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

Abstract

The photoluminescence (PL) spectra of ${\mathrm{Ar}}^{+}$-ion irradiated single crystals of ${\mathrm{SrTiO}}_{3}$ (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 ${\mathrm{Ar}}^{+}$-beam irradiation activates three distinctly visible PL peaks: blue ($\ensuremath{\sim}430$ nm), green ($\ensuremath{\sim}550$ nm), and infrared ($\ensuremath{\sim}820$ nm) at room temperature, making the photoluminescence multicolored. The abrupt changes in PL properties below $\ensuremath{\approx}100$ K are discussed in relation with the antiferrodistortive structural phase transition in ${\mathrm{SrTiO}}_{3}$ 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 $\ensuremath{\sim}20%$ in PL intensity under back gating of ${\mathrm{SrTiO}}_{3}$.