Size confinement of Si nanocrystals in multinanolayer structures

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.

Photoluminescence Quantum Yield in Ensembles of Si Nanocrystals

“…[33] In that case, the relative contribution of smaller NCs, whose absorption sets in at a higher energy, will gradually increase with the excitation energy. According to the "volcano plot," [3,4] the PL QY of Si NCs increases with decreasing size (as the radiative recombination rate grows due to quantum confinement), reaches a maximum around d ≈ 4 nm, and then decreases again, as the quality of the Si/SiO 2 interface deteriorates due to the high curvature. Nevertheless, this plausible and frequently considered possibility is not supported by the current experimental results.…”

“…[1] For these purposes, ensembles of Si NCs are considered and their high photoluminescence (PL) quantum yield (QY), which is defined as the ratio between the numbers of emitted and absorbed photons, is an essential parameter for photon emission and conversion applications. [4] In general, the optical properties of solid-state dispersions of Si NCs are determined by those of individual NCs and by cooperative processes. Optical properties of Si NCs are influenced by a combination of quantum confinement and the surface condition, and therefore depend on the NC size.…”

“…Due to different initial Si excess and annealing temperatures, the Si/SiO 2 segregation process proceeds differently, resulting in a variety of size distributions of Si NCs in individual structures which are typically characterized by a log-normal size distribution. [4] for further details. [24] Here we have used this relation to determine the average NC size within the produced samples.…”

Luminescent Properties of Silicon Nanocrystals:Spin on Glass Hybrid Materials

“…In the first method, temperatures of around 1100 • C are needed to promote the nucleation and activation of radiative Si nanopoints, crystalline or not [3,4,19,20]. Examples of these materials are the silicon rich oxides obtained by chemical vapor deposition (CVD), Ionic implantation of Si in a dielectric matrix, and thermally treated chemical solutions [4,[21][22][23].…”

“…However, if high energy is used to stimulate them, the range emission can be extended to the blue side of the electromagnetic spectrum. However, the high temperature annealing usually needed to promote the silicon-emissive center formation [3,4,19,20] complicates their potential incorporation in CMOS (complementary metal-oxide-semiconductor) integrated circuits.…”