Free to Bound Recombination and Screening Effects in CdS

The luminescence spectrum of CdS consists of a number of sharp lines from 2.43 eV to the band edge at 2.58 eV and a number of broad bands below 2.43 eV. The highest energy broad band is due to a free electron recombining with a hole bound at a shallow acceptor (free to bound). The dependence of the free to bound peak of energy on temperature and excitation intensity is investigated in the interval 4.2–80 K. Increasing the excitation intensity results in a shift to higher energy. This is interpreted in terms of free carrier screening which reduces the binding energy of the acceptor. An energy shift due to the recombination of hot electrons is also considered. An often neglected donor–acceptor interaction term is discussed and found to be significant for an accurate measurement of the acceptor binding energy. Experimental observations suggest that screening of excitons due to free carriers is ineffective.