Doping
An epitaxial layer can be doped during deposition by adding impurities to the source gas, such as arsine, phosphine or diborane. The concentration of impurity in the gas phase determines its concentration in the deposited film. As in CVD, impurities change the deposition rate. Additionally, the high temperatures at which CVD is performed may allow dopants to diffuse into the growing layer from other layers in the wafer ("out-diffusion"). Also, dopants in the source gas, liberated by evaporation or wet etching of the surface, may diffuse into the epitaxial layer ("autodoping"). The dopant profiles of underlying layers change as well, however not as significantly.
Read more about this topic: Epitaxy