Phosphorus doping of Si nanocrystals embedded in silicon oxynitride determined by atom probe tomography

Abstract

Silicon nanocrystals (SiNCs) embedded in a silicon oxide matrix were studied by 3D atom probe tomography (APT). The distribution of the SiNC diameter was found to have a mean value of 3.7 ± 0.8 nm. The elemental composition of these particles was determined by employing two different approaches: (i) The proximity histogram method and (ii) a cluster identification algorithm based on maximum-atom separations. Both approaches give very similar values in terms of the amount of P, O, and Si within the SiNCs: the mean atomic concentrations are cP = 0.77% ± 0.4%, cO = 12.3% ± 2.1%, and cSi = 85.3% ± 2.1%. A detailed cluster analysis implies that, on average, a 4.5-nm SiNC would contain around 30 P atoms, whereas a 2.0-nm SiNC would contain only around 3 P atoms. Radial concentration profiles obtained for these SiNCs indicate that the P content is inhomogeneous and possibly enhanced at the boundary as compared to the interior of the NCs. About 20% of the P atoms are found to be incorporated into the SiNCs, whereas roughly 30% are trapped within the interfacial layer (with a thickness of ∼ 0.8 nm); the remainder resides in the surrounding matrix. Cluster-size dependent P concentrations support the view of self-purification in the Si nanostructures.