Band-edge luminescence from SrTiO3: No polaron effect

We studied the band-edge photoluminescence (PL) of electron-doped SrTiO3 and strongly photoexcited SrTiO3 crystals. Two band-edge PL peaks are clearly observed at low temperatures. The PL peak energies coincide with the high- and low-temperature onsets of the optical absorption, corresponding to the phonon-assisted band-to-band optical transition. This result is the direct evidence that the free electrons and holes exist in the conduction and valence bands, and radiative recombination of free electrons and free holes causes the band-edge PL.