The deposition of in situ heavily boron-doped polycrystalline silicon (poly-Si) films was studied using an ultrahigh vacuum/chemical vapor deposition system. Fully activated carrier concentrations up to 3×1020 cm-3 were obtained for the as-deposited films grown at 550°C. For boron concentration beyond this level, the crystallinity of poly-Si films degraded with increasing boron concentration, which resulted in an anomalous rise in resistivity. This crystallinity degradation occurred at a higher rate for films grown on a SiO2 surface than those grown on an undoped poly-Si surface. It is attributed to the preferential adsorption of boron atoms on the SiO2 surface. Under a high B2H6 flux condition, a large amount of boron atoms would accumulate on the SiO2 surface before the formation of Si nuclei, and thus disturbs the subsequent film deposition and grain growth processes.