Near and far infrared transitions of Fe2+ in cubic II-VI semiconductors: Dynamic Jahn-Teller interaction

Optical-absorption and emission measurements of doubly ionized iron in CdTe, ZnTe, ZnSe and ZnS have been analyzed by means of a vibronic coupling model. The model [1] is based on the crystal-field theory, including spin-orbit and spin-spin interactions and Jahn-Teller couplings of the orbital doublet and triplet states of Fe2+ with overtones of phonons of Gamma(3) and Gamma(5) symmetries, respectively. Starting from the 25 spin-orbit wave functions appropriate to the orbital doublet and triplet manifolds, the symmetry-adapted vibronic basis is constructed and used to diagonalize the Hamiltonian matrix formed out of five blocks of symmetry Gamma(1), Gamma(2), Gamma(3), Gamma(4) and Gamma(5). Phonon overtones up to N=6 for the Gamma(3) phonon and up to N=10 for the Gamma(5) phonon were included to ensure convergence of the energy values and eigenfunction of the vibronic states. The available measured positions and relative intensities of the spectral lines in the near and far infrared are accounted for within the experimental accuracy. It is shown that the above mentioned iron based II-VI compounds (ZnS:Fe2+ excepted) exhibit a strong dynamic Jahn-Teller coupling of the upper orbital triplet state with phonons of Gamma(5) symmetry and that the spin-orbit interaction between the orbital doublet and triplet states of Fe2+ is essential to explain the data.