JAHN-TELLER EFFECT OF CU2+ IN II-VI COMPOUNDS

High-resolution spectroscopy of weakly Cu-doped CdS and polymorphic Zn S crystals leads to the detection of new Cu2+ (E-2 - T-2(2)) zero-phon on lines which have been investigated as a function of temperature and in magnetic fields up to 15 T. As a result, an unique termscheme for Cu2+ in the sulfide host compounds is presented. We interpret the obse rved Cu2+ transitions in ZnS and CdS crystals by a moderate Jahn-Telle r coupling of the electronic states to a local vibrational mode of E s ymmetry and a Huang-Rhys factor of S = 0.8 for ZnS and S = 1.1 for CdS . We report on parameter-free calculations of the magnetic-field split ting of Cu2+ centers in II-VI compounds, which show a general agreemen t with the observed spectra. For ZnS:Cu2+ the agreement between the ca lculated and observed g values is excellent, whereas some differences for the T-2(2) ground-state g factors occur. This is due to the neglec t of the T-2 mode coupling in the Jahn-Teller calculation. The local v ibrational modes of copper in ZnS and CdS are investigated and fitted in a cluster calculation using the valence-force model of Kane includi ng Coulomb forces. An E mode at 32.4 meV is found to be the dominant c oupling mode in ZnS whereas a T-2 mode at 21.3 meV is obtained for CdS .