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 () are fairly well unraveled; corresponding information for silicon nitride () 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 ‐ 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 ‐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 ‐ versus ‐covered NCs due to the delocalizing versus self‐localizing impact of the dielectric onto the exciton. We find higher optical absorption rates for ‐embedded NCs versus ‐embedded NCs. However, SRH interface recombination renders the PL of ‐embedded NCs inferior to their ‐embedded counterparts. Finally, we explain a discrepancy in PL gaps of free‐standing oxidized versus ‐embedded NCs by considering adequate phononic boundary conditions.