Ultrafast relaxation of photoexcited carriers in ZnSe/ZnMgSSe multiple-quantum wells

By means of femtosecond pump and probe spectroscopy we have investigated well-width-dependent behaviors of intraband relaxation and exciton formation in ZnS/ZnMgSSe multiple-quantum well (MQW) structures with well-widths of 3, 5 and 13 nm. The spectral change of exciton line induced by the pumping pulse is composed of broadening, peak shift and change in absorption area. The red shift coming from the band gap renormalization appears immediately after the pulse excitation, and subsequently, the peak is shifted to the higher-energy side. The blue shift is interpreted in terms of the exciton-exciton exchange interaction and its temporal behavior reflects a kinetics of exciton formation following the intraband relaxation. Time evolution of additional broadening also reflects the carrier relaxation within bands with emission of longitudinal optical (LO) phonons. The results show that the electron-LO phonon scattering rate is larger for the smaller well-width, which is consistent with the theory taking into account both confined phonons and interface phonons. (C) 1998 Elsevier Science B.V. All rights reserved.