Electronic excitations of substitutional transition-metal ions in II-VI semiconductors: CdTe:${\mathrm{Fe}}^{2+}$ and CdSe:${\mathrm{Fe}}^{2+}$

The near-infrared electronic transitions of substitutional ${\mathrm{Fe}}^{2+}$ in the tetrahedral environment in the zinc-blende II-VI semiconductor CdTe and in the trigonal environment in its wurtzite counterpart CdSe are studied with Fourier-transform spectroscopy. In CdTe:${\mathrm{Fe}}^{2+}$, the transitions correspond to initial states in the ${}_{}{}^5{}_{}{}^{}\mathrm{\Gamma }_3^{}{}_{}{}^{}$ orbital multiplet whose separation involve σ=(‖λ‖/Δ) and (${\lambda }^2$/Δ), where λ is the spin-orbit coupling and Δ, the crystal-field splitting. The final states are sublevels of the ${}_{}{}^5{}_{}{}^{}\mathrm{\Gamma }_5^{}{}_{}{}^{}$ orbital multiplet, lying Δ above ${}_{}{}^5{}_{}{}^{}\mathrm{\Gamma }_3^{}{}_{}{}^{}$; their separations are in the main fixed by the magnitude of λ. The appearance and the increasing intensity of several electronic transitions, as the temperature is changed from 2.0 to ∼12.0 K, can be accounted for in terms of the magnetic levels which lie above the nonmagnetic ${\mathrm{\Gamma }}_1$ ground state, taking into account their increasing thermal population as well as their degeneracies. The corresponding energy levels in CdTe:${\mathrm{Fe}}^{2+}$ and CdSe:${\mathrm{Fe}}^{2+}$ show additional splittings in the latter due to the lower trigonal symmetry. The electronic transitions in CdSe:${\mathrm{Fe}}^{2+}$ show splittings and polarization effects associated with the trigonal symmetry. Phonon-assisted electronic transitions are observed in both CdTe:${\mathrm{Fe}}^{2+}$ and CdSe:${\mathrm{Fe}}^{2+}$.