No abstract

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.

Excitation measurements of the 2E(D)-2T2(D)Cu2+ luminescences in ZnS and CdS crystals are presented, revealing new excitation processes of the centres. In ZnS sharp resonances observed on the low energy onset of the charge transfer band at 1.17 eV enable one to identify a transient shallow acceptor state of the Cu2+ centre in ZnS. An energy transfer process by photoexcited holes between Cu2+ and Fe2+ centres yields an accurate value for the deep Cu2+-acceptor position in ZnS of (1.293+or-0.005) eV. In CdS the deep Cu2+-acceptor position is (1.20+or-0.02) eV above the valence band. Additional excitation bands just below the excitonic bandgap are observed and interpreted as transitions between the conduction band and the transient shallow acceptor state. The binding energies of these states are determined to be 119 and 94 meV for ZnS and CdS, respectively. The recombination energy of the transient shallow acceptor state is transferred efficiently to the excited 2E(D) state, leading to the internal Cu2+ luminescence.

We report on optical investigations of a near-infrared luminescence band in hexagonal GaN with a single zero-phonon line (ZPL) at 1.1934 eV. It is attributed to the spin-forbidden internal d-d transition 'E (D)-A 2(F) of a defect with a d electronic configuration. This assignment is based on the observed Zeeman splittings, which agree with the ground and excited states being threefold and twofold degenerate, respectively. This interpretation is supported by the observed small full width at half maximum (FWHM) of the ZPL, the weak phonon sideband, and the weak temperature dependence of the luminescence band. With increasing temperature, the ZPL shifts towards lower energies but maintains its FWHM of about 200 peV up to 60 K. The observed luminescence lifetime of 65 ps indicates a strong mixing of the E (D) with the Tz(F) multiplet at slightly higher energies by spin-orbit interaction. Photoluminescence excitation spectra show intracenter absorption into the higher excited T, states at 1.62 and 2.8 eV in n-type samples, proving the defect to be in the luminescent charge state in n-type material. Thus, we propose Ti + as the luminescence center responsible for the 1.19-eV transition. Implications for the band o6'set between GaN and GaAs are discussed.

We report an optical investigation of the Zeeman behavior of the deep iron acceptor in GaN grown on 6H–SiC. The characteristic ground state splitting of the near-infrared luminescence transition at 1.2988 eV allows for an unambiguous assignment to Fe3+previously proposed on the basis of ODMR results. The observed luminescence lifetime of 8 ms as well as the fine structure of the excited state are consistent with a 4T1(G)–6A1(S) transition. The 4T1(G) state is found to couple only weakly to ε-type phonon modes.