Phosphorus incorporation during selective silicon epitaxy using the phosphine (PH3), disilane (Si2H6). and chlorine (Cl-2) chemistry in a cold-wall ultrahigh vacuum rapid thermal chemical vapor deposition reactor was investigated, We have studied the dependence of silicon growth rate and phosphorus incorporation on phosphine partial pressure and temperature in the range of similar to 10(-9) to 10(-6) Torr, and 650 to 800 degrees C, respectively. Even at such low partial pressures. phosphorus concentration above 10(18) cm(-3) was obtained due to the high sticking coefficient of phosphine. Phosphorus incorporation was found to be a strong function of temperature. Two possible incorporation mechanisms have been discussed in detail: surface electronic effects created by silicon becoming extrinsic at high phosphorus concentrations and high phosphorus surface coverage in the form of P-P dimers. A reduction in silicon growth rate was observed due to phosphine. Doping concentration was found to be uniform in the films at low temperatures (650-750 degrees C) accompanied with by phosphorus peaks at interfaces for growth temperatures above 800 degrees C. A significant chamber memory effect was observed in the process which prohibits intrinsic silicon deposition following an in situ phosphorus-doped layer.