In situ phosphorus doped Si epitaxial layers have been grown at 600 degrees C in a very low pressure chemical vapor deposition system, using SiH4 and PH3 diluted in H-2. The presence of H-2 was found to decelerate the epitaxial growth rates. Secondary-ion mass spectrometry measurement shows that the constant phosphorus concentration with depth for a steady flow of PH3 was achieved. Namely, dopant concentration is a function of gas phase dopant concentration. Chemical concentration as high as 2.5x10(20) P/cm(3) was obtained in Si epitaxial layers though with very low growth rate. Epilayers with constant doping levels from 1.5x10(18) P/cm(3) to 1.2x10(20) can readily be grown. Despite the relatively low growth rate, there is no evidence of a time-dependent accumulation of phosphorus on the growth surface or the reactor wall. The reasons accounting for this phenomenon are discussed.