Theory of optical spectra of polar quantum wells: Temperature effects

Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-two-dimensional polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on and off resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tuning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green’s function formalism at finite temperature. The effective exciton-phonon interaction is included in the Bethe-Salpeter equation. Numerical results are illustrated for ZnSe-based quantum wells. At low temperatures, both a single-exciton peak and a continuum resonance state are found in the optical absorption spectra. By constrast, at high enough temperatures, a splitting of the exciton line due to real phonon absorption processes is predicted. Possible previous experimental observations of this splitting are discussed.