Efficiency of the coherent biexciton admixture mechanism for multiple exciton generation in InAs nanocrystals

We study the coherent mixing between two-particle (single exciton) and four-particle (biexciton) states of a semiconductor nanocrystal resulting from the coulomb coupling between states with different numbers of electron-hole pairs. Using a simple model of the nanocrystal wave functions and an envelope function approach, we estimate the efficiency of the multiple exciton generation (MEG) process resulting from such coherent admixture mechanism, including all the relevant states in a very broad energy interval. We show that in a typical ensemble of nanocrystals with 3~nm average radius, the onset of the MEG process appears about 1~eV above the lower edge of the biexciton density of states and the efficiency of the process reaches 50\% for photon energies around 5~eV. The MEG onset shifts to lower energies and the efficiency increases as the radius grows. We point out that the energy dependence of the MEG efficiency differs considerably between ensembles with small and large inhomogeneity of nancrystal sizes.