Size confinement of Si nanocrystals in multinanolayer structures

Si nanocrystals (NCs) are often prepared by thermal annealing of multiple stacks of alternating sub-stoichiometric SiO_x and SiO₂ nanolayers. It is frequently claimed that in these structures, the NC diameter can be predefined by the thickness of the SiO_x layer, while the NC concentration is independently controlled by the stoichiometry parameter x. However, several detailed structural investigations report that the NC size confinement to within the thickness of the SiO_x layer is not strictly obeyed. In this study we address these contradicting findings: based on cross-correlation between structural and optical characterization of NCs grown in a series of purposefully prepared samples of different stoichiometry and layer thickness, we develop a comprehensive understanding of NC formation by Si precipitation in multinanolayer structures. We argue that the narrow NC size distribution generally observed in these materials appears due to reduction of the Si diffusion range, imposed by the SiO₂ spacer layer. Therefore, both the SiO_x layer thickness and composition as well as the actual thickness of the SiO₂ spacer play an essential role in the NC formation.