Mixed orientation PbTiO₃/BaTiO₃ superlattices: X-ray diffraction and raman spectroscopy

Abstract

We have used pulsed laser ablation to grow a series of PbTiO₃/BaTiO₃ (PTO/BTO) multilayers with a modulation wavelength Λ that varies between 50Å<Λ<360 Å. 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 is in contrast to individual thin films of PTO in which the c-axis is perpendicular to the plane of the film. Raman measurements of the multilayers reinforce this x-ray model determination. We also observe that the soft mode Raman line shifts abruptly in frequency above Λ=240Å. We ascribe this to the strain relaxation in the multilayer, in which misfit dislocations appear at a critical wavelength Λ_c. A calculation of Λ_c for a-axis PTO/c-axis BTO multilayers based on elasticity theory yields a value of 260Å, in good agreement with the Raman results.

Keywords:

• multilayers
• x-ray diffraction
• Raman spectroscopy