Polarized Raman Spectra of BaTiO 3 /SrTiO 3 Superlattices.

BaTiO 3 /SrTiO 3 superlattices were grown on (001)MgO substrates by pulsed-laser-deposition technique. The thickness of each layer varied from 6 to 25 unit cells. Polarization-dependent Raman spectra confirmed tetragonal symmetry of the superlattices with the polar axis being normal to the layers. The observed optic modes in the superlattices showed intermediate frequencies between corresponding bulk BaTiO 3 and SrTiO 3 values, while the E( TO 1 ) soft mode exhibited upward shift due to significant two-dimensional clamping caused by the lattice mismatch of the constituting layers. The observed downward shift of the soft mode revealed relaxation of two-dimensional stresses with increasing the stacking periodicity of the superlattices. The excitation of folded transverse acoustic phonons in the Raman spectra due to the new periodicity of the Brillouin zone was revealed. The observed data are quantitatively explained by a folding of the corresponding acoustic branch. It is shown that micro-Raman spectroscopy provides determination of a spatial variation of the layer thickness in graded superlattices. [ABSTRACT FROM AUTHOR]