Lattice Dynamics and Ferroelectric Phase Transitions in BaTiO$_3$/SrTiO$_3$ Superlattices Studied by Ultraviolet Raman Spectroscopy

D. A. TENNE; A. BRUCHHAUSEN; A. FAINSTEIN; R. S. KATIYAR; A. CANTARERO; A. SOUKIASSIAN; V. VAITHYANATHAN; W. TIAN; D. G. SCHLOM; Y. L. LI; L. Q. CHEN; S. M. NAKHMANSON; K. M. RABE; C. B. EOM; H. P. SUN; X. Q. PAN; X. X. XI

Pennsylvania, USA

Conferencia; 48th Electronic Materials Conference; 2006

Recent advances in epitaxial technology made possible the growth of multilayer oxide structures with monolayer-precision control of thicknesses and atomically flat interfaces, opening a way for synthesis of new materials, such as periodic multilayer ferroelectric structures - superlattices (SLs). These artificially engineered nanostructures were predicted by theoretical investigations to possess essentially new properties compared to their bulk constituents, and provide an opportunity to manipulate and enhance the ferroelectric properties of these materials. Vibrational (Raman and infrared) spectroscopies can provide valuable information for understanding the behavior of nanoscale ferroelectrics, as the lattice dynamics determines the fundamental ferroelectric properties. However, conventional Raman or infrared spectroscopies fail to measure the phonon spectra of nanoscale ferroelectric heterostructures because of extremely weak signals from nanometer-thick films and the overwhelming substrate signals. Here we present the first experimental study of short-period BaTiO3/SrTiO3 SLs by ultraviolet (UV) Raman spectroscopy. SLs were grown by molecular beam epitaxy on (001)-oriented SrTiO3 substrates. Using the UV excitation allowed us to overcome the problem of overwhelming substrate contributions in Raman spectra and made possible the observation of phonons in superlattices having the ferroelectric BaTiO3 layers as thin as 1 unit cell. The ferroelectricparaelectric phase transitions have been observed. Depending on the thickness of the BaTiO3 layers and strain, the phase transition temperature Tc varies by hundreds of degrees from ~140 K to 640 K, i.e from over 250° below to over 200° above the Tc in bulk BaTiO3. Below the Tc, the superlattices remain in the single (tetragonal) ferroelectric phase down to 7 K, i.e. the low-temperature phases characteristic for bulk BaTiO3, are suppressed by strain. The experimental data are in good agreement with the results of the thermodynamic calculations of polarization in superlattices as a function of temperature.