Absorption and Photoluminescence of Silicon Nanocrystals Investigated by Excited State DFT: Role of Embedding Dielectric and Defects

Abstract

Absorption and photoluminescence (PL) properties of silicon (Si) nanocrystals (NCs) covered with silicon dioxide ( SiO 2 ) are fairly well unraveled; corresponding information for silicon nitride ( Si 3 normalN 4 ) coverage is scarce. We elucidate important optical and electronic features depending on the embedding dielectric and interface defect (dangling bond, DB) properties. Using density functional theory (DFT) and time‐dependent (TD‐) DFT for ground state (GS) and excited state (ES) properties, respectively, we compute fully NH 2 ‐ and OH‐covered NCs of 11–26 Å size, enabling comparisons with experimental data. Our non‐radiative Shockley‐Read‐Hall (SRH) recombination model of DBs at NC/dielectric interfaces demonstrates that SRH recombination is substantially higher for Si 3 normalN 4 ‐covered NCs. An ensemble TD‐DFT calculation of the eight lowest fundamental transitions accurately describes the absorption edge. Exciton binding energies are significantly smaller in SiO 2 ‐ versus Si 3 normalN 4 ‐covered NCs due to the delocalizing versus self‐localizing impact of the dielectric onto the exciton. We find higher optical absorption rates for Si 3 normalN 4 ‐embedded NCs versus SiO 2 ‐embedded NCs. However, SRH interface recombination renders the PL of Si 3 normalN 4 ‐embedded NCs inferior to their SiO 2 ‐embedded counterparts. Finally, we explain a discrepancy in PL gaps of free‐standing oxidized versus SiO 2 ‐embedded NCs by considering adequate phononic boundary conditions.

Origin and Quantitative Description of the NESSIAS Effect at Si Nanostructures

“…Possibly identical low nanoscale intrinsic Si NWell systems near the low end of the one-digit nm range embedded in different dielectrics pose a challenge in experiment. The common perception of Si 3 N 4 as an inferior dielectric on grounds of interface defect density [10] and its more complex technology as opposed to SiO 2 [11] are likely reasons for the literature on low nanoscale Si embedded in or coated with Si 3 N 4 being rather scarce. Indeed, standard Si 3 N 4 has an interface defect density to low nanoscale Si which exceeds values of SiO 2 /Si interfaces ca.…”

“…Indeed, standard Si 3 N 4 has an interface defect density to low nanoscale Si which exceeds values of SiO 2 /Si interfaces ca. 13-fold, [10,12,13] though refined preparation techniques for high-quality H-passivated Si 3 N 4 -coatings rival trap densities on SiO 2 /Si interfaces. [14] This complex situation may explain why the NESSIAS might have been overlooked in the past.…”