Photoluminescence at High Exciton Densities in CdS

This report is concerned with exciton interactions in CdS at 4.2 °K in a range of concentrations where exciton densities are very large (∼ ${10}^{19}$/${\mathrm{cm}}^3$) and the independent-particle description of the exciton is no longer valid. The exciton interactions are studied by monitoring the photoluminescence emission from this compound when excited by intense laser radiation. The exciton-emission spectra of CdS were studied as a function of excitation power in the region of the $M$ band (4904 Å). As the laser power is increased from 100 ${\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ to 8 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$, several new emission bands appear on the long-wavelength side of the $M$ band shifted by several meV from the $M$ band. Both of these new bands show a superlinear dependence on laser power. Several theories for the behavior of high-density exciton systems are examined, and it is shown that the formation and subsequent decay of an excitonic molecule can consistently explain the observed photoluminescence behavior. We deduce a molecular binding energy of 3-4 meV for the excitonic molecule in CdS.