Defect-induced photoluminescence of strontium titanate and its modulation by electrostatic gating

The photoluminescence (PL) spectra of Ar⁺-ion irradiated single crystals of SrTiO₃ (STO) excited by the 325 nm line of a He-Cd laser are compared with those of pristine crystals, epitaxial films, and amorphous layers of STO at several temperatures down to 20 K. The 550 eV Ar⁺-beam irradiation activates three distinctly visible PL peaks: blue (~430 nm), green (~550 nm), and infrared (~820 nm) at room temperature, making the photoluminescence multicolored. The abrupt changes in PL properties below ≈100 K are discussed in relation with the antiferrodistortive structural phase transition in SrTiO₃ from cubic to tetragonal symmetry, which makes it a direct bandgap semiconductor. The photoluminescence spectra are also tuned by an electrostatic gate field in a field-effect transistor geometry. At 20 K, we observed a maximum increase of ~20% in PL intensity under back gating of SrTiO₃.