PbTiO3-based multilayers: growth anomalies, x-ray analysis, and Raman spectroscopy

We have used pulsed laser ablation to grow a series of PbTiO 3 /BaTiO 3 (PTO/BTO) multilayers with a modulation wavelength (Lambda) that varies between 48 angstrom 3 (STO). By modeling the x-ray diffraction patterns we have determined that the PTO layers are a-axis oriented and the BTO layers are c-axis oriented. This result is in contrast to individual thin films of PTO in which we find the c-axis to be perpendicular to the plane of the film, as well as with PTO/STO superlattices we have grown on the same buffered substrates. Raman spectroscopy reveals only PTO modes in the spectra of seven different PTO/BTO multilayers which, from symmetry arguments, is consistent with the x-ray model determination of a-PTO/c-BTO oriented superlattices. We also observe that the soft mode and the A 1 (2TO) Raman line shift abruptly in frequency above (Lambda) equals 240 angstrom. We ascribe this to the possible strain relaxation in the multilayer, in which misfit dislocations appear at a critical wavelength (Lambda) c . The E(2TO) line in the vicinity of 200 cm -1 is (Lambda) dependent as well as symmetric and we have modeled it as a confined mode: a signature of the modulated structure.