Ferroelectricity in nonstoichiometric SrTiO3 films studied by ultraviolet Raman spectroscopy

Abstract

Homoepitaxial ${\text{Sr}}_{1+x}{\text{TiO}}_{3+\delta }$ films with $-0.2\le x\le 0.25$ grown by reactive molecular-beam epitaxy on ${\text{SrTiO}}_3$ (001) substrates have been studied by ultraviolet Raman spectroscopy. Nonstoichiometry for strontium-deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of a spontaneous polarization with a paraelectric-to-ferroelectric transition temperature above room temperature. Strontium-rich samples also show a strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution of the disorder-activated mechanism in Raman scattering.