Micro-Raman investigation of stress variations in lead titanate films on sapphire

Using the sol-gel method, PbTiO3 films of 21, 64, 128, 210, 310, and 420 nm thicknesses were obtained on (0001) sapphire. Raman scattering and x-ray d iffraction techniques were used to study the correlation between the film t hickness and the structural changes on these films. The Raman and x-ray int ensities in 21 nm film were too weak to reveal any structural information, while all other films showed tetragonal structure. At room temperature, the variation of lowest E(1TO) phonon mode frequency with film thickness was o bserved due to compressive stresses in the films. The lattice parameters an d the degree of a axis orientation values of tetragonal PbTiO3 have been ev aluated as a function of film thickness. The changes in lattice parameters thus obtained were used to estimate the stress at each thickness. An excell ent agreement was found between the stress values obtained using Raman and x-ray results. An exponential decrease in stress with increasing film thick ness was observed because of the structural changes in the lattice. Accordi ngly, a downshift in the tetragonal-cubic transition temperature with decre asing film thickness has been interpreted using the conventional Landau-Dev onshire approach. (C) 1999 American Institute of Physics. [S0021-8979(99)02 014-9].