Phonon Coupling in Edge Emission and Photoconductivity of CdSe, CdS, andCd(SexS1−x)

In mixtures of $\mathrm{Cd}\left({\mathrm{Se}}_x{\mathrm{S}}_{1-x}\right)$ two longitudinal optical (LO) phonons (and their additive combinations) couple to the edge-emission centers and to the conduction electrons. The phonon frequencies as a function of the CdSe/CdS ratio—which are observed in the edge emission spectra—agree well with the eigenfrequencies calculated for a linear chain of Cd-Se-Cd-S-Cd-...atoms. The spectral response of the photoconductivity of pure CdS and CdSe shows oscillations at the high-energy side of the absorption edge. The energy separation between successive photocurrent maxima or minima corresponds approximately to the LO phonon energy of each crystal lattice. The minima are explained by a shortened electron lifetime at the respective energies, because electrons having such energies may easily drop to a recombination center (exciton or impurity near band edge) by the emission of one or several LO phonons. It is shown that in mixed crystals the two LO phonons (and their combinations) shorten the lifetime of the conduction electrons. Thus, when excitation occurs via the conduction band, the recombination center will be populated faster in mixed crystals than in pure crystals. Hurwitz's observation, namely, that the lasing threshold in Cd (SeS) mixtures is lower than that of either CdSe or CdS, might therefore be accounted for.