Effects of surface-bond saturation on the luminescence of silicon nanocrystals

The effect of surface-bond saturation on the luminescence of Si nanocrystals is studied by the tight-binding cluster model. It is found that the energy gaps increase when the diameter decreases, in accordance with the quantum confinement effect. The energy gap becomes smaller in the case of incomplete saturation of surface bonds, which are closer to the experimental results. The lifetimes decrease simultaneously one or two orders of magnitude, which explains the high luminescence efficiency of porous Si and Si nanocrystals. The lowest unoccupied molecular orbital (LUMO) A1 state turns into a surface state as the surface saturation strength is weakened, while the highest occupied molecular orbital T2 state changes little. The lowest LUMO state may be the A1 or T2 state, depending on the cluster shape. The energy difference between the two states is several or several tenths of a meV, which may explain the high luminescence efficiency of porous Si at room temperature. It seems that our theory can qualitatively unify the quantum confinement model and the surface-localized-state model for the luminescence mechanism.