  The blue-green photoluminescence emitted by pure and electron-doped strontium
titanate under intense pulsed near-ultraviolet excitation is studied
experimentally, as a function of excitation intensity and temperature. Both
emission spectra and time-resolved decays of the emission are measured and
analyzed in the framework of simple phenomenological models. We find an
interesting blue-to-green transition occurring for increasing temperatures in
pure samples, which is instead absent in doped materials. The luminescence
yield and decay rate measured as a function of temperature can be modeled well
as standard activated behaviors. The leading electron-hole recombination
process taking place in the initial decay is established to be second-order, or
bimolecular, in contrast to recent reports favoring a third-order
interpretation as an Auger process. The temporal decay of the luminescence can
be described well by a model based on two interacting populations of
excitations, respectively identified with interacting defect-trapped (possibly
forming excitons) and mobile charges. Finally, from the measured doping and
sample dependence of the luminescence yield, we conclude that the radiative
centers responsible for the luminescence are probably intrinsic structural
defects other than bulk oxygen vacancies.
