Excitonic saturation in ZnSe at low temperatures: blue versus red shift

Summary form only given.The Mott transition in bulk semiconductors is well understood as a consequence of screening of the Coulomb interaction between carriers. While the energetic position of the exciton stays nearly unchanged, the exciton peak disappears due to bandgap shrinkage. This was explained qualitatively by a strong compensation of gap shrinkage and weakening of the Coulomb interaction due to screening. Recently, a considerable blue shift of the exciton was found for resonant excitation at low temperatures in bulk ZnSe, where the influence of many-body effects on the exciton can be observed much more pronounced than in III-V semiconductors due to the larger exciton binding energy. We extend our pump-and-probe experiments to different temperatures, densities of carriers, created by 100-fs pump pulses, and delays up to 100 ps. The results clearly show that the weak blue shift of the exciton observed below 20 K turns into a red shift for higher temperatures. Describing the excitonic absorption of the probe pulse by solving the semiconductor Bloch equations including phase-space filling and screening, a simple static approximation of screening fails completely resulting almost in a strong red shift of the exciton. We demonstrate that, besides carrier-carrier scattering, coherent effects, in particular carrier-polarization scattering and dynamical screening, have to be taken into account to explain the crossover of the exciton shift from blue to red.