Impact ionization and auger recombination rates in semiconductor quantum dots

Impact ionization and Auger recombination in nanoscale spherical quantum dots (QDs) have been studied theoretically. It is shown that due to the strong quantum confinement of both electrons in the conduction band and holes in the valence band, impact ionization and Auger recombination energies in these QDs can be on the order of a few millielectronvolts when various selection rules are fulfilled, which are much higher than spontaneous radiative emission energies. This explains the experimentally reported high occurrence rates of the multiple exciton generation (MEG) effect in QDs. However, due to quantum confinement, the energy states are discrete in QDs, especially for low-energy states where the densities of states are low. This implies that only a limited number of high-energy electron states can interact with (i.e., impact ionize) low-energy hole states in QDs having certain values of radii due to the energy conservation requirement. This explains the vastly scattered experimental data and difficulties in utilizing the MEG effect in practice. © 2010 American Chemical Society.