An oligosilane bridge model for the origin of the intense visible photoluminescence of porous silicon

Oligosilanes bridging microgaps between silicon microcrystallites were investigated as a possible model for the origin of the intense visible photoluminescence of porous silicon by means of the semiempirical molecular-orbital method. The incomplete structural relaxation of the oligosilane bridge after the photoexcitation was found to be a key factor to give the visible photoluminescence. The calculated structure-insensitive photoexcitation energy around 3.3 eV and the structure-sensitive light emission energy around 1.2–2.1 eV are consistent with the experimental evidence. The sufficient transition probabilities between the concerning electronic states support the high efficiency of photoluminescence. Durabiliy of the structure under the photoexcitation was also suggested. The model is valid even if the silicon microcrystallites are partly or thoroughly replaced by silicon oxides particles as is more realistic for the porous silicon exposed to the air.