Boron doping in Si layers grown by molecular beam epitaxy (MBE) at 500–700 °C using a HBO2source has been studied. The maximum boron concentration without detectable oxygen incorporation for a given substrate temperature and Si growth rate has been determined using secondary‐ion mass spectrometry analysis. Boron present in the Si MBE layers grown at 550–700 °C was found to be electrically active, independent of the amount of oxygen incorporation. By reducing the Si growth rate, highly boron‐doped layers have been grown at 600 °C without detectable oxygen incorporation.