Atomic layer doping of P (P-ALD) in Si is investigated using reduced pressure chemical vapor deposition (RPCVD). For P-ALD, PH3 exposure on Si (100) surface followed by Si cap layer deposition using SiH4 or Si2H6 is performed. P adsorption is suppressed by hydrogen-termination of the Si surface. On the hydrogen-free Si surface, the P adsorption is increasing with increasing PH3 exposure temperature saturating at temperatures above 600°C. P adsorption is also increasing with increasing PH3 exposure time. It tends to saturate at long exposure time indicating a self-limitation of the process. By Si deposition using Si 2H6, higher P peak concentration and higher P doping level in the Si cap layer is observed compared to those with SiH4 based Si deposition. For both SiH4 and Si2H6 based capping process lower P segregation is observed by lowering the growth temperature.