Two-step growth has been developed to achieve in situ phosphorous-doped epitaxial and polycrystalline Si films, and the characteristic transitions in microstructures have been investigated for different phosphorus concentrations and annealing conditions of Si buffers. Our experimental results showed microstructures of Si films were controlled significantly by phosphorus doping and annealing of Si buffers grown at low temperature. While undoped Si buffers grown at 500 degrees C were deposited as amorphous, Si buffers with phosphorus doping above similar to 10(20) cm(-3) converts the growth modes from amorphous to single crystalline. Moreover, we thought that heavily doped phosphorus could dissociate native oxide at Si buffers/substrate interface by kinetically activated process during annealing. Using heavily phosphorus doped buffers, we can get high quality epi Si films. In addition, uniform polycrystalline Si films grew on undoped Si buffers.