Atom probe tomography of size-controlled phosphorus doped silicon nanocrystals

Doping of silicon nanocrystals is essential to control their electronic and optical properties. The incorporation of an impurity into a silicon nanovolume is a nontrivial task due to the self-purification effect. Here, a systematic atom probe tomography study of the phosphorus distribution and incorporation in size-controlled silicon nanocrystals embedded in silicon dioxide is presented. Qualitatively, it turns out that the phosphorus distribution in the system follows a universal, nanocrystal-size independent trend: phosphorus-enrichment at the interface with a substantial phosphorus-incorporation in the silicon nanocrystal as small as 2 nm in diameter. This clearly contradicts strict self-purification. These observations are explained by the bulk-solubility and -segregation behaviour, kinetic effects related to the diffusion lengths, and nanoscale interface strain. The quantitative determination of the amount of phosphorus atoms per quantum dot enables a systematic understanding of phosphorus-induced effects on optical and electronic properties of silicon nanovolumes.