A microscopical model is proposed, describing the origin and properties of three closely spaced zero-phonon lines observed in the green Cu band in ZnO:Cu crystals labelled and . These excitations are known to be formed by a charge-transfer reaction with hole bound states. These lines are shown to originate from an intermediately bound exciton of acceptor type, . This sort of exciton, in which both carriers are captured at intermediate-radius orbitals, results from the wurzite-type symmetry of the ZnO:Cu system. The electronic structure obtained for these three intermediately bound excitons enables us to explain their magneto-optic behaviour and to calculate their g-values. Additionally, we determined the quantum efficiency of both intracentre and exciton transitions by using time-resolved and calorimetric absorption spectroscopy. While no luminescence is observed in ZnS, the exciton states in ZnO are purely radiative only to the ground state, . The picture of an intermediately bound exciton explains the recombination channels and also makes clear the difference between copper states in the ZnS and ZnO systems.

Isotope shifts for various lines associated with excitations of transition-metal impurities in semiconductors are considered. Special attention is paid to ZnO:Cu, for which experimental results are presented. Isotope shifts are measured for the so-called photoluminescence ␣ and ␤ zero-phonon lines associated with excitations of bound excitons, and of the zero-phonon line associated with the intracenter Cu 2ϩ (2 T 2 Ϫ 2 E) transition. These shifts appear to be negative and nearly equal. A theoretical model explaining these results is proposed, which incorporates the mode softening mechanism and the covalent swelling of the impurity d electron wave functions. It is shown that, contrary to transitions in simple neutral impurities, this mechanism works both for the excited and ground states of all processes in transition-metal impurities considered here. Using reasonable values of the parameters of the system, we are able to explain both the sign and value of the isotope shifts. ͓S0163-1829͑98͒04115-0͔

A possibility of intermediately bound excitons in semiconductors doped with transition metal impurities is considered. These sorts of exciton 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 strongly suppressed due to a symmetry consideration. It is shown that the exciton hole in ZnS:Ni (zinc blende type) is bound by the Coulomb field of the exciton electron and may be considered within the framework of the hydrogen-like model. As for CdS:Ni (wurzite type) the orthogonalization central cell pseudopotential appears to be the leading attracting potential for the hole. These differences in the binding mechanisms account for striking differences measured experimentally in the structure of the exciton spectra and values of the g-factors of these two presumably similar systems.

A theory of magnetic impurities in a two-dimensional electron gas quantized by a strong magnetic field is formulated in terms of Friedel-Anderson theory of resonance impurity scattering. It is shown that this scattering results in an appearance of bound Landau states with zero angular moment between the Landau subbands. The resonance scattering is spin selective, and it results in a strong spin polarization of Landau states, as well as in a noticeable magnetic field dependence of the g factor and the crystal field splitting of the impurity d levels.

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