Exciton-Phonon Interaction Effects in II-Vi Compound Semiconductor Quantum Wells

In this thesis, we report on two specific examples of exciton-LO phonon Frohlich interaction effects, namely, hot carrier relaxation and temperature dependent exciton linewidth broadening. These phenomena are considered in the context of quasi-two dimensional excitons in strongly polar II-VI semiconductor quantum wells. Hot-exciton luminescence phenomena are investigated in a single quantum well of ZnTe/MnTe where tunneling through thin MnTe barrier layers suppresses the formation of thermalized luminescence. For near resonant photoexcitation, the secondary emission spectrum is modulated by distinct LO-phonon peaks, which, for sufficiently high order of scattering ( >=4), behave like hot luminescence (HPL) as opposed to resonant Raman scattering. This is confirmed by time-resolved spectroscopy as well as by steady-state characteristics such as linewidth broadening and lack of polarization memory. Several novel observations are made: (1) The LO-phonon intermediated energy relaxation involves Coulomb-correlated pairs, i.e. hot excitons, as opposed to independently-relaxing free electrons and holes. (2) The additional weak disorder originating from QW thickness fluctuations plays a major role in the details of the HPL spectra. The major contribution to the ground state exciton linewidth at room temperature originates from LO phonon -intermediated exciton scattering to higher exciton states. A measure of the effect is given by the parameter Gamma_{LO} which increases with the polarity of the material and is independent of dimensionality provided that the LO phonon energy is greater than the exciton binding energy. Measurements of Gamma_{LO} are performed in two quantum well systems: CdTe/MnTe and (Zn,Cd)Se/ZnSe. In the latter system, a strong reduction of Gamma _{LO} is observed as the quantum well width becomes comparable to the three-dimensional exciton Bohr radius. This is explained in terms of a model where quasi-2D confinement effects increase the exciton binding energy to a value greater than the LO phonon energy and hence reduce the available phase space for the exciton -LO phonon scattering process. Direct confirmation of our interpretation is found in magneto-transmission experiments.