Rôle of the Jahn-Teller Coupling in the Luminescence Spectra of Fe²⁺ in Zinc-Blende Compounds

The luminescence spectra of Fe²⁺ in zinc-blende-type II-VI and III-V compounds do not show an equally spaced set of emission lines as predicted by spin-orbit interaction in a plain crystalline field. The unequal separation between these lines is a signature of the Jahn-Teller effect in these systems. Attention is focused here on the general trend of the ⁵E-derived energy levels providing the end states for the emission transitions. The interval 1 between the second and third energy levels (γ₄ γ₃) is employed as a sensitive test based on the two following characteristics: First, this is the spacing that varies the most; second, the emissions to these levels are usually quite sharp as they involve energies not overlapping with phonon-assisted transitions. This property is studied in the plane [ωE_JT] (energy of the coupling phonon and the Jahn-Teller energy which is directly related to the coupling strength). The general behaviour is then studied under different theoretical conditions, in particular those that maximize the effect. Application of this theory to each real compound is thus possible by choosing the right combination of the two variables. To this end, the examples of luminescent substitutional Fe²⁺ ions in ZnS, ZnTe, and CdTe are discussed based on published spectra. The main emphasis is placed on new precise measurements of the ZnSe:Fe²⁺ emission. With crystals containing different iron concentrations, changing line shapes, including self-inversion of several emission lines, have been obtained in the 2600 to 2800 cm^-1 spectral range. The properties of the four host/impurity systems are satisfactorily explained while an overall description emerges for the whole family of these compounds from a compilation of the derived coupling parameters.