Coexistence of excitons and free carriers in Cd1−xMnxTe/ZnTe multiple-quantum wells

Optical pump–probe experiments were carried out in Cd 1− x Mn x Te/ZnTe multiple-quantum wells to study the carrier dynamics and the exciton formation in these materials. Starting from 15 ps after the pump excitation, the bleaching of the absorption of the heavy-hole excitons in the wells yields a strong negative pump–probe signal. But before this occurs, the pump–probe signal has a completely different lineshape resembling a lying `S'. The time evolution of the pump–probe signal is understood to have arisen from a coexistence of carriers and excitons. We treated the excitons in the wells as Mahan excitons and obtained a good fit to the experimental data. The cooling of the carriers after the pump excitation can be monitored as an increasing Fermi energy of the `cold’ carriers at the band edge. The Fermi sea is then depleted starting from 15 ps after the pump excitation due to the formation of excitons.