ISOTOPE SPLITTING OF THE ZERO-PHONON LINE OF FE2-V SEMICONDUCTORS( INCUBIC III)

A theoretical study of the isotopic-mass dependence of the internal tr ansitions of Fe2+ at a cation site in a cubic zinc-blende semiconducto r is presented. The model used is based on crystal-field theory and in cludes the spin-orbit interaction and a weak dynamic Jahn-Teller coupl ing between the (5) Gamma(5) excited manifold of Fe2+ and a local vibr ational mode (LVM) of Gamma(5) symmetry. The mass dependence of the LV M frequency is described, in the harmonic approximation, within two di fferent limits: the rigid-cage model and a molecular model. In the rig id-cage model, the Fe2+ ion undergoes a displacement but the rest of t he lattice is fixed. In this case, a simple M-1/2 dependence of the fr equency is obtained and the Jahn-Teller energy, E-JT, is independent o f the mass. In the molecular model, the four nearest neighbors of the magnetic ion are allowed to move and the LVM then behaves as the Gamma (5) mode of a MX4 tetrahedral molecule leading to a more complicated d ependence of the frequency on the isotopic mass and to a mass-dependen ce of E-JT. The theoretical results obtained with these two models are compared with the observed isotopic shifts of the zero-phonon lines i n InP:Fe and GaP:Fe corresponding to an optical transition between the vibronic Gamma(1) ground state and the lowest Gamma(5) state originat ing from the (5) Gamma(5) excited orbital multiplet. A prediction of t he isotopic shifts of the zero-phonon line in GaAs:Fe is also presente d. (C) 1997 Elsevier Science Ltd.