Size dependence of lifetime and absorption cross section of Si nanocrystals embedded in SiO2

Photoluminescence lifetimes and optical absorption cross sections of Si nanocrystals embedded in ${\mathrm{SiO}}_2$ have been studied as a function of their average size and emission energy. The lifetimes span from 20 μs for the smallest sizes (2.5 nm) to more than 200 μs for the largest ones (7 nm). The passivation of nonradiative interface states by hydrogenation increases the lifetime for a given size. In contrast with porous Si, the cross section per nanocrystal shows a nonmonotonic behavior with emission energy. In fact, although the density of states above the gap increases for larger nanocrystals, this trend is compensated by a stronger reduction of the oscillator strength, providing an overall reduction of the absorption cross section per nanocrystal for increasing size.