First principles study on phosphorus or boron doping in si nanocrystals with various sizes embedded in SiO2 matrix

Doping in Si nanocrystals (Si NCs) is an interesting topic since the doping behaviors in the nanoscale are quite complicated compared with the case in bulk Si. In our present work, we use the first-principles calculation to study Phosphorus (P) or Boron (B) doping in Si NCs with the size of 2-8 nm embedded in SiO2 matrix by taking into account the existence of dangling bonds on the interfacial region. It is found that both P and B impurities tend to stay at the interfacial region to passivate the dangling bonds when the dot size is as small as 2 nm. However, P impurities exhibit the possibility to occupy the inner sites of Si NCs while B impurities are more difficult to be introduced into Si NCs due to the large formation energy. Our detailed study suggests that P or B impurities preferentially stay at the intermediate sites between Si and oxygen to form stable bonding configurations. With increasing the dot size from 2 nm to 8 nm, both P and B impurities can enter into the Si NCs more easily due to the relaxation of stress in the larger-sized Si NCs. Our theoretical results are in good agreement with the experimental observations.