Size-dependent electron-hole exchange interaction in Si nanocrystals

Silicon nanocrystals with diameters ranging from »2 to 5.5 nm were formed by Si ion implantation into SiO₂ followed by annealing. After passivation with deuterium, the photoluminescence (PL) spectrum at 12 K peaks at 1.60 eV and has a full width at half maximum of 0.28 eV. The emission is attributed to the recombination of quantum-confined excitons in the nanocrystals. The temperature dependence of the PL intensity and decay rate at several energies between 1.4 and 1.9 eV was determined between 12 and 300 K. The temperature dependence of the radiative decay rate was determined, and is in good agreement with a model that takes into account the energy splitting between the excitonic singlet and triplet levels due to the es from 8.4 meV for large nanocrystals (»5.5 nm) to 16.5 meV for small nanocrystals (»2 nm). For all nanocrystal sizes, the radiative rate from the singlet state is 300–800 times larger than the radiative rate from the triplet state.