Ferroelectricity in Homoepitaxial Off-Stoichiometric SrTiO3 Films Studied by Ultraviolet Raman Spectroscopy
Dr. Dmitri Tenne
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. Here we study the effect of off-stoichiometry on the ferroelectric behavior of SrTiO3 thin films. We apply ultraviolet Raman spectroscopy, which is a useful tool for the study of ferroelectric thin films, as it probes crystal lattice vibrations, which are directly related to the ferroelectricity. Homoepitaxial SrTiO3 films with Sr off-stoichiometry from 0.8 to 1.25 studied here were grown by molecular-beam epitaxy on SrTiO3 (001) substrates. Off-stoichiometry for Sr-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 spontaneous ferroelectric polarization with the Curie temperature above room temperature. Strontium-rich samples also show 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.
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