Following the theory of optical transition in semiconductors [32],
the low- and high-temperature onsets of the absorption spectra
correspond to band-to-band transitions involving phonon absorption
(Egþ:oph) and phonon-emission (Eg :oph) processes,respectively, where Eg and :oph are the band-gap energy and
phonon energy. The PL peak energies coincide with the phononassisted
optical transition energies. Thus, we can conclude that the
PL peaks at 3.22 and 3.27 eV are band-to-band optical transitions
(i.e., recombination of free electrons and holes) involving phonon
absorption and emission. The temperature dependent spectral shape
of the band-edge PL also supports this conclusion [19]. The phonon
energy of 25 meV is close to the transverse optical (TO2) phonon
energy at zone boundary [33].
Finally, we comment on unique PL properties in SrTiO3. The
observation of band-edge PL indicates the existence of free electrons
and holes in SrTiO3. Although LiTaO3, LiNbO3, KTaO3, and
BaTiO3 are d0
-type and indirect-gap semiconductors and show no
excitonic effects similar to SrTiO3 [19–23], no band-edge PL is
observed in perovskite semiconductors other than SrTiO3. At
present stage, we believe that the band-edge PL of SrTiO3 is related
to the fact that SrTiO3 shows significantly high carrier mobility
compared to other oxide semiconductors [34,35]. It is likely that in
other perovskite semiconductors carriers would be localized and
no band-edge PL is observed. Our findings will provide new insight
into the nature of carriers in wide-gap perovskite semiconductors.