Doping of Si nanocrystals is expected to be crucial in order to tailor the properties of these nanostructures and to implement their technological applications. In this work, phosphosilicate ultra-thin films (P δ-layers) were buried in an SiO/SiO2 multilayer structure, and the redistribution of P atoms during high-temperature (800–1100 °C) thermal treatments was studied by means of ToF-SIMS depth profiling. We demonstrated that the presence of the surrounding SiO2 matrix provides a strong barrier to P diffusion and, for temperatures equal or above 1000 °C, induces P segregation in the SiO regions, where two-dimensional layers of Si nanocrystals are formed during the thermal treatment. Such an effect is qualitatively in agreement with the P diffusivity data reported in the literature. The amount of P atoms incorporated in the Si nanocrystal region is directly controlled through a limited source process by properly adjusting the thickness of the P δ-layer interposed between the SiO and SiO2 films. Copyright © 2012 John Wiley & Sons, Ltd.