The doping of polycrystalline silicon films is known to structurally change the material, producing larger grains after annealing. The grain growth is especially enhanced during high‐temperature doping using a PBr3 or POCl3 dopant source. In this paper, we report our observation of the PBr3‐doping induced epitaxial growth in polycrystalline silicon films deposited on silicon substrates and on the silicon in windows etched in the oxide. Epitaxial films contained twins induced by the microcrystals at the substrate‐film interface. Models are proposed to explain possible mechanisms for grain growth of undoped and PBr3‐doped polysilicon films. Also proposed are possible explanations for the observed microcrystalline growth at the epitaxy‐substrate interface and in the epitaxial film itself.