Zhen Yin

Size-driven phase transition in stress-induced ferroelectric thin films

Abstract We have developed a concise phenomenological theory to investigate phase transitions and dielectric properties of ferroelectric thin films under stress. When the polarization at the surface is greater than that in the interior of the film, competition between the ‘superpolarized’ surface and tensile stress leads to a ferroelectric–paraelectric transition above a critical stress. This model is appropriate for the data on stress-induced barium–strontium titanate (BST) thin-film ferroelectric memory devices [Jpn. J. Appl. Phys. 36 (1997) 5846]. Stress–thickness phase diagrams are presented for PbTiO 3 and BaTiO 3 ferroelectric thin films.

Dielectric properties of ferroelectric strained superlattices BaTiO3/SrTiO3

Abstract We have developed a phonon–pseudospin coupled model in double-time Green’s function theory to investigate dielectric properties of the ferroelectric strained superlattices BaTiO3/SrTiO3. We have calculated the influence of the phonon–pseudospin interaction on the dielectric susceptibility. The calculation results show that the peak of the dielectric susceptibility shifts up to higher temperature with increasing the pseudospin interaction, which means larger pseudospin interaction corresponds to higher transition temperature. As the phonon–pseudospin interaction is strengthened, the intensity and position of the susceptibility peak go up and shift to a higher temperature, respectively. The peak of the dielectric susceptibility shifts to a higher temperature with a rather high intensity when the BaTiO3/SrTiO3 superlattice period increases to a period 5/5, which is consistent with experimental results.

Phase structures and stability in barium titanate ferroelectric ultrathin films

Abstract Under taking into account both stress and surface effects, an extended Landau–Devonshire theory is developed to investigate the phase structures and ferroelectric stability in barium titanate ultrathin films. The phase diagrams of temperature-stress, film thickness-stress and polarization-film thickness are calculated. For the surface polarization smaller than the bulk, the paraelectric phase starts at temperature below the bulk Curie temperature under small stresses and the ferroelectric phases can be stabilized in the ultrathin films under large stresses. For the surface polarization larger than the bulk, the paraelectric phase appears at temperature above the bulk Curie temperature and the ferroelectricity can be sustained in the ultrathin films by the “superpolarized” surface layer.

Stress Effect on Critical Thickness in Ferroelectric Thin Films

Taking into account surface and stress effects simultaneously we have extended a modified Landau-Devonshire theory to study how stresses between films and substrates affect the critical thickness in ferroelectric thin films. Film thickness-stress phase diagrams have been obtained. The diagrams can interpret why the reported critical thickness varies in a wide range, or even no critical thickness exists in some epitaxial thin films. Our results can also help understand that ferroelectricity can be sustained in a monolayer thin film. Calculation shows that the inverse ferroelectric critical thickness is dependent on stress and temperature linearly. We have got emperical equations to calculate critical thickness under various stresses and temperatures.

Theoretical study of dielectric susceptibility in ferroelectric thin films

Abstract Based on a pseudospin–phonon interaction model dielectric susceptibility in ferroelectric thin films was studied by using double-time Greens functions method. The dielectric susceptibility peak shifts up to higher temperature with increasing the pseudospin–phonon coupling. Critical pseudospin–phonon couplings were obtained. Below the critical coupling Curie temperature for the film is lower than that for the bulk and the transition temperature for the film decreases with decreasing the film thickness; above the critical coupling Curie temperature for the film is higher than that for the bulk and the transition temperature for the film increases with decreasing the film thickness.

Theoretical investigation of ferroelectric transitions in epitaxial ferroelectric films grown on a substrate

We have employed a pseudospin-phonon interaction model to investigate ferroelectric transitions in epitaxial ferroelectric films on substrates. Using the double-time Greens function method, we have calculated a pseudospin-phonon spectrum, effective pseudospin interaction constants and effective tunneling frequency. Numerical calculations give phase diagrams under various parameters. Without taking into account the pseudospin-phonon interaction in the interface, the Curie temperature at the surface, T c , was lower than that for the bulk, T b . Taking into account the pseudospin-phonon interaction in the interface, T c changed from the case of T c > T b to that of T c T b as N ≥ 2, leading to a critical effective pseudospin interaction in the interface. The lattice mismatch led to stresses in the interface between the film and substrate and led to changes in both the pseudospin-phonon interaction and tunneling frequencies, thus altering the phase transition temperature of films.