Role of surface self-trapped excitons in the energy relaxation of photoexcited silicon nanocrystals

A new mechanism of the energy relaxation of hot charge carriers in silicon nanocrystals embedded in the SiO$_2$ matrix is suggested. Effective energy exchange between “hot” excitons in a nanocrystal and the surface state of a self-trapped exciton leads to the excitation of vibrations in the Si–O surface defect. Relaxation of the vibration energy gives rise to the emission of local phonons, which, in turn, transfer the energy to the SiO$_2$ matrix and transform into phonons with lower energies as a result of anharmonism. Simulation by the Monte Carlo method shows, due to this mechanism, “hot” localized charge carriers lose their energy within $\sim$ 100 ps after excitation. It is also shown that a broad band of energy distribution of “hot” charge carriers is formed rapidly (within 5–10 ps) after excitation of a nanocrystal. The maximum of the band shifts during the course of relaxation.