Spectroscopic ellipsometry study of the diluted magnetic semiconductor system Zn(Mn,Fe,Co)Se

Single-crystal films of (001)Zn_1-x{A}ₓSe (A=Mn, Fe, Co) (0≤x0.14) grown by molecular-beam epitaxy on (001)GaAs have been studied by spectroscopic ellipsometry in the 3.5--5.5 eV photon-energy range. Using fits of the{E}₁$ and ${E}₁$+${{{Δ}}}₁$ peaks with a standard analytic expression, we find that the linewidths increase with x for all samples, the energies increase with x for ${Zn}_{1{{-}}x}FeₓSe and Zn_1-x{Co}ₓ$Se, and the energies decrease with x for ${Zn}_{1{{-}}x}MnₓSe. A model describing the effects of the sp-d exchange interaction on the L point band-gap energy is developed and applied. We find that the strength of the energy correction due to this interaction, which is proportional to the product of the square of the exchange integrals and the magnetic susceptibility of the material, is largest in Mn-doped and smallest in Co-doped ZnSe. While the sp-d exchange interaction model is consistent with the composition dependence of the E₁ and E₁+Δ₁ band-gap energies in Zn_1-x{Mn}ₓ$Se, it does not describe the behavior observed in ${Zn}_{1{{-}}x}FeₓSe and Zn_1-x{Co}ₓ$Se. We show that an sp-d hybridization model, which includes the location of the energy levels of the magnetic impurity d levels, can account for the composition dependence of ${E}₁$ and ${E}₁$+${{{Δ}}}₁$ band-gap energies of all three materials.