Localized states in germanate glasses. Study of luminescence

The nature of electronic states of germanate glasses at the intrinsic absorption threshold is studied by the luminescence method. Three types of luminescence were observed. A broad luminescence band at 1.8 eV excited by ionizing radiation only in pure germania glass is ascribed to a self-trapped exciton. The already known photoluminescence (PL) band at 2.9 eV of the twofold-coordinated germanium centers was observed in germanate glasses obtained under reduced or normal melting conditions. The third is the PL band at 2.8 eV, excited in the strong absorption range of all the glasses studied. It was observed irrespective of the redox conditions. The nature of this luminescence is ascribed to localized states with the tetragonal 'rutile'-type structure of the short-range order of germanate glasses. The luminescence intensity increases exponentially with decrease of temperature to 4.5 K. The luminescence decay takes place in the time range from several ns to 10 ms and its kinetics are essentially non-exponential (power law t^-1). The decay kinetics are almost independent of temperature.