Two-step growth in reduced pressure chemical vapor deposition has been successfully developed to achieve in situ phosphorus-doped silicon epitaxial layers, and the characteristic evolution of their microstructures has been investigated using scanning electron microscopy, transmission electron microscopy, and secondary ion mass spectroscopy. The two-step growth, which employs crystallization of amorphous silicon (alpha-Si) layer grown at low temperature, offers crucial advantages in manipulating crystal structures of in situ phosphorus-doped silicon. Our experimental results suggest that an epitaxial or polycrystalline film is readily obtained by initiating growth either without or with annealing to crystallize the ct-Si layers. Kinetic processes for crystallization were accompanied by stimulated dispersion of native oxides in alpha-Si buffers. We also observed that a high phosphorus doping level plays an important role in promoting the dispersion of native oxides, and that the shape of doping profiles depends upon the evolution of crystal structures with V-shaped defects. Thus, the phosphorus doping concentration remains uniform in high-quality single-crystalline Si films obtained by the two-step growth.