Exciton Structure and Magneto-Optical Effects in ZnS

The exciton structure in hexagonal ZnS has been studied in transmission and interpreted in terms of an effective-mass formalism. The $n_1=1,2,3,4,\mathrm{and}5$ states of the first (${\Gamma }_7-{\Gamma }_9$) series, and the $n_2=1,2,\mathrm{and}3$ states of the second (${\Gamma }_7-{\Gamma }_7$) series have been observed and identified. From the series limits, a band gap of 31 543 ${\mathrm{cm}}^{-1\mathrm{}}$ and a crystal-field splitting of $\approx 250$ ${\mathrm{cm}}^{-1\mathrm{}}$ has been measured. Using a semiempirical theory of exciton structure to interpret the behavior of the excited states in the presence of an external magnetic field, band parameters for the ${\Gamma }_7$ conduction band and the ${\Gamma }_9$ valence band have been obtained. We find an isotropic electron effective mass $m_e=0.28\mathrm{}\pm 0.03$, and an anisotropic hole mass with $m_{\mathrm{hx}}=0.49\mathrm{}\pm 0.06$, $m_{\mathrm{hz}}=1.4\mathrm{}$, where $m_e$ and $m_h$ are expressed in terms of the free-electron mass. In addition, the conduction-band electron is found to have a $g$ value approximately equal to 2. The most interesting feature of the exciton spectra is that interpretation of the first-series excited states requires the assignment of so-called forbidden symmetry. It is shown that the occurrence of very strong forbidden transitions in ZnS is in line with the trend exhibited by the exciton spectra of CdSe and CdS.