Intrinsic luminescence from self-trapped excitons in Bi_4Ge _3O_(12) and bi_(12)geo_(20): decay kinetics and multiplication of electronic excitations

摘要

The intrinsic luminescence appearing at 500 nm in Bi_4Ge _3O_(12) (e-BGO) and that at 450 nm in Bi_(12)GeO _(20) (s-BGO) have been studied over a wide range of temperature T = 5-300K by using a Nd:YAG laser and synchrotron radiation as excitation light sources. Luminescence decay curves in e-BGO depend dramatically on the laser power; they are composed of three decay components under high-density excitation, while they show a single exponential decay at low-density excitation. From temperature dependences of the decay time and emission intensity, it is clarified that the triplet state of a self-trapped exciton (STE) responsible for the e-BGO luminescence consists of a pair of closely spaced sublevels with separation energy of 5.7 meV. The decay curves of s-BGO luminescence are essentially nonexponential, irrespective of the excitation power. Time-resolved luminescence measurements of s-BGO suggest the existence of a singlet state lying higher than the triplet STE state. The excitation spectra for the intrinsic luminescence bands have been measured up to 35 eV (35 nm) at 5K. From the obtained spectra, it is obvious that the multiplication of electronic excitations takes place efficiently in both BGOs. The production processes of multiple excitons are discussed by referring to a recent study on the electronic structures.