Grundlegendes Verständnis der Lumineszenzeffizienz von Silizium Quantenpunkten
Zusammenfassung der Projektergebnisse
The project investigated in details the luminescence efficiency of size controlled Si quantum dots. We used our detailed knowledge to prepare Si NCs in single-layer and multilayer structures with variable thickness of the oxide barriers. The absorption cross section (ACS) of Si NCs was systematically determined based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We were able to show that an optimum separation barrier thickness of ca. 1.6 nm is needed to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by the smaller occupation number of phonons and the expansion of the band gap of Si NCs at low temperatures. Further, the photoluminescence quantum yield (QY) of the Si NCs of oxynitride (SRON) was studied. It was shown that the PL QY is sensitive mostly to the thickness of SRON and barrier oxide layers and to the passivation procedures. Annealing in hydrogen improved the QY proportionally to the NC surface area mainly by passivating the NC/oxide interface defects present at a surface density of about 2.5 × 10^12 cm^−2. A maximum external QY of nearly 30% was found in well-passivated multilayers with a SiNC size of about 4 nm and a SiO2 barrier thickness of 2 nm or larger. The existence of an extended near-infrared tail of the PL spectra was revealed, whose weak intensity anti-correlates with the external QY. The relative intensity of this emission increases with temperature as well as for strong excitation above the PL saturation level and may be related to excitation energy transfer to the structural defects near NCs. Based on the presented results the mechanisms reducing the EQY of SiNCs in oxynitride films can be summarized as: (1) Strong excitation effects, (2) Defective (dark) nanocrystals, i.e. energy transfer to defect centers by the weak extended infrared emission, whose relative intensity anti-correlates with EQY values. (3) Absorption by defects, (4) Thermal effects. In addition, the nearly perfect near-infrared luminescence efficiency of Si nanocrystals was investigated and a comprehensive quantum yield study employing the Purcell effect was presented. Optimized layers of Si NC in oxide matrix fabricated by the PECVD (Si NC sizes ~ 4.5 nm, SiO2 barrier thickness 3 nm) reveal an external quantum yield (QY) close to 50% which is near to the best chemically synthetized colloidal Si NC. Internal QY is determined using the Purcell effect. For the first time we performed these experiments at variable temperatures. The complete optical characterization and knowledge of both internal and external QY allow to estimate the spectral distribution of the dark and bright NC populations within the Si NC ensemble. We show that the Si NCs emitting at around 1.2–1.3 eV are mostly bright with internal QY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes (below 100 K internal QY approaches (100%). The mechanisms of non-radiative decay are discussed based on their temperature dependence.
