  Zinc sulfide is a wide bandgap semiconductor which crystallizes in either the
wurtzite modification (a-ZnS), the zincblende modification (b-ZnS) or as one of
several similar tetrahedrally coordinated polytypes. In this work, we report a
photoluminescence study of different samples of isotopically pure b-ZnS
crystals, and crystals with the natural isotopic abundances, at 15 and 77 K.
The derivatives of the free and bound exciton energies on isotopic mass have
been obtained. They allow us to estimate the contribution of the zinc and
sulfur vibrations to the bandgap renormalization energy by electron-phonon
interaction. A two-oscillator model based on the zinc and sulfur
renormalization energies has been used to account for the temperature
dependence of the bandgap energy in ZnS. The results are compared with those
found for other tetrahedrally coordinated semiconductors.
