JPS6140639B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method of growing silicon ribbon crystals using a die.

[Technical background of the invention and its problems]

Conventionally, silicon ribbon crystals are pulled by the method shown in FIG. That is, a silicon material is placed in a crucible 1 made of quartz glass, and heated by a pair of opposing plate heaters 2a and 2b to form a silicon melt 3. A pair of dies 4 made of carbon are placed in this silicon melt 3.
Set up a and 4b. Since carbon is a material that gets wet with the silicon melt, the silicon melt 3 is applied to the die 4.
It rises through the slots of dies 4a and 4b due to capillary action and reaches the tips of dies 4a and 4b. By bringing a seed crystal into contact with this rising silicon melt 3 and pulling up the seed crystal, the die 4
A silicon ribbon crystal 5 defined by slots a and 4b is grown.

By the way, there are many silicon particles on the surface of silicon ribbon crystals manufactured using such conventional equipment.
Carbide grains are present. A pair of dies 4a and 4b shown in FIG. 1 are usually made of carbon.
In this case, carbon, which is a constituent material of the dies 4a and 4b, dissolves into the silicon melt and exists as silicon carbide grains due to a eutectic reaction with the silicon melt. These silicon carbide grains are deposited at the tips of the dies 4a, 4b, which are at a relatively low temperature. The precipitated silicon carbide grains are then incorporated into the growing silicon ribbon crystal. Even if an attempt is made to form a PN junction by diffusing impurities into such a crystal, the diffusion coefficient will be different due to the silicon carbide grains, making it impossible to form a predetermined flat PN junction. For this reason, when used as a solar cell element, the electrical conversion efficiency will differ depending on the part of the silicon ribbon crystal.
It is not possible to obtain a solar cell element with a predetermined power. Moreover, because of the silicon carbide grains,
A current-voltage characteristic with extremely large leakage current is observed. Furthermore, when printing surface electrodes with pressure, problems such as damage to the mask and damage due to the printing pressure tend to occur.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and
By preventing silicon carbide grains from precipitating on one side of the silicon ribbon crystal and forming a PN junction on the side where silicon carbide does not precipitate, photoelectric conversion efficiency can be improved even when used as a solar cell element, for example. It is an object of the present invention to provide a method for growing silicon ribbon crystals that does not cause a decrease in the silicon ribbon crystal and has almost no leakage current.

[Summary of the invention]

In the present invention, a pair of dies each having a slit are set upright in a crucible containing a silicon melt, a seed crystal is brought into contact with the silicon melt rising inside the slit, and the ribbon crystal is pulled up. A method for growing a silicon ribbon crystal, characterized in that each of a pair of dies is made of graphite having different thermal conductivity, for example, pyrolytic graphite.

[Embodiments of the invention]

The inventors of the present invention have conducted intensive research to prevent silicon carbide grains from precipitating on one side of a silicon ribbon crystal, and have found that it is effective to provide a temperature difference between the tips of a pair of dies. Dies 4a, 4b and the solid-liquid interface 6 in the ribbon thickness direction in Fig. 1
The shape of is shown in Figure 2. In the case of Figure 2, die 4
The temperatures at the tips of a and 4b are the same, and the shape of the solid-liquid interface 6 is flat. In this case, silicon carbide grains are formed on both sides of the ribbon crystal.

There is a method shown in FIG. 3 as a means for creating a temperature difference at the die tip. The method shown in FIG. 3 uses the temperature gradient of the die to create a difference in the height of the tip of the die. According to this method, the die 4b' on one side (low end) is hot and the die 4b' on the other side is hot.
a′ (higher at the tip) becomes low temperature, and the solid-liquid interface 6′
is inclined, and the silicon carbide grains adhere only to the ribbon crystal surface on the low temperature side and not on the high temperature side. However, in this method, silicon carbide grains do not adhere to one side of the ribbon crystal, but the pulling yield is significantly reduced, making it unsuitable for mass production.

As a result of further research, the present inventors discovered a method of constructing a pair of dies with graphite having different thermal conductivities as a means of creating a temperature difference at the tip of the die, and used pyrolytic graphite as the die material. Adopted. Pyrolitic graphite is produced using hydrocarbon gas as a raw material, and carbon atoms are deposited in layers. Therefore, the anisotropy of physical properties is large in the direction parallel to and perpendicular to the deposition surface. In particular, when comparing the thermal conductivity, pyrolytic graphite has a thermal conductivity of 470 Kcal/m in the direction parallel to the deposition surface.
hr・℃, and in the direction perpendicular to the deposition plane of pyrolitic graphite, there is a large difference of 11 Kcal/m・hr・℃. Therefore, using this physical anisotropy,
One side of the die was made of material cut in a direction parallel to the deposited surface of pyrolytic graphite, and the deposited surface was configured to be perpendicular to the horizontal direction, and the other side of the die was cut in a direction perpendicular to the deposited surface. If the material is configured so that the deposition surface is horizontal, a temperature difference will occur at the die tip due to the large difference in thermal conductivity, and silicon carbide will adhere only to the low temperature side and not to the high temperature side. Does not stick.

That is, as shown in FIG. 4, the die 4a'' is
The material is cut in a direction perpendicular to the deposition surface 7 of the pyrolytic graphite, and the material is constructed so that the deposition surface 7 is horizontal, and the die 4b'' is used to cut the pyrolytic graphite in a direction parallel to the deposition surface 7. The depositing surface 7 is constructed in such a way that the depositing surface 7 is perpendicular to the horizontal direction.

As shown in FIG. 1 of the conventional example, the heating sources for the die are carbon heaters 2a and 2b, and the hot zone structure is configured symmetrically with respect to the die.
Therefore, the amount of radiation incident on the dies 4a'' and 4b'' is the same. In Figure 1, the thermal conductivity of dies 4a and 4b is the same, so the temperatures at the tips of dies 1a and 4b are the same, but in Figure 4, the thermal conductivity of dies 4a'' and 4'' is significantly different. Because of the difference, the heat transfer on the die 4a'' side is Q1, and the amount of heat transmitted to the tip of the die 4a'' by thermal conduction is extremely reduced. However, die 4
On the b'' side, the heat transfer is Q2, and the amount of heat transferred to the tip of die 4b'' by thermal conduction is significantly reduced compared to die 4a''. Therefore, only a small amount of heat is transferred to the tip of die 4a''. Therefore, a large amount of heat is transferred to the tip of the die 4b''. Therefore, the tip of the die 4a'' becomes low temperature, the tip of the die 4b'' becomes high temperature, and the solid-liquid interface 6'' becomes The silicon carbide grains adhere only to the silicon ribbon surface on the die 4a'' side and do not adhere to the silicon ribbon surface on the die 4b'' side. Also, dies 4a″, 4
Since the height of the tip of "b" is the same, there is no interruption in pulling, and the pulling yield is the same as in the case shown in FIG.

〔Effect of the invention〕

As described above, according to the present invention, a pair of dies are made of pyrolytic graphite, one die is provided with the deposition surface in the horizontal direction, and the other die is provided with the deposition surface in the horizontal direction. By arranging the temperature difference in the direction perpendicular to , it is possible to create a temperature difference between the tips of the dies on both sides and prevent silicon carbide grains from adhering to the silicon ribbon surface on the low temperature side. Therefore, impurities are diffused onto the surface where silicon carbide grains do not adhere to form P-N.
When the junction is formed, a flat PN junction is formed. Therefore, when used as a solar cell element,
There is an advantage that a solar cell element having a predetermined power can be obtained.

Furthermore, according to this invention, impurity atoms tend to adhere to the slit surface on the side where a material with low thermal conductivity is used. Therefore, the average impurity concentration in the ribbon crystal is When not used, the amount is several tens of ppm, and when the present invention is used, it is reduced to 1/10 ppm, which is a significant reduction. Therefore, the present invention can significantly improve the purity of ribbon crystals and, in turn, provide highly efficient solar cell elements.

Further, in the present invention, since the thermal conductivity of each of the pair of dies is different, the temperature of each die is different. Therefore, thermal strain exists in the thickness direction in the pulled ribbon crystal.
By lowering the temperature during pulling or by simple heat treatment, it is possible to divide the crystal into two along with thermal strain, and two ribbon crystals can be obtained at the same time.

[Brief explanation of the drawing]

FIG. 1 is a configuration explanatory diagram showing a conventional silicon ribbon crystal growth apparatus, FIGS. 2 and 3 are cross-sectional views each showing a conventional die structure, and FIG. 4 is an example of a die structure according to the present invention. FIG. 1...Quartz crucible, 2a, 2b...Heater, 3
...Silicon melt, 4a, 4b, 4a', 4b', 4
a″, 4b″…Die, 5…Ribbon crystal, 6,
6', 6''...solid-liquid interface, 7...deposition surface of pyrolyte graphite.

Claims (1)

[Claims] 1 A pair of dies having slits are set up in a crucible containing a silicon melt, and a silicon melt is brought into contact with the silicon melt rising inside the slit to pull up a ribbon crystal. In the method for growing ribbon crystals, one side of the pair of dies is provided so that a deposition surface of pyrolytic graphite is in a horizontal direction;
and the other side of the pair of dies is provided so that the deposition surface of pyrolytic graphite is perpendicular to the horizontal direction.