Ultraviolet Raman spectroscopy of SrTiO₃ single crystalline membranes

Free-standing single-crystalline SrTiO₃ membranes grown by hybrid molecular beam epitaxy using selectively dissolved sacrificial layers have been studied by variable temperature Raman spectroscopy with ultraviolet excitation (325 nm). . Bulk SrTiO₃ has a cubic perovskite structure with all phonon modes being symmetry forbidden in the first order Raman spectra. The membranes show behavior characteristic of ferroelectrics at low temperatures. Strong first order optical phonon modes have been observed, including the lowest-frequency modes, which exhibit characteristic softening with increasing temperature. From the temperature evolution of Raman intensities of the hard modes the ferroelectric phase transition temperature, T_c is determined to be in the range 250–300 K, consistent with the temperature dependence of the soft mode frequencies.