Polycrystalline silicon rod and process for production thereof

A process for producing polycrystalline silicon rods by introducing a reaction gas comprising a silicon-containing component into a reactor, as a result of which polycrystalline silicon is deposited on thin rods up to a target rod diameter, said process comprising:

• (a) in a first step, depositing a core A having a porosity of 0 to less than 0.01 onto each of the thin rods up to a rod diameter of 15-60 mm, the rod temperature being 1000° C. to 1150° C., the concentration of the silicon-containing component in the reaction gas being 20 to 60 mol % and a feed rate of the silicon-containing component being 100 to 550 kg/h per 1 m2 of rod surface area;
• (b) in a second step, beginning at a rod diameter of at least 10% up to a rod diameter of at most 90% of the target rod diameter, depositing a layer B having a porosity of 0.06 to 0.23 onto core A, the rod temperature being 1030° C. to 1130° C., the concentration of the silicon-containing component in the reaction gas being 20 to 40 mol % and the feed rate of the silicon-containing component being 80 to 200 kg/h per 1 m2 of rod surface area;
• and
• (c) in a third step, beginning at a rod diameter of at least 50% up to a rod diameter of at most 100% of the target rod diameter, depositing a layer C having a porosity of 0.01 to 0.1 onto layer B, the rod temperature being 960° C. to 1030° C. and being at least 20° C. lower than the rod temperature during the second step, the concentration of the silicon-containing component in the reaction gas being 15 to 35 mol %, and the feed rate of the silicon-containing component being 10 to 130 kg/h per 1 m2 of rod surface area.