JPH0912394A - Production of planar crystal - Google Patents

Abstract

PURPOSE: To stably obtain a planar crystal having high purity and high accuracy at a low cost by holding a columnar raw material rod in such a manner that its axis is held in a perpendicular direction and heating and melting the top end thereof to form a melt layer, disposing a cap body having a slit in the upper part thereof and pulling up the crystal through the slit. CONSTITUTION: This method is preferably executed while the raw material rod is rotated around the shaft and heating and melting are preferably executed by high-frequency heating. Further, the heating and melting are preferably executed in an inert gas (e.g. argon, helium, nitrogen) or under a reduced pressure. The impurities in the atmosphere gas is preferably <=1,000ppm and the moisture content <=10<-3> atm under a vapor pressure (25 deg.C). The planar silicon crystal is formed by using the raw material rod consisting of silicon for an added effect. The raw material rod 12 of the columnar silicon is held in the lower part as shown in Fig. and the top end is melted by high-frequency induction heating to form the molten layer. The cap 15 having the slit-like aperture 14 is disposed thereon and the planar crystal 13 is pulled up through the slit 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for producing a plate crystal, particularly a plate silicon crystal useful as a substrate material for solar cells.

[0002]

2. Description of the Related Art Solar cells, especially solar cells using a semiconductor material such as silicon, are attracting attention as a clean alternative energy source from the viewpoint of environmental measures such as global warming in recent years. As a silicon solar cell, single crystal, polycrystal,
A number of proposals have been made for the three types of amorphous, and also for the configuration and structure of the solar cell, and some of them have already been put into practical use, but there are many problems in view of the characteristics of solar cells and the cost of alternative energy. I have it. As a conventional silicon solar cell, as shown in FIG.
On the surface of the silicon semiconductor substrate 1 of the mold, an n layer 2 in which P or the like is diffused, a surface electrode 4 is formed thereon via an antireflection film 3, and a layer formed by printing an Al paste or the like is formed on the back surface. In general, the p ⁺ layer 5 is fired and the back electrode 6 is formed on the p ⁺ layer 5 by printing an Al paste. When this solar cell is irradiated with sunlight 7, light electrons incident on silicon generate free electron and hole pairs, and a current can be taken out to an external circuit between the back electrode 6 and the front electrode 4. Solar energy can be directly converted into electrical energy.

However, in order to reduce the cost of the crystal substrate in the cost of the solar cell, a method for producing thin ribbon-shaped or plate-shaped silicon has been researched and proposed as a typical method. EF for growing a plate crystal by raising the silicon melt by capillarity using a jig having a slit-shaped opening and crystallizing at the upper end thereof
G (Edge-defined Film fed Growth) method, Gompertz-Stepanov method of directly pulling out and crystallizing a silicon melt from the slit-shaped opening, or a modification of these methods, a method of pulling out a silicon melt downward, or a silicon melt. Research and proposals have been made on methods such as a dendrite growing method for growing plate crystals without using a jig while keeping the liquid surface in a supercooled state, or a Web method. However, in any of these methods, it is difficult to keep the temperature of the solid-liquid interface where the plate-like silicon grows stable over the entire area, so that plate-like silicon with a constant thickness is continuously stable. It has the drawback of being extremely difficult to grow.

That is, in the plate-like silicon manufacturing method according to the prior art, the growth rate is determined by the heat of solidification generated and the heat from the solid-liquid interface determined by the temperature distribution near the solid-liquid interface between the plate-like silicon and the melt. Determined by the amount of movement.
Therefore, in order to grow plate-like silicon stably at a constant rate, it is necessary to precisely control the temperature at the solid-liquid interface and the temperature distribution in the vicinity of it, and measure the width and thickness of the plate-like silicon during growth. It is desirable to control the temperature and its distribution by feedback.However, considering the structure of the growth furnace and the heat capacity of the members, the time constant of the temperature control system is generally large and the response is inferior. However, the growth rate must be kept low as a result. Further, for example, in the case of the EFG method, since the supply of the melt to the solid-liquid interface depends on the capillary phenomenon at the opening of the jig, the growth rate also depends on the properties of the interface between the jig and the melt. Therefore, stable manufacturing becomes more difficult. Furthermore, in the method using the jig, the quality and shape of the jig surface layer may change due to the chemical reaction between the silicon melt and the jig material, and the purity of the silicon may decrease due to contamination from the jig material. Inevitably, when growing a single crystal in particular, it is difficult to obtain a stable single crystal plate-like silicon which becomes crystalline and has many crystal defects due to these causes.

In each of these methods, the raw material silicon melt is housed in a heat-resistant container, and plate crystals are grown from this, so that the silicon is contaminated from the container material as in the case of the above-mentioned jig, and the growth of the crystal is promoted. As the amount of silicon in the container decreases with the progress, the thermal condition near the solid-liquid interface of crystal growth changes, and it becomes extremely difficult to form a plate-shaped crystal having a constant shape. In order to overcome such drawbacks of the conventional plate crystal growth method, an RTR (Ribon to Ribon) method of growing a plate crystal from a plate material has been proposed. In this method, a laser is incident on the side surface of the plate-shaped raw material from a direction perpendicular to the lengthwise direction of the plate-shaped raw material to melt a part of the plate-shaped raw material into an extremely thin band shape, and this melting zone is fed from one end of the raw material plate to the other end. A plate-shaped crystal is created by moving it toward the plate. In this method, since a plate-shaped crystal to be grown is uniformly heated by a laser beam over the entire width, a high-power laser light source and a complicated and precise An optical system is required. In addition, special equipment is required to secure the stability of the laser beam for a long time, which makes the device extremely expensive, and the efficiency of the laser output energy with respect to the input energy is extremely low. There are many problems in the economical production of plate crystals.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a plate crystal, particularly a plate silicon crystal useful as a substrate material for solar cells, with high quality, high accuracy and low cost. The purpose is to obtain at high cost and high productivity.

[0007]

The present inventors have succeeded in stably producing a high quality plate crystal from a raw material rod, and the gist thereof is that a circle is formed so that the axis is in the vertical direction. A column-shaped raw material rod is held, an upper end portion thereof is heated and melted to form a melt layer on the cylindrical raw material rod, and a lid having a slit is arranged on the upper portion of the melt layer. It is a method of producing a plate crystal by pulling up through the slit. This method is preferably performed while rotating the raw material rod about its vertical axis, and the heating and melting may be performed by high frequency heating. And these methods are preferably carried out in an atmosphere of an inert gas, for example, at least one of argon, helium and nitrogen, or under reduced pressure, the impurities of the atmosphere gas used are 1,000 ppm or less, and the water content in the atmosphere is The vapor pressure at 25 ° C is preferably 10 ^-3 atm or less. Furthermore, in these methods, the shape of the plate crystal produced can be controlled by high-frequency heating power, the solidification rate of the plate crystal, or both. Further, it is useful for forming a plate-shaped silicon crystal by using the raw material rod as silicon. In this case, it is possible to adjust the conductivity and conductivity of the plate-like silicon produced by adding an element that imparts n-type or p-type conductivity to silicon in the silicon raw material rod or the atmosphere gas or both of them. It is possible. Since the plate-shaped silicon crystal obtained by this method has high quality and low cost, it can be used as a substrate for solar cells, and a high-efficiency, low-cost solar cell is provided.

Hereinafter, the present invention will be described in detail. The present invention
It is intended to provide a new manufacturing method capable of overcoming the above-mentioned problems of the prior art and significantly reducing the cost of a plate crystal, particularly a plate silicon crystal. Hereinafter, the present invention will be described focusing on the case of producing a plate crystal of silicon, but the present invention is not limited to silicon. In the method of the present invention, as shown in FIG. 1, a cylindrical raw material rod 12 is held at the lower part so that the axis of the cylindrical column is in the vertical direction, and its upper end is heated and melted by, for example, high frequency induction heating,
A molten layer of silicon is formed on the solid raw silicon rod, a lid 15 having slit-shaped openings 14 is arranged on the melt layer, and a plate-shaped silicon crystal 13 is provided through the slit 14. Is to raise. That is, above the melt layer formed on the raw material silicon rod, there is a slit for pulling out the plate-shaped silicon crystal in the central part, and the lid body so that the slit and the raw material silicon columnar rod are concentric. The plate-like silicon crystal serving as a seed is fused from the upper side through the slit with the melt on the surface of the molten layer, and then the seed is pulled upward to grow the plate-like silicon crystal.

At this time, the lid having the slit is extremely important for controlling the temperature of the melt layer and forming the temperature distribution,
Regarding the shape, it is necessary to carefully determine the outer diameter dimension, thickness, etc. in consideration of this point. Further, it is important to determine the material of the lid body in consideration of not only the above temperature control but also the influence on the quality of the plate-shaped silicon crystal to be produced. Examples include high melting point oxides such as alumina and yttria, SiC, carbon, silicon nitride and BN.
Or ceramics such as oxynitride, Mo,
High-melting-point materials such as Ta, Pt, W, and alloys thereof can be used, but when a metal material or a material having high electric conductivity at high temperature is used, it is necessary to consider the influence of the high-frequency electric field for heating.

In addition to high frequency induction heating, heating can be performed by a heater provided around the raw material rod, or light such as a laser or a valence particle beam, but a conductive material or silicon. In the case of semiconductors such as those that can obtain a certain degree of conductivity, high frequency induction heating is most suitable from the viewpoint of energy efficiency, controllability thereof, equipment cost, and the like. In practice, for semiconductor materials such as silicon, an auxiliary ring or other device may be used to preheat the upper end portion of the raw material bar to enhance conductivity and facilitate high frequency induction. In order to keep the temperature of the melt layer formed on the raw material rod uniform, it is desirable to rotate the raw material rod about its vertical axis. This is important in the heating method using an electron particle beam. Further, contamination from the growing atmosphere is also a problem, and in order to obtain an extremely high-purity plate-like silicon crystal, the pressure is reduced, for example, 1 Torr or less, preferably 10 ^-3 Torr.
The following is used, or an atmosphere such as a rare gas such as Ar or He or an inert gas atmosphere such as N ₂ is used to remove impurities such as oxygen, hydrogen or hydrocarbon contained therein.
It is preferable to control it to 1,000 ppm or less. Moisture in the atmosphere is preferably 10 ^-3 atm or less in vapor pressure at 25 ° C. Furthermore, the electrical characteristics of the plate-shaped silicon, that is, its conductivity type and resistance value, are obtained by previously doping a raw silicon rod with an element that imparts n-type or p-type conductivity to silicon, or when growing plate-shaped silicon. In the atmosphere of these elements in the form of compounds (eg B ₂ H ₆ , PH
It is possible to control it arbitrarily by including it in ₃ ).

In order to accurately control the shape of the plate-shaped silicon crystal to be created, the shape and dimensions of the plate-shaped silicon crystal growth interface are enlarged and observed or measured by, for example, a CCD camera or the like, and the fluctuation amount is measured at a high frequency. The shape and growth rate of the generated plate-shaped silicon crystal are fed back to the relative movement speed (solidification speed of the plate-shaped crystal) of the power or the raw material silicon rod and the generated plate-shaped silicon crystal, or by feedback control to both of them. Can be controlled extremely stably.

As a matter of course, the method of the present invention is based on the same principle as the floating zone method (FZ method for short) which is one of the methods for producing a silicon single crystal ingot. It is possible to single crystallize a silicon crystal and to highly purify a silicon rod as a raw material. Therefore, in the method of the present invention, it is desirable that a material other than silicon, for example, a metal material such as Fe, Cu, Al, etc. is required to have a high degree of purification, or single crystallization or special crystal grains are grown. It can also be applied to cases.

The method of the present invention is superior to the conventional methods for producing plate-like silicon and the plate-like silicon obtained by those methods, particularly the plate-like silicon used as a substrate for solar cells, in the following points. You can do it. (1) Since crucibles and other jigs that directly contact the silicon melt are not used, contamination from these crucibles and jig materials does not occur. (2) At present, the columnar polycrystalline silicon used for manufacturing a silicon single crystal ingot can be used as it is to directly manufacture a solar cell substrate. Moreover, there is no material loss as in the case of manufacturing a substrate wafer by cutting a cylindrical silicon single crystal, and the cost of the substrate can be significantly reduced. (3) The present invention can be carried out by an apparatus similar to the FZ method which is generally used for producing a silicon single crystal ingot at present, and basically, a cylindrical raw material polycrystal holder and a seed holder. , A heating high-frequency coil and a lid with a slit, and a container for accommodating them and forming an atmosphere at the time of forming a plate-shaped silicon crystal, because the device configuration is relatively simple and auxiliary materials are extremely small, Also in terms of cost reduction. (4) Since the relative positions of the raw material silicon rod, the heating coil, and the silicon melt layer are kept constant during the growth of the plate-shaped silicon crystal, the raw material silicon rod must be extremely short unless the raw silicon rod becomes extremely short. The temperature conditions of the melt layer and the plate-shaped silicon forming part are kept constant. Therefore, as in the conventional plate-shaped silicon crystal production method, the shape of the grown plate-shaped silicon crystal gradually changes due to the decrease of the silicon melt and the temperature change of the silicon melt and the plate-shaped silicon crystal forming part. Nothing happens, and it is possible to stably form a plate-shaped silicon crystal having a constant shape.

[0014]

【Example】

(Example 1) A columnar polycrystalline silicon having a diameter of 120 mm and a length of 300 mm and exhibiting n-type conductivity was held on a lower rotation axis so that the axis of the column was vertical, and in an argon atmosphere.
It was rotated at 0.5 rpm, and its upper end was heated and melted by a high frequency induction heating coil (frequency 2.0 MHz). Width 5 mm, thickness 0.6 mm, length 50 mm, crystal orientation [211] in the length direction, and the seed crystal (dendrite) whose plate surface is the (111) plane is vertical in the length direction at the lower end of the upper moving axis. I attached it like this.
The lid was made of alumina with an outer diameter of 160 mm and a thickness of 2 mm, and a slit with a length of 30 mm and a width of 4 mm was provided at the center. After the seed crystal was contact-fused to the upper surface of the molten layer through this slit, the temperature of the molten layer was controlled by high frequency power for heating, and the average
The seed crystal was pulled up at a speed of 10 mm / min to form a plate-like silicon crystal with a length of 420 mm. The obtained plate-like silicon crystal had a width of about 14 mm and a thickness of 210 μm, and the plate plane having the (111) plane as a twin crystal interface was a twin crystal of the (111) plane. This plate crystal showed a p-type resistivity of 10 Ωcm.

(Example 2) The silicon crystal plate obtained in Example 1 was cut to a size of 10 mm x 100 mm, phosphorus was diffused at 850 ° C to form a pn junction, the back diffusion layer was removed, and TiO 2 was removed. ₂
An antireflection film was formed on the n-type surface, an Al-Si front surface electrode was formed by photolithography, and an Al-Si rear surface electrode was similarly formed on the rear surface to fabricate a solar battery cell as shown in FIG. . This solar cell is AM 1.5, 100mW / cm ² , 28 ℃
The conversion efficiency was measured under the conditions of 1 and a value of 13.5% was obtained.

[0016]

According to the present invention, highly pure and highly precise plate crystals, particularly plate silicon crystals, can be stably obtained at low cost. Therefore, it contributes to the cost reduction of the solar cell made from such a plate-like silicon crystal, which can contribute to the spread of the solar cell.

[Brief description of the drawings]

FIG. 1 is a schematic view of a method for producing a plate-shaped material of the present invention. (A) Sectional view (b) Sectional view in another embodiment (c) Perspective view showing an enlarged fusion zone in (a)

FIG. 2 is a cross-sectional view of a solar cell.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... P-type silicon semiconductor substrate 11 ... High frequency heating coil 2 ... N layer 12 ... Raw silicon rod 3 ... Antireflection film 13 ... Plate-shaped silicon crystal 4 ... Surface electrode 14 ... Slit 5 ... P ⁺ layer 15 ... Lid 6 ... Back electrode 7 ... Sunlight

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masatsugu Ueoka 3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture Corporate Research Center, Shin-Etsu Chemical Co., Ltd. (72) Teruhiko Hirasawa, Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture 3-2-1, Shin-Etsu Chemical Co., Ltd. Corporate Research Center

Claims (13)

[Claims] 1. A cylindrical raw material rod is held so that its axis is in a vertical direction, and an upper end portion thereof is heated and melted to form a melt layer on the cylindrical raw material rod, and an upper portion of the melt layer is formed. A method for producing a plate crystal by arranging a lid having a slit on the substrate and pulling it up through the opened slit. 2. The method for producing a plate crystal according to claim 1, wherein the cylindrical raw material rod is rotated about its vertical axis. 3. The method for producing a plate crystal according to claim 1, wherein the heating and melting are performed by high frequency heating. 4. The method for producing a plate crystal according to claim 1, 2 or 3, which is carried out in an inert gas atmosphere. 5. The method according to claim 1 or claim 2 or claim 4, wherein the inert gas is at least one of argon, helium and nitrogen. 6. The method according to claim 1 or claim 2 to claim 5 carried out under reduced pressure. 7. The method according to claim 1 or claim 2 or claim 6, wherein the impurities in the atmospheric gas are 1,000 ppm or less. 8. The method according to claim 7, wherein the water content in the atmosphere is 10 ⁻³ atm or less in vapor pressure at 25 ° C. 9. The method according to claim 1, wherein the shape of the plate crystal to be produced is controlled by high-frequency heating power, the solidification rate of the plate crystal, or both of them. 10. The method according to claim 1, or 2 to 9, wherein the raw material rod is silicon. 11. The conductivity and the conductivity of the plate-shaped silicon are adjusted by including an element which imparts n-type or p-type conductivity to silicon in the silicon raw material rod or the atmosphere gas or both of them. the method of. 12. A plate-like silicon crystal produced by the method according to claim 10. 13. A solar cell manufactured by using the plate-shaped silicon crystal according to claim 12.