Doping of Si nanocrystals is an important topic in the emerging field of Si nanocrystals based all-Si tandem solar cells. Boron-doped Si nanocrystals embedded in a silicon dioxide matrix were realized by a co-sputtering process, followed by high temperature annealing. The x-ray photoelectron spectroscopy B 1s signal attributable to Si-B (187 eV) and/or B-B (188 eV) indicates that the boron may exist inside Si nanocrystals. A higher probability of effective boron doping was suggested for Si-rich oxide films with a low oxygen content, Then, structural and optical properties were characterized with a focus on the effects of the boron content on Si quantum dots. The results show that as the boron content increases, the nanocrystal size is slightly reduced and the Si crystallization is suppressed. The photoluminescence intensity of the films is decreased as the boron content increases. This is due to boron-induced defects and/or Auger processes induced by effective doping. These results can provide optimal conditions for future Si quantum dot based solar cells.