Phase Transition And Domain Observation Of Ferroelectric Thin Film Under The Applied Field

Current interest in ferroelectric thin film results from the numerous potential applications in micro electronics, micro electromechanical system (MEMS) and information storing that utilize the unique ferroelectric, dielectric, pyroelectric, electro-optic, acousto-optic, and piezoelectric properties of the material. Nano-indentation and scanning force microscopy (SFM) piezoresponse imaging method, which represent useful techniques to study the characterization of ferroelectric thin films on nanometer scale, are accompanied by external field between the electrode and tip/cantilever system. On the other hand, the inhomogenity and mechanical restriction may lead to domain structure of ferroelectric thin film. The physical properties of ferroelectric thin film are depended on the domain structure, domain type, domain size and the rule of domain motion. In situ observation on ferroelectric domain is the very important for us to study properties of ferroelectric thin film. In this paper, the effects of external stress on the phase diagrams and physical properties of ferroelectric thin films are theoretically studied. The ferroelectric domain is observed in situ by scanning probe microscopy (SPM). The experimental result is compared with that of the theoretical calculation. The main contents are given as follow.A nonlinear thermodynamic theory is used to investigate the effect of external stress and composition on phase diagrams and physical properties of single-domain Pb(Zr1-xTix)O3 (PZT) thin films epitaxially grown on dissimilar cubic substrates. The"external stress-misfit strain"phase diagrams are constructed for PZT thin films with different compositions (x=0.4, 0.5, 0.6, 0.7, 0.8, and 0.9). For the phase diagrams, the most remarkable theoretical prediction is that the external stress may lead to phase transitions, and the stress-induced ferroelectric to paraelectric phase transition may take place in PZT thin films even at room temperature. There is a monoclinic gap separating the stability ranges of the tetragonal out-of-plane (c-phase) and orthorhombic in-plane (aa-phase) polarization states. Under the external stress, this gap narrows, and the phase boundary between the c-phase and aa-phase lengthens with the increase of Ti content. For the dielectric and piezoelectric properties, they are very sensitive to the phase transitions, and reach very high values by adjusting appropriate external stress. The results can be used to explain the experimental phenomena observed via the indentation and scanning force microscopy of ferroelectric thin films.The nonlinear thermodynamic theory, which is employed study the ferroelectric thin film grown on the cubic substrate, is modified to investigate the effect of external stress on phase states and physical properties of epitaxial ferroelectric thin films grown on anisotropic substrates which induce nonequally biaxial misfit strains in the films plane. The"misfit strain-external stress"and"external stress-temperature"phase diagrams are constructed for single-domain PbTiO3 (PT) and BaTiO3 (BT) thin films grown on anisotropic substrates. It is shown that the external stress may lead to the rotation of the spontaneous Ps and a gradual change of its magnitude. The dependence of the stability of polarization on the stress is studied, which enables the determining the limits of applicability of thin films during the device operation. The interesting theoretical prediction is that stress-induced ferroelectric to paraelectric phase transition which may take place in ferroelectrics thin films grown on cubic substrates is forbidden in thin films grown on anisotropic substrates at room temperature. It is also shown that the anisotropic misfit strains in the film plane may lead to the appearance of new phases. These new phases involve the ai-phase (i=1, 2), where the spontaneous polarization Ps is oriented along one of the in-plane edges of the prototypic unit cell and the triclinic r-phase, where the Ps is rotated away from the diagonal of this cell. The dielectric and piezoelectric responses of single-domain PT and BT are very sensitive to the phase transition. The dielectric and piezoelectric responses could be improved by applying an appropriate external stress to the film.Scanning probe microscopy (SPM) is used to observe the domain timely and perform nanoscale studies of the domain switching behavior of ferroelectric thin films. Applied a polarized voltage to the single grain of the ferroelectric, the piezo-phase images are obtained and reveal that the domain can be switched at a dc bias of 6 V. The grain boundaries are visible in the piezo-response images. The nucleation domains that are opposite to the direction of the applied electric field, and their formations are ascribed to the grain boundaries which also impede domain-wall movement. Additionally, the mechanism concerning the dependence of ferroelectric properties on domain switching was discussed.