Fe3+as near-infrared luminescence center in ZnS

A richly structured luminescence is observed in the 1.2-\ensuremath{\mu}m spectral region in ``pure'' and Fe-doped ZnS crystals for the first time. By means of emission, excitation, and magneto-optical measurements an unambiguous assignment to the $^{4}$${\mathit{T}}_{1}$(G)${\mathrm{\ensuremath{-}}}^{6}$${\mathit{A}}_{1}$(S) transition of isolated ${\mathrm{Fe}}^{3+}$ on ${\mathrm{Zn}}^{2+}$ lattice sites becomes possible. Basic arguments are the sixfold degeneracy of the ground state with an isotropic g factor of 2.026\ifmmode\pm\else\textpm\fi{}0.010, the long decay time of about 4 ms, and the influence of additional iron doping. The excitation mechanism can be described by an energy transfer via free holes from ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ centers and a charge-transfer process ${\mathrm{Fe}}^{3+}$long-${>}^{\mathit{h}\ensuremath{\nu}}$${\mathrm{Fe}}^{2+}$+${\mathit{h}}_{\mathrm{VB}}$ (VB denotes valence band). A comparison with the well-known isoelectronic ${\mathrm{Mn}}^{2+}$ centers reveals striking similarities. The numerous zero-phonon lines at T=1.8 K are due to the polytypic structure of the investigated crystals, offering several different ${\mathrm{Zn}}^{2+}$ lattice sites. The observed fine structures of the $^{4}$${\mathit{T}}_{1}$(G) terms and their behavior in a magnetic field indicate a strong Jahn-Teller coupling to E modes as is known for the ${\mathrm{Mn}}^{2+}$ centers.