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where exciton densities are very large ({~} 10¹⁹/cm³) and the independent-particle description of the exciton is no longer valid.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 {}). As the laser power is increased from 100 kW/cm² to 8 MW/cm², 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.