Stress manipulation in ferroelectric thin films and superlattices

The integration of ferroelectric oxides into microelectronic devices requires a better understanding of stress and size effects onto ferroelectric properties of epitaxial thin films and superlattices which are generally imposed by the underlying substrate due to misfits in their lattice parameters and their thermal expansion coefficients. We have utilized polarized micro-Raman spectroscopy to investigate such stress effects in epitaxially grown Ba0.7Sr0.3TiO3 (BST-0.3) thin films and BaTiO3/SrTiO3 (BT/ST) superlattices with different periodicities as a function of temperature. In BST-0.3 films, the mismatch of the thermal expansion coefficients of the film and the substrate induces significant two-dimensional (2D) clamping and governs the domain structure formation. Moreover, the 2D stresses increase the Curie temperature remarkably and even change the phase-transition sequence in thin films, creating new phases, which were never observed in bulk materials. It is shown that in the epitaxial BST-0.3 films and BT/ST superlattices the E(TO) soft mode is very sensitive internal probe of 2D clamping and can be used for precise characterization.