Z.-G. Ban

Phase diagrams and dielectric response of epitaxial barium strontium titanate films: A theoretical analysis

We develop phase diagrams for single-domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau–Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. The biaxial epitaxy-induced internal stresses enable phase transformations to unusual ferroelectric phases that are not possible in single crystals and bulk ceramics. The dielectric response of the films is calculated as a function of the misfit strain by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that by adjusting the misfit strain via substrate selection and film thickness, a high dielectric response can be obtained, especially in the vicinity of structural instabilities. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films on (001) Si, MgO, LaAlO3, and SrTiO3 substrates as a function of misfit strain and film t...

Optimization of the tunability of barium strontium titanate films via epitaxial stresses

The tunability of epitaxial barium strontium titanate films is analyzed theoretically using a phenomenological model. The relative dielectric constant of Ba0.5Sr0.5TiO3 (BST 50/50) films as a function of the applied external electric field is calculated and an electric field–misfit strain phase diagram is developed to assist in the interpretation of the behavior. On the basis of these results, the tunability of BST 50/50 films as a function of the misfit strain is provided and compared with the experimental data in the literature. Analysis shows that a high tunability can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. The misfit strain in epitaxial films can be controlled with the selection of a substrate material or variations in the film thickness. The film thickness dependence is due to misfit dislocation formation at the film growth temperature. A critical thickness to attain the maximum tunability can be defined for BST 50/50 films on MgO (...

Pyroelectric response of ferroelectric thin films

A thermodynamic formalism is developed to calculate the pyroelectric coefficients of epitaxial (001) Ba0.6Sr0.4TiO3 (BST 60/40) and Pb0.5Zr0.5O3 (PZT 50/50) thin films on (001) LaAlO3, 0.29 LaAlO3:0.35(Sr2TaAlO6) (LSAT), MgO, Si, and SrTiO3 substrates as a function of film thickness by taking into account the formation of misfit dislocations at the growth temperature. The role of internal stress is discussed in detail with respect to epitaxy-induced misfit and thermal stresses arising from the difference between the thermal expansion coefficients of the film and the substrates. It is shown that the pyroelectric coefficients steadily increase with increasing film thickness for BST 60/40 and PZT 50/50 on LSAT and SrTiO3 substrates due to stress relaxation by misfit dislocations. Large pyroelectric responses (∼1.1 μC/cm2 K for BST 60/40 and ∼0.3 μC/cm2 K for PZT 50/50) are theoretically predicted for films on MgO substrates at critical film thicknesses (∼52 nm for BST 60/40 and ∼36 nm for PZT 50/50) correspo...

Structural characteristics of ferroelectric phase transformations in single-domain epitaxial films

Structural characteristics of phase transformations in epitaxial ferroelectric films are analyzed via a Landau–Devonshire thermodynamic formalism. It is shown that the phase transformation temperature, the lattice parameters, and the order of the phase transformation are a strong function of the misfit strain and are considerably different compared to unconstrained, unstressed single crystals of the same composition. Depending on the internal stress state, it is possible that the structural aspects of the paraelectric–ferroelectric phase transformation may be completely obscured in the presence of epitaxial strains. The thickness dependence of epitaxial stresses due to relaxation by misfit dislocations during film deposition is incorporated into the model using an “effective” substrate lattice parameter. There is a good quantitative agreement between the theoretical analysis and experimental observations reported in the literature on the variations in the lattice parameters and the phase transformation te...

STRUCTURAL PHASE TRANSITION IN EPITAXIAL PEROVSKITE FILMS

Three different film systems have been systematically investigated to understand the effects of strain and substrate constraint on the phase transitions of perovskite films. In

The role of thermally-induced internal stresses on the tunabilityof textured barium strontium titanate films

\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}

Dependence of the pyroelectric response on internal stresses in ferroelectric thin films

films, the phase transition temperature

Effective pyroelectric response of compositionally graded ferroelectric materials

{T}_{c}

Large piezoelectric strains from polarization graded ferroelectrics

was determined by monitoring the superlattice peaks associated with rotations of

Thermodynamic analysis of temperature-graded ferroelectrics

\mathrm{Ti}{\mathrm{O}}_{6}