A polycrystal silicon rod is generally produced by a chemical vapor deposition (to be abbreviated as CVD hereinafter). That is, the CVD is generally carried out by bringing hydrogen and one member of silane gases such as monosilane, dichlorosilane, trichlorosilane and the like or a mixture of two or more of these gases into contact with a core material kept at a high temperature, in a gaseous atmosphere diluted with an inert gas as required, to deposit silicon on the surface of the core material. Out of methods for depositing polycrystal silicon by the CVD method, there is a method for producing a polycrystal silicon rod by using silicon as a core material and thickening the rod, in particular. This method is also called a Siemens method, and is generally and widely employed.
Meanwhile, an attempt is made to melt a polycrystal silicon rod produced by the above Siemens method, as it is, and convert it into single crystals by recharging. Japanese Laid-Open Patent Publication 7-277874 teaches a technology for producing single crystal silicon by supplying the silicon rod directly as a rod for recharging.
This Laid-open Publication discloses the necessity for reducing the residual stress of a polycrystal silicon rod to prevent a fall caused by the cracking of the polycrystal silicon rod as a raw material during the preparation of single crystals. As specific means of reducing the residual stress, there is disclosed a method of producing a polycrystal silicon rod from monosilane as a raw material. This publication teaches also that since a polycrystal silicon rod obtained from raw materials other than monosilane has large residual stress, a heat treatment such as annealing is carried out before melting of the rod to remove the residual stress.
However, a polycrystal silicon rod produced from monosilane as a raw material on an industrial scale generally has low crystallinity. In other words, the polycrystal silicon rod produced from monosilane has a half value width of a peak (may be also referred to as "peak (111)" hereinafter) in the vicinity of 2.theta.=28.5.degree. of an X-ray diffraction pattern obtained using copper as a target of around 0.4.degree. to 0.50.degree..
Therefore, the polycrystal silicon rod produced from such monosilane as a raw material has a low crystallinity and in consequence, such trouble is invited that an amorphous portion remains as a hole and an etching solution remains therein when a treatment for obtaining high-purity single crystal silicon, particularly a treatment for etching the surface thereof with a view to prevent inclusion of heavy metals, is carried out.
This can be presumed from Japanese Laid-Open Patent Publication 8-169797 which teaches that fine powders formed through a homogenous reaction are contained in a polycrystal silicon rod produced by the deposition of monosilane.
Japanese Laid-Open Patent Publication 8-67510 discloses that the surface area of the above polycrystal polysilicon having a low crystallinity is increased when it is etched.
In contrast to this, a polycrystal silicon rod produced using trichlorosilane as a raw material has a high crystallinity and does not contain fine powders because it is free from a homogenous reaction.
Therefore, when trichlorosilane is used as a raw material for the production of a polycrystal silicon rod, the resulting polycrystal silicon rod does not deteriorate in quality by etching because it has a smooth surface even after etching. Further, as trichlorosilane is much more inexpensive than monosilane, the use of a polycrystal silicon rod produced from trichlorosilane for recharging has been desired.
However, since a polycrystal silicon rod produced from trichlorosilane as a raw material has large residual stress in the rod, as also disclosed in Japanese Laid-Open Patent Publication 7-277874, it has been considered to be unsuitable for use as a rod for FZ or recharging.
Further, when the polycrystal silicon rod is subjected to a heat treatment such as annealing before melting treatment for the purpose of removing the residual stress of the polycrystal silicon rod, its purity is greatly reduced by contamination and it cannot be used in the production of single crystals any longer.
That is, since the heat treatment of the polycrystal silicon rod before melting is carried out after the polycrystal silicon rod is taken out from a deposition reactor, its surface is contaminated with iron at a concentration of around 1.times.10.sup.15 atoms/cm.sup.2 by its contact with impurities contained in the air during its transfer. This surface contamination spreads into the interior of the rod during heat treatment with the result of a final contamination of about 7 ppba.
Moreover, since this heat treatment is carried out at a temperature of not lower than 1,100.degree. C, there is the possibility that the interior of the rod is contaminated by dopant impurities and heavy metal impurities released from a heater or container.
Even when such contamination on the exterior surface of the polycrystal silicon rod subjected to the above heat treatment is removed by etching in a clean room before melting in order to use it in the production process of single crystal silicon, the rod is never restored to a clean state and exerts a bad influence on the purity of the obtained single crystal silicon.
To prevent the contamination of the polycrystal silicon rod, it is conceivable to protect the rod with a quartz glass tube. However, it cannot be said that this method is effective because the quartz glass is softened at a temperature of not lower than 1,000.degree. C.