Optical transitions in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se: Strong concentration-dependent effective p-d exchange

Results of photoluminescence (PL) and resonant Raman scattering from single crystalline epitaxial films of ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$Se are presented. Samples with Co concentrations up to x=0.1 and Fe concentrations in the range 0<x<0.75 were studied. For x<0.03 (Co) and x<0.30 (Fe) the optical emission associated with the 3d deep levels are observed as discrete transitions that broaden very rapidly and weaken as the doping further increases. It is proposed that the increase in concentration x leads to an increase in the exchange between low-lying crystal-field split d levels and band electrons, and thus to the changes in the photoluminescence. Concomitant with the onset of broadening of the PL, a continuum of Raman excitations extending from zero to about 4000 ${\mathrm{cm}}^{\mathrm{-}1}$ is observed which is resonantly enhanced as the energy of the exciting radiation approaches the alloy band gap from below. The Raman-scattering features are discussed in terms of electronic scattering within the 3d-ion band manifold located in the gap of the insulating host. The role of strong disorder and correlations on the electronic Raman excitations in these insulating structures are briefly addressed.