Doping efficiency and confinement of donors in embedded and free standing Si nanocrystals

Doping semiconductor nanocrystals (NCs) is a promising way to tailor the optical and electronic behavior of these materials to enable their use in (opto)electronic applications. Yet the practical exploitation of doping requires an understanding of its efficiency, and dependence on external environment, and of the electronic localization of dopant states due to confinement effects. Here, we experimentally probe the efficiency of doping of Si NCs grown in amorphous SiO2 by means of phase segregation method. We estimate a P doping efficiency of these Si NCs of about 30% and from this we infer that most P dopants are incorporated at substitutional sites of the NCs lattice and thus act as donors. We further show that the doping efficiency in Si NCs varies by several orders of magnitude depending on their external environment. Charge traps associated with air molecules adsorbed to the NCs surface give rise to a strong compensation of donors. We observe that this process can be reverted by desorbing the molecules from the NCs surface under vacuum. Moreover, we experimentally assess the confinement energy of isolated donors in Si NCs from the temperature dependence of their magnetic resonance. From this, we provide experimental evidence for the confinement-induced increase of ionization energy of dopants with decreasing NC size previously predicted with ab initio calculations of doped Si NCs.