We also performed PL measurements of all samples at 10 K.
Fig. 3 shows the PL spectra of undoped and Arþ-irradiated samples.
Note that PL measurement in undoped BaTiO3 crystals was not
performed at low temperatures because undoped BaTiO3 single
crystals can be broken to pieces due to rapid volume change at the
phase transition temperature. Here, the PL intensities are normalized
at their peak intensities. Undoped samples show broad PL
bands, similar to those of Arþ-irradiated samples. In undoped and
Arþ-irradiated LiTaO3 and LiNbO3, no significant spectral changes
are observed in a wide temperature range between 10 K and 300 K,
indicating that the origins of the broad PL bands at 10 K and room
temperature are identical. On the other hand, in Arþ-irradiated
KTaO3, SrTiO3, and BaTiO3 crystals, a broad PL band red-shifts at low
temperatures. We consider that in these crystals the origin of
efficient PL at 10 K is different from that at room temperature.
Temperature dependence of the PL intensity of (a) undoped
samples and (b) Arþ-irradiated samples is summarized in Fig. 4.
The PL intensity is normalized at 10 K. All the undoped samples
show Arrhenius-like temperature dependence. Undoped LiTaO3
and LiNbO3 crystals show gradual decrease of PL intensity with an
increase of temperature and weak PL is observed at room
temperature. On the other hand, the PL intensity of undoped
SrTiO3 and KTaO3 samples shows thermal quenching below 100 K
and no room temperature PL is observed. Arþ-irradiated KTaO3
and SrTiO3 samples also show a decrease of PL intensity at low
temperatures. However, their PL intensities keep almost constant
values above 150 K unlike the case of undoped samples. These
results indicate that electron doping by Arþ irradiation enhances
the broad PL intensity at high temperatures.
It is worth mentioning that the low-temperature PL spectrum
of Arþ-irradiated SrTiO3 samples show significant difference
compared to other samples: band-edge PL is observed only in
Arþ-irradiated SrTiO3 samples, as shown in Fig. 3. The band-edge
PL peak has been reported in chemically electron-doped SrTiO3
and strongly photoexcited SrTiO3 [19]. Fig. 5 shows an enlarged
view of PL spectra of Arþ-irradiated SrTiO3 crystals under cw
excitation at 15 and 100 K. Two PL peaks are observed at
approximately 3.22 and 3.27 eV, and shows spectral broadening
and blueshift with an increase of temperature. The square root of
the optical absorption coefficient of undoped SrTiO3 crystals, a, at
8 and 85 K are also shown. Since a1/2 varies linearly with energy,
we can conclude that SrTiO3 is an indirect-gap semiconductor and
no excitons are formed because of the large dielectric constant.