Polariton Relaxation and Bound Exciton Formation in PbI₂ Studied by Excitation Spectra

Abstract

Excitation spectra for the luminescence of free and bound excitons in PbI₂ single crystal have been investigated in the exciton absorption region. An oscillatory structure in the excitation spectrum for the free exciton luminescence is explained in terms of phonon scattering process and spatial distribution of polaritons. It is found that trapping of polaritons to form the bound excitons takes place efficiently by virtue of emission of optical phonons. The profile of the excitation spectrum for the bound exciton luminescence is different from those in other semiconductors. This is interpreted to arise from the difference in the trapping mechanism, which depends on the magnitude of optical phonon energies relative to the binding energy of bound excitons. Formation process of bound excitons is discussed by comparing the cases of PbI₂, CdS and HgI₂.