High purity crystalline silicon is used as a raw material of silicon for a solar cell and a raw material of silicon for a semiconductor. Particularly in recent years, the situation that popularization of solar cells is being expanded to a large extent is followed by that demand to high purity crystalline silicon, which is a raw material of solar cells increases as well.
The existing situation is that a residue remaining in a crucible after pulling up silicon for a semiconductor and scraps such as wastes generated in cutting silicon ingots are used as high purity crystalline silicon, which is a raw material of silicon for a solar cell. Accordingly, high purity crystalline silicon used for a solar cell depends on the trend of the semiconductor industry in terms of a quality and a quantity, and as a result thereof, it resides in the situation that it is chronically short.
A representative process for producing high purity crystalline silicon which is a raw material of silicon for a semiconductor includes a Siemens process. In this Siemens process, high purity crystalline silicon is obtained by hydrogen reduction of trichlorosilane (HSiCl3) (refer to, for example, a patent document 1). In a general Siemens process, a seed bar of silicon is set in a bell jar type reactor cooled with water, and an electric current is applied to this seed bar of silicon to heat the seed bar to about 1000° C.; trichlorosilane (HSiCl3) and a hydrogen (H2) of a reducing agent are introduced into the reactor to reduce silicon chloride, and silicon produced is adhered selectively onto a surface of the seed bar, whereby bar-like crystalline silicon is obtained. In addition to the advantage that the raw material gas is vaporized at a relatively low temperature, the above Siemens process has the advantage in terms of an apparatus that since the reactor itself is cooled with water, the environment is readily sealed, and therefore it has so far widely been popularized and employed.
In the Siemens process, however, the seed bar is heated by applying an electric current, and therefore as bar-like silicon grows by adhesion of crystalline silicon produced by reductive reaction to allow an electric resistance to go down gradually, a large amount of electricity has to be applied in order to heat the seed bar. Accordingly, the growth limit is present due to balance with the energy cost, and the production facilities are operated by a batch system, so that involved therein are the problems that the production efficiency is inferior and that an electric power consumption rate accounts for a large proportion in a cost of high purity crystalline silicon, which is the product. Further, special facilities such as a reactor for exclusive use, an equipment for pulling up the crystal, a cutting equipment and the like and special techniques are required in preparing the seed bar, and therefore the seed bar itself becomes expensive.
A process for producing crystalline silicon other than the Siemens process includes, for example, a process for producing it by reduction of silicon tetrachloride (SiCl4) using a metal reducing agent (refer to, for example, patent documents 2 and 3). To be specific, it is a process in which gases of silicon tetrachloride and zinc (Zn) are supplied into a quarts-made lateral reactor heated to about 1000° C. to thereby grow crystalline silicon in the reactor.
Based on the assumption of enabling to separate zinc chloride (ZnCl2) by-produced in the process described above into zinc and chlorine by electrolysis and the like, use again zinc obtained as a reducing agent, react chlorine obtained with metal silicon to synthesize silicon tetrachloride and use it as a raw material gas, a process of a circulating type is constituted, and therefore provided is the possibility that crystalline silicon can be produced at a low cost
However, involved in the above process are the problems that the crystalline silicon obtained by the reaction grows on a wall of the reactor and therefore is susceptible to an influence of contamination exerted by the reactor and that the reactor itself is broken due to a difference in a thermal expansion coefficient between the reactor and the crystalline silicon. Accordingly, the above process has scarcely been carried out in an industrial scale. A production process in which a vertical reactor is used to inhibit an influence of contamination exerted by a material of the reactor is proposed (refer to, for example, a patent document 4), and this has enhanced a purity of crystalline silicon produced by reduction of silicon tetrachloride (SiCl4) to a large extent. However, requirements to a purity of crystalline silicon are further elevated in every application field, and desired are a rise in a quality of the raw materials such as silicon tetrachloride and metal zinc which are used for reduction and further improvement in the process for producing crystalline silicon and the respective steps related to the quality, such as a method for washing crystalline silicon.