JP2008063194A - METHOD FOR PRODUCING Si BULK POLYCRYSTAL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simple method for producing a Si bulk polycrystal, by which the orientations of crystal grains in the Si bulk polycrystal can be made to be only ä110} plane in a cast growth method. <P>SOLUTION: The method for producing the Si bulk polycrystal having a uniform crystal grain orientation comprises developing a dendrite crystal extending in the <112> direction along the bottom surface of a crucible at an initial growth stage by adding Ge into a Si melt so as to make the upper surface of the dendrite crystal to be ä110} plane in the melt growth of the Si bulk polycrystal using the crucible, and then growing the Si bulk polycrystal on the upper surface of the dendrite crystal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing Si bulk polycrystal used for solar cells and the like.

  As a measure against global warming, high-efficiency solar cells are produced at low cost using abundant and safe Si resources in order to fully promote the use of environmentally friendly solar cells. Technology development that can be done is necessary. Currently, a method of growing large-diameter bulk polycrystals from Si melt using a cast method, cutting it into a thin plate, and making it into a solar cell has become the mainstream in practical use in Japan and overseas. However, the biggest problem of Si bulk polycrystals grown by the cast growth method is that the crystal orientations of many crystal grains are scattered or are aligned in the vicinity of the {111} plane, and the grain boundaries are dense and random. It is arranged.

  If the crystal orientation is random or aligned in the vicinity of the {111} plane, a texture structure cannot be created on the crystal surface by chemical etching like a bulk single crystal solar cell, so that part of the sunlight is reflected and sufficient Cannot be absorbed, and conversion efficiency does not increase. In addition, since the crystal grain boundary density is high, carriers are recombined at the crystal grain boundary, leading to deterioration of characteristics. In order to solve these problems, it is important that the surface orientation is aligned to one orientation other than the {111} plane and the crystal grain size is several centimeters or more.

  Already for the above purpose, in the cast growth method of Si bulk polycrystal, the degree of supercooling of the melt is controlled at the initial stage of growth, and a dendrite crystal is developed along the bottom surface of the crucible, and Si is formed on the upper surface of the dendrite crystal. A technique for growing bulk polycrystals to produce Si bulk polycrystals with controlled crystal grain orientation and crystal grain size has been reported (see, for example, Patent Document 1). However, in this method, dendrite growth was not performed along the bottom surface of the crucible, and the crystal grain orientation of the Si bulk polycrystal was centered on the {111} plane because the direction was not controlled in the growth melt. In addition, the orientation relationship is aligned in an orientation within 30 ° from the normal of the surface. Further, since the growth direction of crystal grains formed at the initial stage of growth is not controlled, crystal grain boundaries are randomly arranged.

  Usually, since the most stable surface of Si is the {111} plane, even if dendrite growth is not performed, it can be aligned with the {111} plane when grown under conditions close to thermal equilibrium conditions. For this reason, it is unlikely that the {111} plane has been obtained by using the growth from the upper surface of the dendrite crystal, and there is no point in using the dendrite growth to align the {111} plane. In addition, the method of producing the surface texture structure by chemical etching cannot form an optimal texture structure because the crystals are aligned on the {111} plane.

  In order to produce a surface texture structure by chemical etching, it is necessary to align crystal grains in a plane orientation other than the {111} plane. As such a growth technique, dendrite growth at the initial growth stage is grown along the bottom of the crucible. A method has been proposed by the present inventors (Patent Document 2).

According to this method, it is possible to align the orientation to the {112} plane or the {110} plane, but it is difficult to limit the orientation to only one direction. Therefore, there has been a demand for a technique for aligning the orientation of the Si bulk polycrystal only in the {110} plane with a simple method.
JP-A-2005-132671 Japanese Patent Application No. 2005-345042

  An object of the present invention is to provide a simple Si bulk polycrystal production method capable of aligning the orientation of the Si bulk polycrystal only to the {110} plane in the cast growth method.

Means for solving the problems are as follows.
(1) In melt growth of Si bulk polycrystal using a crucible, Ge is added to the Si melt, and a dendrite crystal extending in the <112> direction along the bottom surface of the crucible is expressed at the initial stage of growth. Is a {110} plane, and then a Si bulk polycrystal is grown on the upper surface of the dendrite crystal.
(2) In melt growth of Si bulk polycrystal using a crucible, Ge is added to the Si melt, and a dendrite crystal extending in the <112> direction along the bottom of the crucible is expressed at the initial stage of growth, and the upper surface of the dendrite crystal Is a {110} plane, and then the Si bulk polycrystal is solidified and grown in one direction on the upper surface of the dendrite crystal.
(3) The crucible containing the Si melt is moved to a low temperature side at a moving speed of 0.2 mm / min to 2.0 mm / min in a temperature gradient controlled in a range of 5 ° C./cm to 50 ° C./cm. (2) The method for producing a Si bulk polycrystal having a uniform crystal grain orientation, wherein the Si bulk polycrystal is solidified and grown in one direction.
(4) In melt growth of Si bulk polycrystal using a crucible, Ge was added to the Si melt, and the temperature of the crucible containing the Si melt was controlled in the range of 5 ° C / cm to 50 ° C / cm. First, the temperature of the bottom surface of the crucible is moved 2 ° C. or more lower than the melting point of Si at a moving speed of 1 mm / min to 10 mm / min in the gradient, and kept at that location for 1 minute or longer along the bottom surface of the crucible <112. > The dendrite crystal extending in the direction is developed to crystallize the melt immediately above the bottom of the crucible, and then moved to the low temperature side at a moving speed of 0.2 mm / min to 2.0 mm / min, thereby obtaining a single Si bulk polycrystal. A method for producing a Si bulk polycrystal having uniform crystal grain orientation, characterized by solidifying and growing in a direction.
(5) Si with uniform crystal grain orientation according to any one of (1) to (4), wherein the amount of Ge added to the Si melt is in the range of 0.1 mol% to 1 mol% Bulk polycrystalline production method.
(6) Production of Si bulk polycrystal with uniform crystal grain orientation according to any one of (1) to (5), characterized in that quartz is used as a material for the crucible containing a bulk polycrystal growth melt. Method.

  According to the present invention, the addition of a small amount of Ge allows dendrite growth to appear along the bottom surface of the crucible even at a temperature slightly lower than the melting point of Si due to the effect of compositional supercooling. The top surface orientation of the dendrite crystal grown from this low supercooled melt is limited to the {110} plane. After that, the temperature of the entire melt is cooled below the melting point, and epitaxial growth is performed on the polycrystalline structure at the initial stage of growth, thereby obtaining a high-quality Si bulk polycrystal having a uniform orientation on the {110} plane. it can.

  The present invention provides a high-efficiency solar cell crystal technology, the cast growth method. In order to grow a high-quality Si bulk polycrystal whose orientation is aligned on a plane other than the {111} plane, In this case, the upper surface of the dendrite crystal is made to be a {110} plane by adding a small amount of Ge, and then an Si bulk polycrystal is grown epitaxially on the surface. It is characterized in that the growth orientation of is aligned only on the {110} plane.

  That is, by adding a small amount of Ge, dendrite growth appears along the bottom surface of the crucible even at a temperature slightly lower than the melting point of Si (1414 ° C.) due to the effect of compositional supercooling. The top surface orientation of the dendrite crystal grown from this low supercooled melt is limited to the {110} plane. After that, the temperature of the entire melt is cooled below the melting point, and epitaxial growth is performed on the polycrystalline structure at the initial stage of growth, thereby obtaining a high-quality Si bulk polycrystal having a uniform orientation on the {110} plane. it can.

  First, the relationship between the dendrite crystal growth condition and the dendrite crystal growth direction along the bottom surface of the crucible was examined by direct observation experiments of the melt growth process. In a quartz crucible, raw materials Si and Ge were adjusted so as to have a Ge concentration of 0.5 mol%, completely melted at 1450 ° C., and a Si melt containing 0.5 mol% of Ge was added at 5 ° C. / The crystal was grown by cooling at min. For comparison, the pure Si melt was also cooled under the same conditions to grow crystals.

FIG. 1 is a photograph showing the change over time of dendrite crystal growth when a Si melt containing 0.5 mol% of Ge (Si _99.5 Ge _0.5 melt) is cooled at 5 ° C./min. As shown in FIG. 1, dendrite growth is observed in the Si _99.5 Ge _0.5 melt.

  As a comparative example, a photograph showing a change with time of crystal growth of the Si melt when cooled at 5 ° C./min is shown in FIG. It can be seen from FIG. 2 that dendrite growth does not occur in the pure Si melt. Thus, in the pure Si melt, when the cooling rate is slow, the degree of supercooling at the start of crystal growth is small, so dendritic growth does not occur, but by adding a small amount of Ge, due to the effect of compositional supercooling, Dendritic growth is likely to occur. As a result of orientation analysis by the EBSP method, it was found that the orientation of the upper surface of the dendrite crystal grown from such a low supercooled melt is limited to the {110} plane.

  Based on this knowledge, cast growth of Si bulk polycrystal was performed. Si raw material 450 g and Ge raw material 3.3 g were inserted into a quartz crucible having an inner diameter of 80 mm, and heated to 1450 ° C. in an Ar atmosphere to be completely dissolved, thereby preparing a Si melt. The Ge concentration of this melt is 0.28 mol%. A dendrite crystal extending in the <112> direction along the bottom of the crucible is developed by holding the temperature of the bottom of the crucible at a position of 1410 ° C. for 40 minutes in the initial stage of growth, and the upper surface of the dendrite crystal is set to the {110} plane, Was moved through a temperature gradient of 20 ° C./cm at a rate of 0.3 mm / min to solidify and grow in one direction.

  FIG. 3 is a photograph showing the orientation analysis result of the cast-grown Si and SiGe (Ge concentration; 0.28 mol%) bulk polycrystalline ingot bottom. After the growth was completed, the lower part of the Si bulk polycrystal added with Ge was cut and polished, and the orientation was analyzed by the EBSP method. For comparison, Si bulk polycrystals were grown under the same conditions, and the same analysis was performed. The Si bulk polycrystal without addition of Ge has a random orientation distribution (FIG. 3 left), but the Si bulk polycrystal with addition of Ge has approximately 70% orientation on the {110} plane (FIG. 3). right).

  FIG. 4 is a photograph showing the orientation analysis result of the bottom portion of the cast-grown SiGe (Ge concentration: 1.0 mol%) bulk polycrystalline ingot. As in the case of FIG. 3, when a Si polycrystal added with 1.0 mol% of Ge was grown, the orientation in the growth direction was 70% or more aligned on the {110} plane. This is because the orientation of the upper surface of the dendrite grown along the bottom surface of the crucible in the early stage of growth was limited to the {110} plane.

In the bulk Si polycrystal grown according to the present invention, a bulk polycrystal having a uniform orientation in the {110} plane was obtained. FIG. 5 shows the results of measuring the surface reflectivity after texturing the {110} surface and the {110} surface often found in normal Si polycrystals by alkali etching.
As can be seen from FIG. 5, by converting the {110} plane into a texture, the surface reflectance is reduced, so that the conversion efficiency of the solar cell is improved.

  According to the present invention, it becomes possible to align the crystal grains of the Si bulk polycrystal having a columnar structure in the <110> orientation. Therefore, an appropriate texture structure can be produced using a wafer cut out from the crystal. For this reason, in the solar cell produced using the Si polycrystal wafer which has this texture structure, the conversion efficiency can be improved significantly. Moreover, since these Si bulk polycrystal growth methods can be performed using a practical and inexpensive cast growth method, the conversion efficiency of solar cells using Si bulk polycrystals can be greatly improved and the cost can be reduced. The effect which can be realized simultaneously is acquired. According to the present invention, a high-efficiency and low-cost practical solar cell, which has been eagerly desired to be realized, can be produced using high-quality Si bulk polycrystal, and an immense effect on the spread of solar cells is expected. it can.

  The important point in the present invention is the amount of Ge added to the Si melt. It is well known that when Ge enters the Si crystal, the open circuit voltage may drop when a solar cell is fabricated. However, when a small amount of Ge is added, the open-circuit voltage is hardly lowered and the solar cell characteristics are hardly changed. For this reason, the Ge addition amount which does not deteriorate the solar cell characteristics is an important point of the present invention.

The addition of Ge is meaningless in such a small amount that it does not help the orientation control of the dendrite crystal. That is, when the amount of Ge added is less than 0.1 mol%, there is no effect on the orientation control of the dendrite crystal. In order for the addition of Ge to be useful for controlling the orientation of the dendrite crystal, the amount of Ge added to the Si melt needs to be 0.1 mol% or more.
On the other hand, when the Ge addition amount exceeds 1 mol%, defects such as dislocations increase in the crystal, so that the lifetime of the carrier is lowered and the effect of lowering the open circuit voltage of the solar cell appears, thereby deteriorating the solar cell characteristics.
Therefore, the optimum Ge addition amount in the Si melt is in the range of 0.1 mol% to 1 mol%.

  Next, in the examples, dendrite crystals extending in the <112> direction along the bottom of the crucible are expressed by holding the crucible bottom at a temperature of 1410 ° C. for 40 minutes in the initial stage of growth. The crucible is first moved in a temperature gradient controlled in the range of 5 ° C./cm to 50 ° C./cm to a place where the temperature at the bottom of the crucible is 2 ° C. lower than the melting point of Si at a moving speed of 1 mm / min to 10 mm / min. The dendrite crystals that extend in the <112> direction along the bottom surface of the crucible can be developed by holding at that location for 1 minute or longer.

In the examples, the crucible was moved in a temperature gradient of 20 ° C./cm at 0.3 mm / min and solidified and grown in one direction. The temperature gradient was 5 ° C./cm to 50 ° C./cm. It may be in the range.
This is because dendrite crystals do not develop when the temperature gradient is less than 5 ° C./cm, and when the temperature gradient exceeds 50 ° C./cm, the dendrite crystals do not align with the bottom of the crucible.

  Furthermore, as for the moving speed of the crucible, when it is slower than 0.2 mm / min, the productivity is industrially deteriorated, and when the speed exceeds 2.0 mm / min, the quality of the obtained crystal is deteriorated. Needs to be in the range of 0.2 to 2.0 mm / min.

  Since the present invention can grow Si bulk polycrystals oriented in the {110} plane by using an inexpensive cast growth method, a surface texture structure can be easily formed by chemical etching, and a highly efficient solar cell can be obtained. realizable. Moreover, since it is based on a practical cast growth method, it has the advantage of being easy to commercialize.

The Ge of 5 ° C. upon cooling in / min is a photograph showing the time course of dendrite crystal growth of Si melt was added 0.5 mol% (Si _99.5 Ge _0.5 melt). It is a photograph which shows a time-dependent change of the crystal growth of Si melt when it cools at 5 degrees C / min. It is a photograph which shows the orientation analysis result of the Si (Ge density | concentration; 0.28 mol%) bulk polycrystal ingot which added Si and Ge which carried out the cast growth. It is a photograph which shows the azimuth | direction analysis result of Si (Ge density | concentration; 1.0 mol%) bulk polycrystalline ingot which added Ge which carried out cast growth. It is drawing which shows the surface reflectance after {110} and {111} surface texture formation.

Claims (6)

  In the melt growth of Si bulk polycrystal using a crucible, Ge is added to the Si melt, and a dendrite crystal extending in the <112> direction along the bottom of the crucible is expressed in the initial stage of growth, and the upper surface of the dendrite crystal is {110 }, A bulk Si crystal is grown on the upper surface of the dendrite crystal.   In the melt growth of Si bulk polycrystal using a crucible, Ge is added to the Si melt, and a dendrite crystal extending in the <112> direction along the bottom of the crucible is expressed in the initial stage of growth, and the upper surface of the dendrite crystal is {110 }, A Si bulk polycrystal having a uniform crystal grain orientation is obtained by solidifying and growing the Si bulk polycrystal in one direction on the upper surface of the dendrite crystal.   By moving the crucible containing the Si melt in the temperature gradient controlled in the range of 5 ° C./cm to 50 ° C./cm to the low temperature side at a moving speed of 0.2 mm / min to 2.0 mm / min, The method for producing a Si bulk polycrystal with uniform crystal grain orientation according to claim 2, wherein the Si bulk polycrystal is solidified and grown in one direction.   In the melt growth of Si bulk polycrystal using a crucible, Ge was added to the Si melt, and the temperature of the crucible containing the Si melt was controlled in the range of 5 ° C / cm to 50 ° C / cm. First, at a moving speed of 1 mm / min to 10 mm / min, the temperature of the bottom surface of the crucible is moved to a place lower than the melting point of Si by 2 ° C. or more, and held in that place for 1 minute or longer in the <112> direction along the bottom surface of the crucible. Si bulk polycrystals are solidified in one direction by developing an extended dendrite crystal and crystallizing the melt just above the bottom of the crucible, and then moving to a low temperature side at a moving speed of 0.2 mm / min to 2.0 mm / min. A method for producing a Si bulk polycrystal having a uniform crystal grain orientation, characterized by growing.   The Si bulk with uniform crystal grain orientation according to any one of claims 1 to 4, wherein an addition amount of Ge in the Si melt is in a range of 0.1 mol% to 1 mol%. A method for producing a polycrystal.   The method for producing a Si bulk polycrystal with uniform crystal grain orientation according to any one of claims 1 to 5, wherein the material of the crucible containing the melt for growing the bulk polycrystal is quartz. .