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

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 N 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 angstrom 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 N 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 N 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 N 4 -embedded NCs versus SiO 2 -embedded NCs. However, SRH interface recombination renders the PL of Si 3 N 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.

Automated summary

This study investigates the optical and electronic properties of silicon nanocrystals under different dielectric coverings. It reveals the influence of dielectric and interface defects on the properties. The results show that silicon nitride-covered nanocrystals have higher optical absorption rates but inferior photoluminescence due to interface recombination. Additionally, the exciton binding energy is larger in silicon nitride-covered nanocrystals.