Fast relaxation of hot carriers by impact ionization in semiconductor nanocrystals: Role of defects

We present calculations predicting that defects at the surface of semiconductor nanocrystals have a strong influence on the dynamics of hot carriers after photoexcitation. The relaxation of excited carriers by impact ionization of carriers trapped on deep levels is efficient, in particular in the presence of a band of defect states in the gap. Impact ionization of defects can also induce single-carrier multiplication but carriers generated in this way have a 1-100 ps lifetime due to multiphonon capture by the defects. These results are used to discuss recent experimental studies on carrier relaxation and multiplication in nanocrystals.