゴットの内部に金属不純物や、酸素や炭素などの不純物が混入することが知られている。 Generally, the raw material Si for producing the Si polycrystalline ingot is a high-purity raw material that eliminates metal impurities as much as possible. However, the process for producing the Si polycrystalline ingot using a unidirectional solidification casting apparatus Therefore, metal impurities, impurities such as oxygen and carbon, etc. in the Si polycrystal ingot from the cast equipment components, the release agent mainly composed of silicon nitride applied to the inner wall of the crucible, and the atmosphere inside the equipment. Is known to mix.
また、太陽電池の低コスト化のために、もともと金属不純物が多く含まれている安価な原料Ｓｉを用いてＳｉ多結晶インゴットを作製する場合もある。このように、金属不純物が多く含まれる原料Ｓｉを用いた場合は、高純度の原料Ｓｉを用いた場合に比べて高濃度の金属不純物がＳｉ多結晶インゴットの内部に混入することとなる。 In addition, in order to reduce the cost of solar cells, there are cases where a Si polycrystalline ingot is manufactured using inexpensive raw material Si that originally contains a large amount of metal impurities. Thus, when raw material Si containing a large amount of metal impurities is used, a higher concentration of metal impurities is mixed into the Si polycrystalline ingot than when high-purity raw material Si is used.
いずれの原料を用いた場合においても、Ｓｉ多結晶インゴットの内部で不純物が局所的に多く存在する（不純物が局所偏析した）部分が存在すると、この部分でキャリアの再結合が起こり、太陽電池のエネルギー変換効率を低下させる原因となる。これまでの研究により、一方向凝固キャスト法で作製されたＳｉ多結晶インゴットにおいては、結晶粒界に不純物が局所偏析していることが知られている（例えば、非特許文献１および２参照）。不純物が局所偏析した結晶粒界はキャリアの再結合サイトとなり、太陽電池特性を低下させることが知られている（例えば、非特許文献３参照）。 Regardless of which raw material is used, if there is a portion where a large amount of impurities exist locally (impurities segregated locally) inside the Si polycrystalline ingot, carrier recombination occurs in this portion, and the solar cell It becomes a cause to reduce energy conversion efficiency. According to previous studies, it is known that impurities are locally segregated at grain boundaries in Si polycrystalline ingots produced by the unidirectional solidification cast method (see, for example, Non-Patent Documents 1 and 2). . It is known that a crystal grain boundary where impurities are segregated locally serves as a carrier recombination site and deteriorates solar cell characteristics (see, for example, Non-Patent Document 3).
Ｓｉ多結晶インゴットを作製する過程で結晶粒界に不純物が局所偏析する理由として、一方向凝固中に不純物が結晶粒界に取り込まれる、あるいは、一方向凝固が終了した後の冷却過程で不純物が結晶粒界に拡散する、などの理由が考えられているが、明確な原因はわかっていない。 The reason why impurities are segregated locally at the grain boundaries in the process of producing a Si polycrystal ingot is that impurities are incorporated into the grain boundaries during unidirectional solidification, or impurities are removed during the cooling process after unidirectional solidification is completed. There are thought to be reasons such as diffusion to the grain boundaries, but no clear cause is known.
次に、一般的な太陽電池用Ｓｉ多結晶インゴットの製造方法について詳しく説明する。太陽電池用Ｓｉ多結晶インゴットは、一方向凝固法を利用したキャスト法によって製造されている。キャスト法では、原料Ｓｉとｐ型またはｎ型ドーパントとを石英ルツボ内に充填し、この石英ルツボをキャスト成長炉内の所定の位置に配置し、炉内の温度をＳｉの融点以上の温度まで上昇させてルツボ内の原料Ｓｉおよびドーパントを完全に溶解した後、温度を下げることによってルツボ内のＳｉ融液からＳｉ多結晶インゴットを成長させる。通常、使用する石英るつぼの内壁には、窒化ケイ素を主成分とした離型剤が塗布されている。ルツボ内のＳｉ融液からＳｉ多結晶インゴットを成長させる方法としては、炉内の温度をＳｉの融点以下の温度まで下げる方法、もしくは、炉内に温度勾配を形成させ、温度の低い方にルツボを移動させることによって一方向凝固させる方法が一般的である。また、あらかじめ石英ルツボの底に種結晶を配置させておいて、種結晶の上に原料Ｓｉとドーパントとを充填し、種結晶を完全に溶解させないように温度を上昇させ、Ｓｉ原料とドーパントと種結晶の一部とを溶解させた後、温度を下げることにより、融け残っている種結晶を起点として一方向凝固させる方法もある。 Next, a general method for producing a Si polycrystalline ingot for a solar cell will be described in detail. The Si polycrystalline ingot for solar cells is manufactured by a casting method using a unidirectional solidification method. In the casting method, raw material Si and p-type or n-type dopant are filled in a quartz crucible, and this quartz crucible is placed at a predetermined position in a cast growth furnace, and the temperature in the furnace reaches a temperature equal to or higher than the melting point of Si. After the raw material Si and the dopant in the crucible are completely dissolved by raising the temperature, the Si polycrystal ingot is grown from the Si melt in the crucible by lowering the temperature. Usually, a release agent mainly composed of silicon nitride is applied to the inner wall of the quartz crucible to be used. As a method of growing a Si polycrystalline ingot from the Si melt in the crucible, the temperature in the furnace is lowered to a temperature below the melting point of Si, or a temperature gradient is formed in the furnace, and the crucible is moved to a lower temperature. A general method is to solidify unidirectionally by moving the. In addition, a seed crystal is arranged in advance at the bottom of the quartz crucible, the raw material Si and the dopant are filled on the seed crystal, the temperature is increased so as not to completely dissolve the seed crystal, and the Si raw material and the dopant are There is also a method in which a part of the seed crystal is dissolved, and then the temperature is lowered to solidify in one direction starting from the unmelted seed crystal.
上記のような方法でＳｉ多結晶インゴットを製造する際、高品質なＳｉ多結晶インゴットを得るために、ルツボ内のＳｉ融液の冷却速度を制御する方法が報告されている。 A method of controlling the cooling rate of the Si melt in the crucible in order to obtain a high-quality Si polycrystalline ingot when producing a Si polycrystalline ingot by the above method has been reported.
例えば、特許文献１では、ルツボ内で溶解させた原料Ｓｉの冷却速度を１℃／分以下にして、Ｓｉ多結晶インゴットを製造する方法が示されている。また、特許文献２では、融液の入ったルツボ全体を０．４℃／分〜５℃／分で冷却してＳｉ多結晶インゴットを製造する工程が示されている。 For example, Patent Document 1 discloses a method of manufacturing a Si polycrystalline ingot by setting the cooling rate of the raw material Si dissolved in the crucible to 1 ° C./min or less. Patent Document 2 discloses a process for producing a Si polycrystalline ingot by cooling the entire crucible containing a melt at 0.4 ° C./min to 5 ° C./min.
特許文献１または２で示されているように、通常は、ルツボ内の原料Ｓｉ全体もしくはルツボ全体の冷却速度を制御してＳｉ多結晶インゴットが製造されている。 As shown in Patent Document 1 or 2, the Si polycrystalline ingot is usually manufactured by controlling the entire raw material Si in the crucible or the cooling rate of the entire crucible.
しかしながら、融液から結晶が成長する過程において、結晶成長は常に融液と結晶との界面（固液界面）でのみ起こっているため、高品質なＳｉ多結晶インゴットを成長させるためには、固液界面の冷却速度を制御することが重要である。一般的に、炉内雰囲気の熱伝導率とルツボの材質またはルツボ内のＳｉの熱伝導率とが異なるため、ルツボ全体の冷却速度と固液界面の冷却速度とは異なっている。また、固液界面では結晶が成長すると凝固潜熱が吐き出されるため温度が上昇するので、通常は、固液界面の冷却速度はＳｉ原料全体もしくはルツボ全体の冷却速度に比べて遅くなる。 However, in the process of crystal growth from the melt, crystal growth always occurs only at the interface between the melt and the crystal (solid-liquid interface). Therefore, in order to grow a high-quality Si polycrystalline ingot, It is important to control the cooling rate at the liquid interface. In general, since the thermal conductivity of the furnace atmosphere and the material of the crucible or the thermal conductivity of Si in the crucible are different, the cooling rate of the entire crucible and the cooling rate of the solid-liquid interface are different. In addition, since the temperature rises because solidification latent heat is discharged when crystals grow at the solid-liquid interface, the cooling rate at the solid-liquid interface is usually slower than the cooling rate of the entire Si raw material or the entire crucible.
従来の、Ｓｉ原料全体もしくはルツボ全体の冷却速度を制御した方法で製造されたＳｉ多結晶インゴットでは、前述したように粒界に不純物が局所偏析しているが、不純物が局所偏析する理由は明確となっていないため、Ｓｉ多結晶インゴット中の粒界における不純物の局所偏析を無くす方法は未だ開発されていない。 In the conventional Si polycrystal ingot manufactured by controlling the cooling rate of the entire Si raw material or the entire crucible, impurities are segregated locally at the grain boundaries as described above, but the reason why the impurities are segregated locally is clear. Therefore, a method for eliminating local segregation of impurities at the grain boundaries in the Si polycrystalline ingot has not been developed yet.
太陽電池用のＳｉ多結晶インゴットの製造技術において、太陽電池の高効率化のためには、前述したように、結晶粒界および結晶粒界近傍における不純物の局所偏析を抑制する手法の開発が不可欠である。本発明は、このような課題に着目してなされたもので、Ｓｉ結晶中の不純物の局所偏析が少ない高品質なＳｉ多結晶インゴットの製造方法およびＳｉ多結晶インゴットを提供することを目的とする。 In the manufacturing technology of Si polycrystal ingots for solar cells, in order to increase the efficiency of solar cells, as described above, it is indispensable to develop a method for suppressing local segregation of impurities at the grain boundaries and in the vicinity of the grain boundaries. It is. The present invention has been made paying attention to such problems, and an object thereof is to provide a method for producing a high-quality Si polycrystalline ingot and a Si polycrystalline ingot with less local segregation of impurities in the Si crystal. .
本発明は、一方向凝固キャスト法によりＳｉ多結晶インゴットを作製する工程において、Ｓｉ多結晶とＳｉ融液との界面（以下、固液界面）の結晶粒界部分における溝の形成を抑制すること、すなわち固液界面の粒界部分の形状を平坦な形状に維持したままＳｉ多結晶インゴットを作製することを特徴としている。これにより、結晶粒界への不純物の局所偏析を抑制して高品質なＳｉ多結晶インゴットを得ることができる。 The present invention suppresses the formation of grooves at the grain boundary portion of the interface between the Si polycrystal and the Si melt (hereinafter, solid-liquid interface) in the process of producing the Si polycrystal ingot by the unidirectional solidification casting method. That is, it is characterized in that a Si polycrystal ingot is produced while maintaining the shape of the grain boundary portion of the solid-liquid interface in a flat shape. Thereby, the local segregation of the impurity to a crystal grain boundary can be suppressed, and a high quality Si polycrystal ingot can be obtained.
前記固液界面の結晶粒界部分における溝の形成を抑制するためには、一方向凝固過程における固液界面の冷却速度を０．５℃／分以下にしなければならない。この条件下では、固液界面の結晶粒界部分に溝が形成されることはなく、平坦な形状に維持されるため、不純物が結晶粒界に局所偏析されることはない。一方、固液界面の冷却速度がこれ以上になると固液界面の結晶粒界部分に溝が形成され、結晶粒界および結晶粒界近傍に不純物が局所偏析されてしまい、高品質のＳｉ多結晶インゴットが得られない。 In order to suppress the formation of grooves in the crystal grain boundary portion of the solid-liquid interface, the cooling rate of the solid-liquid interface in the unidirectional solidification process must be 0.5 ° C./min or less. Under these conditions, no groove is formed in the crystal grain boundary portion of the solid-liquid interface, and the flat shape is maintained, so that impurities are not segregated locally at the crystal grain boundary. On the other hand, when the cooling rate of the solid-liquid interface becomes higher than this, grooves are formed in the crystal grain boundary part of the solid-liquid interface, and impurities are segregated locally in the crystal grain boundary and in the vicinity of the crystal grain boundary. I can't get an ingot.
また、固液界面の冷却速度が０．０１℃／分以下になると、固液界面の結晶粒界部分における溝の形成は抑制できるものの、結晶成長速度が非常に遅くなってしまうため、Ｓｉ多結晶インゴットの生産には適さない。 Further, when the cooling rate of the solid-liquid interface is 0.01 ° C./min or less, the formation of grooves in the crystal grain boundary portion of the solid-liquid interface can be suppressed, but the crystal growth rate becomes very slow. Not suitable for the production of crystalline ingots.
したがって、本発明において、固液界面の結晶粒界部分における溝の形成を抑制する条件として、固液界面の冷却速度を０．０１℃／分〜０．５℃／分の範囲で制御して、Ｓｉ多結晶インゴットを作製することが望ましい。 Therefore, in the present invention, as a condition for suppressing the formation of grooves in the crystal grain boundary portion of the solid-liquid interface, the cooling rate of the solid-liquid interface is controlled in the range of 0.01 ° C./min to 0.5 ° C./min. It is desirable to produce a Si polycrystalline ingot.
また、本発明に係るＳｉ多結晶インゴットは、本発明に係るＳｉ多結晶インゴットの製造方法により製造されるＳｉ多結晶インゴットであって、ドーパントとしてＢもしくはＧａもしくはＰを含有することを特徴とする。 The Si polycrystalline ingot according to the present invention is an Si polycrystalline ingot produced by the method for producing an Si polycrystalline ingot according to the present invention, and contains B, Ga, or P as a dopant. .
本発明により、Ｓｉ多結晶インゴットの一方向凝固過程において、固液界面の結晶粒界部分における溝の形成を抑制し、平坦な形状に維持することにより、結晶粒界における不純物の局所偏析が少ない高品質なＳｉ多結晶インゴットが得られる。 According to the present invention, in the unidirectional solidification process of a Si polycrystal ingot, the formation of grooves in the grain boundary portion of the solid-liquid interface is suppressed, and the flat shape is maintained, thereby reducing local segregation of impurities at the crystal grain boundary. A high-quality Si polycrystalline ingot can be obtained.
本発明による、固液界面の冷却速度を制御することにより、固液界面の結晶粒界部分における溝の形成が抑制され、結晶粒界における不純物の局所偏析が抑制されることを実証するために、Ｓｉ多結晶の一方向凝固過程の固液界面形状の直接観察実験を行った。 In order to demonstrate that by controlling the cooling rate of the solid-liquid interface according to the present invention, the formation of grooves at the grain boundary portion of the solid-liquid interface is suppressed, and the local segregation of impurities at the crystal grain boundary is suppressed. A direct observation experiment of the solid-liquid interface shape during the unidirectional solidification process of Si polycrystals was conducted.
本実験で用いた結晶成長炉は、２つのヒーター（２ゾーンヒーター）の温度を独立に制御することによって、炉内の温度分布およびルツボ全体の温度を制御することが可能な装置である。成長炉には固液界面を観察できるように１５ｍｍφの観察窓を設けており、Ｓｉ融液からＳｉ多結晶が一方向凝固する過程の固液界面形状を直接観察することができる。結晶成長方向の炉内の温度分布は、熱電対を用いて予め測定した。また、結晶成長中の固液界面の温度は、ルツボの下に常時設置している熱電対により測定した。本実験では、２ゾーンヒーターの片方の温度を１３８０℃に設定し、もう片方のヒーターの温度を１４６０℃に設定して、ルツボ内の原料Ｓｉを完全に溶解した。この時、炉内に設置したルツボ内の温度分布は、結晶成長方向に対して５℃／ｍｍであった。その後、２ゾーンヒーターの両方の温度を同じ冷却速度で下げることにより、炉内の温度勾配を５℃／ｍｍに維持したままＳｉ多結晶を一方向凝固させ、固液界面の形状および温度をモニタリングした。２ゾーンヒーターの冷却速度を、０．５℃／分、１０℃／分、１８℃／分、および２０℃／分としてＳｉ多結晶を一方向凝固させた時の固液界面形状を観察した。これらの条件でＳｉ多結晶インゴットが一方向凝固している過程における固液界面の冷却速度は、ルツボの下に設置している熱電対で測定することができる。ルツボ内で一方向凝固しているＳｉ多結晶の固液界面がこの熱電対の位置まで移動すると、凝固潜熱による温度上昇とその後の温度低下とを測定できる。本実験においては、ヒーターの冷却速度が０．５℃／分の時、固液界面の冷却速度が０．０２℃／分、ヒーターの冷却速度が１０℃／分の時に、固液界面の冷却速度が０．１℃／分、ヒーターの冷却速度が１８℃／分の時に、固液界面の冷却底度が０．５℃／分、ヒーターの冷却速度が２０℃／分の時に、固液界面の冷却速度が０．６℃／分であった。 The crystal growth furnace used in this experiment is an apparatus capable of controlling the temperature distribution in the furnace and the temperature of the entire crucible by independently controlling the temperatures of two heaters (two-zone heaters). The growth furnace is provided with a 15 mmφ observation window so that the solid-liquid interface can be observed, and the shape of the solid-liquid interface in the process where the Si polycrystal solidifies in one direction from the Si melt can be directly observed. The temperature distribution in the furnace in the crystal growth direction was measured in advance using a thermocouple. The temperature of the solid-liquid interface during crystal growth was measured with a thermocouple always installed under the crucible. In this experiment, the temperature of one of the two-zone heaters was set to 1380 ° C., and the temperature of the other heater was set to 1460 ° C. to completely dissolve the raw material Si in the crucible. At this time, the temperature distribution in the crucible installed in the furnace was 5 ° C./mm with respect to the crystal growth direction. Then, by lowering the temperature of both zone heaters at the same cooling rate, the Si polycrystal is unidirectionally solidified while maintaining the temperature gradient in the furnace at 5 ° C / mm, and the shape and temperature of the solid-liquid interface are monitored. did. The solid-liquid interface shape was observed when the Si polycrystal was unidirectionally solidified with the cooling rate of the two-zone heater being 0.5 ° C./min, 10 ° C./min, 18 ° C./min, and 20 ° C./min. Under these conditions, the cooling rate of the solid-liquid interface in the process in which the Si polycrystalline ingot is unidirectionally solidified can be measured by a thermocouple installed under the crucible. When the solid-liquid interface of the Si polycrystal solidified in one direction in the crucible moves to the position of this thermocouple, it is possible to measure the temperature rise due to the solidification latent heat and the subsequent temperature drop. In this experiment, when the cooling rate of the heater is 0.5 ° C./min, the cooling rate of the solid-liquid interface is 0.02 ° C./min, and the cooling rate of the heater is 10 ° C./min, When the rate is 0.1 ° C / min, the cooling rate of the heater is 18 ° C / min, the cooling bottom of the solid-liquid interface is 0.5 ° C / min, and the cooling rate of the heater is 20 ° C / min, the solid-liquid The interface cooling rate was 0.6 ° C./min.
本実験による２ゾーンヒーターの冷却速度は、通常のキャスト法におけるＳｉ原料の冷却速度やルツボ全体の冷却速度に相当しており、固液界面の冷却速度とは異なることがわかる。また、ヒーターの冷却速度と固液界面の冷却速度との差は、結晶成長炉の構造やルツボの材質やルツボに充填するＳｉ原料の量に依存することは自明である。 The cooling rate of the two-zone heater according to this experiment corresponds to the cooling rate of the Si raw material in the ordinary casting method and the cooling rate of the entire crucible, and is found to be different from the cooling rate of the solid-liquid interface. Further, it is obvious that the difference between the cooling rate of the heater and the cooling rate of the solid-liquid interface depends on the structure of the crystal growth furnace, the material of the crucible, and the amount of Si raw material filled in the crucible.
図１は、固液界面の冷却速度が０．０２℃／分、０．１℃／分、０．５℃／分および０．６℃／分の時の、Ｓｉ多結晶の一方向凝固過程の固液界面形状の観察結果である。固液界面の冷却速度が０．０２℃／分、０．１℃／分、０．５℃／分の場合は、固液界面の粒界部分に溝は形成されず、平坦な固液界面を維持したまま成長するが、固液界面の冷却速度が０．６℃／分の時は、固液界面の粒界部分に溝が形成されることがわかる。 FIG. 1 shows the unidirectional solidification process of Si polycrystal when the cooling rate of the solid-liquid interface is 0.02 ° C./min, 0.1 ° C./min, 0.5 ° C./min, and 0.6 ° C./min. This is an observation result of the solid-liquid interface shape. When the cooling rate of the solid-liquid interface is 0.02 ° C./min, 0.1 ° C./min, 0.5 ° C./min, no groove is formed in the grain boundary portion of the solid-liquid interface, and the flat solid-liquid interface However, when the cooling rate at the solid-liquid interface is 0.6 ° C./min, it is understood that grooves are formed in the grain boundary portion of the solid-liquid interface.
図２は、固液界面の冷却速度を０．６℃／分および０．５℃／分として一方向凝固させたＳｉ多結晶に対して、少数キャリア拡散長を測定してマッピングした結果である。それぞれの試料において、固液界面形状を観察した領域の、図中の四角で囲った領域に対して、拡散長を測定し測定値のマッピングを行った。固液界面の冷却速度が０．６℃／分の試料では、結晶粒界近傍において局所的に拡散長が低下しており、結晶粒界に不純物が偏析されたことがわかる。一方、固液界面の冷却速度が０．５℃／分の試料では、結晶粒界部分においても拡散長が低下している箇所はなく、不純物の局所偏析が抑制されたことがわかる。 FIG. 2 shows the result of mapping by measuring the minority carrier diffusion length for Si polycrystals unidirectionally solidified at a cooling rate of the solid-liquid interface of 0.6 ° C./min and 0.5 ° C./min. . In each sample, the diffusion length was measured and the measured values were mapped to the region surrounded by the square in the figure in the region where the solid-liquid interface shape was observed. In the sample having a cooling rate of 0.6 ° C./min at the solid-liquid interface, the diffusion length locally decreases in the vicinity of the crystal grain boundary, and it can be seen that impurities are segregated at the crystal grain boundary. On the other hand, in the sample having a cooling rate of 0.5 ° C./min at the solid-liquid interface, there is no portion where the diffusion length is reduced even at the crystal grain boundary portion, indicating that local segregation of impurities is suppressed.
図３は、固液界面の粒界部分が平坦な場合と溝が形成された場合の、固液界面における不純物の偏析挙動を物理的に説明した図である。一般的に、固液界面において融液中に含まれている不純物は、偏析係数にしたがって融液と結晶とに分配される。不純物の平衡偏析係数をｋ０とすると、固液界面で融液中に吐き出される不純物の濃度ＣＬ ＊は、ＣＬ ＊＝ＣＳ ＊／ｋ０となる。ここで、ＣＳ ＊は、結晶に取り込まれる不純物濃度である。固液界面に溝が無く、平坦な場合は、固液界面前方の融液中に含まれる不純物濃度は場所に依らず均一な濃度（ＣＬ ＊）となる。一方、粒界部分に溝が形成されると、溝の部分では結晶粒界の両側の結晶粒から不純物が排出されるため、溝部分には高濃度の不純物が局所偏析されることとなる。このように、不純物が局所偏析した融液が結晶化することにより、粒界部分に不純物が局所偏析することとなる。 FIG. 3 is a diagram physically explaining the segregation behavior of impurities at the solid-liquid interface when the grain boundary portion at the solid-liquid interface is flat and when the groove is formed. In general, impurities contained in the melt at the solid-liquid interface are distributed to the melt and the crystal according to the segregation coefficient. If the equilibrium segregation coefficient of impurities is k 0 , the concentration C L * of impurities discharged into the melt at the solid-liquid interface is C L * = C S * / k 0 . Here, C S * is the concentration of impurities taken into the crystal. When the solid-liquid interface has no groove and is flat, the concentration of impurities contained in the melt in front of the solid-liquid interface becomes a uniform concentration (C L * ) regardless of the location. On the other hand, when a groove is formed in the grain boundary portion, impurities are discharged from the crystal grains on both sides of the crystal grain boundary in the groove portion, so that high-concentration impurities are locally segregated in the groove portion. Thus, when the melt in which impurities are segregated locally is crystallized, the impurities are segregated locally at the grain boundary portion.
したがって、Ｓｉ多結晶インゴットを一方向凝固する際、固液界面の粒界部分の形状を平坦な形状に維持する方法でＳｉ多結晶インゴットを作製しなければならない。 Therefore, when the Si polycrystal ingot is solidified in one direction, the Si polycrystal ingot must be produced by a method of maintaining the shape of the grain boundary portion of the solid-liquid interface in a flat shape.
本発明により、不純物の局所偏析がない高品質なＳｉ多結晶インゴットが得られるため、本Ｓｉ多結晶インゴットは太陽電池用基板として有用であることは自明である。
According to the present invention, a high-quality Si polycrystal ingot free from local segregation of impurities is obtained, and therefore it is obvious that the Si polycrystal ingot is useful as a substrate for solar cells.
Claims (3)
A Si polycrystal ingot produced by the method for producing a Si polycrystal ingot according to claim 1, wherein the polycrystal ingot contains B, Ga, or P as a dopant.
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