INTERMEDIATELY BOUND EXCITONS

A possibility of intermediately bound excitons in semiconductors doped with transition metal impurities is considered. These sorts of excito n can appear in hexagonal (wurzite-type) semiconductors due to strong hybridization of the conduction band states and the impurity d states. In tetrahedral (zinc blende) semiconductors this hybridization is str ongly suppressed due to a symmetry consideration. It is shown that the exciton hole in ZnS:Ni (zinc blende type) is bound by the Coulomb fie ld of the exciton electron and may be considered within the framework of the hydrogen-like model. As for CdS:Ni (wurzite type) the orthogona lization central cell pseudopotential appears to be the leading attrac ting potential for the hole. These differences in the binding mechanis ms account for striking differences measured experimentally in the str ucture of the exciton spectra and values of the g-factors of these two presumably similar systems.