For deep understanding of PL processes, we studied the
excitation density dependence of the broad PL bands.
Fig. 2(a) shows the excitation-density dependence of the PL
intensity of undoped KTaO3 and LiTaO3 crystals under pulse laser
excitation. For undoped KTaO3 crystals, the PL intensity shows
quadratic dependence on the excitation density under weak
excitation, indicating that bimolecular recombination of photoexcited
electrons and holes determines the PL process. Under
high-density excitation above 300 mJ/cm2
, the PL intensity shows
saturation behavior, which is probably due to an increase of
nonradiative three-body Auger recombination rate [15]. Undoped
SrTiO3 and BaTiO3 crystals also show similar excitation-density
dependence (not shown here. For SrTiO3, see Refs. 15 and 16.).
Unlike the case of KTaO3, the PL intensity of undoped LiTaO3
crystals linearly depends on the excitation density. Similar linear
excitation-density dependence is observed in undoped LiNbO3
crystals. Perovskite oxides used in this work have high dielectric
constants [23,29–31], and thus no excitonic effects appear at
room temperature. Then, this linear dependence implies that
carrier tapping determines the PL process. These results mean
that the PL processes of KTaO3, SrTiO3 and BaTiO3 are completely
different from those of LiTaO3 and LiNbO3.
Fig. 2(b) shows the excitation density dependence of PL
intensity in Arþ-irradiated KTaO3 (under cw excitation) and
LiTaO3 (under pulse laser excitation) samples. Arþ-irradiated
KTaO3 sample shows a broad blue PL under weak cw excitation,
whose intensity linearly increases with an increase of excitation
density. This suggests that the PL in Arþ-irradiated KTaO3
samples is dominated by the radiative recombination of doped
electrons and photoexcited holes. As discussed with respect to
Fig. 2(a) and (b), the PL behaviors of undoped and electron-doped
samples are determined by single-carrier trapping, radiative
bimolecular recombination, and nonradiative three-carrier Auger
recombination processes