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 PH, exposure at a partial pressure of 0.26 Pa at 450 degreesC. By subsequent SiH4 exposure at 220 Pa at 450 degreesC, Si is epitaxially grown on the P-adsorbed surface. Furthermore, by 12-cycles of exposure to PH3 at 300-450 degreesC and SiH4 at 450 degreesC followed by 20-nm thick capping Si deposition, the multi-layer P-doped epitaxial Si films of average P concentrations of similar to 10(21) cm(-3) are formed. The resistivity of the film is as low as 2.4 X 10(-4) Omega cm. By annealing the sample at 550 degreesC 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 degreesC 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 degreesC.