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

The effect of surface-bond saturation on the luminescence of Si nanocrystal s is studied by the tight-binding cluster model. It is found that the energ y gaps increase when the diameter decreases, in accordance with the quantum confinement effect. The energy gap becomes smaller in the case of incomple te saturation of surface bonds, which are closer to the experimental result s. The lifetimes decrease simultaneously one or two orders of magnitude, wh ich explains the high luminescence efficiency of porous Si and Si nanocryst als. The lowest unoccupied molecular orbital (LUMO) A1 state turns into a s urface state as the surface saturation strength is weakened, while the high est occupied molecular orbital T2 state changes little. The lowest LUMO sta te may be the A1 or T2 state, depending on the cluster shape. The energy di fference between the two states is several or several tenths of a meV, whic h may explain the high luminescence efficiency of porous Si at room tempera ture. It seems that our theory can qualitatively unify the quantum confinem ent model and the surface-localized-state model for the luminescence mechan ism.