Atomic-layer doping of P in Si epitaxial growth by alternately supplied PH3 and SiH4 was investigated using ultraclean low-pressure chemical vapor deposition. Three atomic layers of P adsorbed on Si(100) are formed by PH3 exposure at a partial pressure of 0.26 Pa at 450 °C. By subsequent SiH4 exposure at 220 Pa at 450 °C, Si is epitaxially grown on the P-adsorbed surface. Furthermore, by 12-cycles of exposure to PH3 at 300-450 °C and SiH4 at 450 °C followed by 20-nm thick capping Si deposition, the multi-layer P-doped epitaxial Si films of average P concentrations of approximately 1021 cm-3 are formed. The resistivity of the film is as low as 2.4×10-4 Ω cm. By annealing the sample at 550 °C and above, it is found that the resistivity increases and the surface may become rough, which may be due to formation of SiP precipitates at 550 °C and above. These results suggest that the epitaxial growth of very low-resistive Si is achieved only at a very low-temperature such as 450 °C.