Band-to-band photoluminescence in SrTiO3

We report the observation of band-edge photoluminescence (PL) in highly photoexcited SrTiO3 and electron-doped SrTiO3 at low temperatures. Two band-edge PL peaks coincide with the low- and high-temperature onsets of optical absorption. This clearly shows that band-edge PL peaks correspond to indirect band-to-band radiative recombination involving phonon emission and absorption processes and allows a determination of the band gap. The PL peaks redshift with increasing carrier density, indicating band-gap shrinkage. The temperature dependence of the band-edge PL and optical absorption spectra are also discussed in conjunction with phonon-assisted optical transitions.