Boron-doped silicon epitaxial layers were grown by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition at 440–510 °C. Reflection high-energy electron diffraction and transmission electron microscopy (TEM) were used to study the effect of boron doping on the crystalline quality of silicon epitaxial layers. At growth conditions where undoped defect-free Si epitaxial layers were successfully obtained at 440 °C, in situ boron-doped epitaxial layers were replete with twins. However, at conditions with increased ion energy flux and at a higher temperature, 470 °C, no twins were observed. TEM analysis revealed the presence of an amorphous phase in the twinned epitaxial layers. It is believed that the amorphous phase formation, presumably from the reaction between B and O during the doping process, appeared to hinder the growth of the epitaxial layer, leading to degradation of the Si crystalline quality. Defect-free boron-doped Si epitaxial layers were able to be obtained by suppressing the amorphous phase formation at conditions with increased growth temperature and higher ion energy flux. © 1999 American Vacuum Society.