A common technique for growing single crystal silicon involves the selective cooling of molten polycrystalline silicon so that solidification occurs in a particular crystal direction. The single crystal silicon is pulled from the "melt" as it grows. This technique is known as the Czochralski method of crystal growth.
In the Czochralski method, a small "seed" crystal crystal is lowered into the molten silicon and raised slowly, allowing the crystal to grow onto the seed. The crystal orientation of the solidifying crystal will be the same as that of the seed. Generally, the crystalline ingot is rotated as it grows to provide a slight stirring of the melt and average out any temperature variations that would cause nonuniform solidification.
The shape of the ingot is determined by a combination of the tendency of the cross section to assume a polygonal shape due to the crystal structure and the influence of surface tension, which encourages a circular cross section.