SrTiO₃ Single Crystalline Membranes Studied by Ultraviolet Raman Spectroscopy

Ferroelectrics are materials possessing a spontaneous and switchable electrical polarization, which are exploited for various electronic devices, such as non-volatile memories or tunable microwave devices. Strontium titanate, SrTiO3 is a material, which remains in a non-ferroelectric state down to liquid helium temperatures. However, a slight perturbation, such as strain, oxygen isotope substitution, or doping can lead to the appearance of ferroelectric polarization. We have studied free-standing single-crystalline SrTiO3 membranes grown by hybrid molecular beam epitaxy using selectively dissolved sacrificial layers 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, Tc is determined to be in the range 250–300 K, consistent with the temperature dependence of the soft mode frequencies. A possible explanation for the observed behavior can be a non-uniform strain in the membranes.