Essentially, a crystal is "pulled" out of a vessel containing liquid Si as shown in Fig 3.10. A seed crystal is dipped into the liquid and is subsequently slowly withdrawn from the melt. The pulling rate (usually a few mm/min) and the temperature profile determines the crystal diameter that can be achieved. The solubility of impurity atoms in the melt is larger than in the solid. As a result, the crystal will be purer than the liquid and crystal growing is simultaneously a purification method. However the distribution of impurities vary along the length of a crystal and a homogeneous doping is difficult to achieve.
Practically only As, P, and B are used for doping because of their segregation coefficient are close to 1. The segregation coefficient in thermodynamic equilibrium gives the relation between the concentration of impurity atoms in the growing crystal and that of the melt. It is usually much lower than 1 because impurity atoms prefer to stay in the melt.
Oxygen is the most important impurity found in silicon and is from the quartz crucible in which the molten silicon is contained. The oxygen is typically at a level of about 5x1017/cm3 1018/cm3 in CZ silicon. Oxygen has three principal effects in the silicon crystal. In an as-grown crystal, the oxygen generally occupies interstitial positions in the silicon lattice and improves the yield strength by 25%. A small amount of the oxygen in the crystal forms SiO4 complexes and act as donors. Even 1016 /cm3 donors can be formed, which is significant to increase the resistivity of lightly doped P-type wafers. During the CZ growth process, the crystal cools slowly through ~500°C temperature and oxygen donors form.