Sulfur isotope effects on the excitonic spectra of CdS

The energies of bound excitons, as well as free excitons associated with the A, B, and C valence band edges, are compared using both photoluminescence spectroscopy and reflectivity spectroscopy for wurtzite CdS crystals containing natural Cd and S of either natural isotopic composition, or highly enriched 3 4 S. In contrast to an earlier study of S isotope effects in CdS, all of the transitions, including the free exciton excited states, show nearly the same energy shifts, demonstrating that it is the isotope dependence of the band gap energy which is the dominant factor in these shifts, and not changes in the free exciton binding energies. The observed dependence of the band gap on the S mass is found to be in good agreement with earlier predictions based on the temperature dependence of the free exciton transition energies.

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