Abstract
Si nanocrystals (NCs) are often prepared by thermal annealing of multiple stacks of alternating sub-stoichiometric SiOx and SiO2 nanolayers. It is frequently claimed that in these structures, the NC diameter can be predefined by the thickness of the SiOx 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 SiOx 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 SiOx layer thickness and composition as well as the actual thickness of the SiO2 spacer play an essential role in the NC formation.
Highlights
These results have been contested by detailed structural characterizations of Si NCs19,28, indicating that the NC size confinement to within the thickness of the SiOx layer, was not strictly obeyed
We start by addressing transmission electron microscopy (TEM) and high-resolution TEM images (HRTEM), which were taken for several Si NC samples and shown in the Supplementary Information (SI)
Based on structural and optical characterization of ML structures of Si NCs, prepared with different growth parameters, we have shown that the previously reported size confinement induced by the thickness of the sub-stoichiometric layer is not valid
Summary
Our results indicate that from solely the diffusion and precipitation of Si we can explain our results, and other observations[17,21,23,27,28], without the need for a NC-size confinement model. Increasing efficiency values are observed for decreasing NC sizes until a threshold value of a mean NC diameter of around 3 nm. The diffusion coefficient of Si within the spacer layer defines the threshold thickness of the spacers (as in Fig. 2) sufficient to prevent the “Si exchange” between neighboring SiOx layers. In line with this reasoning, a larger threshold thickness is observed for the higher annealing temperature, a result which quantitatively coincides with the findings of Roussel et al.[32], shown in the SI
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