Excitonic dynamics in CdTe/ZnTe quantum dots

We study the dephasing and population dynamics of excitons at low temperature, in self-organized CdTe/ZnTe islands grown by atomic layer epitaxy. Using a pump–probe technique, we probe the relaxation dynamics of the electron-hole population after an excitation of the ZnTe barriers. After a fast transfer (<2 ps) of the electron-hole pairs from the barriers into the dots, we observe over 30 ps the thermalization of the pairs to the lowest energy exciton states. The following relaxation is related to the exciton recombination, showing a time constant longer than 500 ps. Degenerate four-wave mixing experiments are performed using femtosecond pulses. Quantum beats are observed, which we attribute to the ground state-exciton–biexciton level system. From the beat period we deduce a biexciton average binding energy of 13 meV, in agreement with micro-luminescence experiments. The four-wave mixing signal is also measured in presence of an electron-hole population created by a pre-pulse pumping the barriers. The dephasing times determined from the bi-exponential decay curves are seen to be constant when increasing the excitation intensity. We interpret these results using a ladder scheme of energy levels corresponding to different numbers of excitons, with constant dephasing times for each transition, in contrast to the exciton collision model used in quantum wells. Our measurements indicate that the dephasing time of the exciton-ground state transition is 19 ps and the one of biexciton-exciton transition is 2.6 ps.