ROLE OF THE JAHN-TELLER COUPLING IN THE LUMINESCENCE SPECTRA OF FE2+ IN ZINCBLENDE COMPOUNDS

The luminescence spectra of Fe2+ in zinc-blende-type II-VI and m-V com pounds do not show an equally spaced set of emission lines as predicte d by spin-orbit interaction in a plain crystalline field. The unequal separation between these lines is a signature of the Jahn-Teller effec t in these systems. Attention is focused here on the general trend of the E-5-derived energy levels providing the end states for the emissio n transitions. The intervall between the second and third energy level s (gamma(4) and gamma(3)) 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 sha rp as they involve energies not overlapping with phonon-assisted trans itions. This property is studied in the plane [<(h)over bar omega>,E-J T] (energy of the coupling phonon and the Jahn-Teller energy which is directly related to the coupling strength). The general behaviour is t hen studied under different theoretical conditions, in particular thes e that maximize the effect. Application of this theory to each real co mpound is thus possible by choosing the right combination of the two v ariables. 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:F e2+ emission. With crystals containing different iron concentrations, changing line shapes, including self-inversion of several emission lin es, have been obtained in the 2600 to 2800 cm(-1) spectral range. The properties of the four host/impurity systems are satisfactorily explai ned while an overall description emerges for the whole family of these compounds from a compilation of the derived coupling parameters.