Sandwich and interdigitated finger electrode ferroelectric nano film capacitors: a comparison of the effect of electrostatic boundary conditions and defects

Abstract

We compare the effect of electrostatic boundary conditions and defects on the electrical properties of epitaxial nano ferroelectric thin film capacitors with various electrode configurations. The capacitor geometries studied are parallel plate, or so called sandwich type electrodes and the interdigitated finger electrodes grown on the ferroelectric film. The defects, when present, are frozen-in dipoles of the p-type. The Landau-Ginzburg-Devonshire free energy coupled with the elastic and electrostatic boundary conditions is used to compute the electrical properties. A switchable ferroelectric polarization lying in the plane of the film for the case of interdigitated electrodes is weakly impacted by the presence of dead layers and asymmetrically distributed defects compared to a classical sandwich type nano film. An out of plane remanent polarization in a sandwich type capacitor, stabilized via compressive in-plane misfit, always forms in a multidomain configuration in ultrathin films with thin dead layers. An in-plane polarization stabilized by a tensile misfit, on the other hand, could be much more effectively used in memory device applications where interdigitated electrodes are possible to utilize. The latter also does not suffer as much from the possible presence of dead layers even in ultrathin structures (Film thickness < 10 nm). Switching fields is a strong function of defects in ultrathin finger electrode type systems. We also show how polarization vector configures itself in favor of the highly inhomogeneous field formed under applied bias in the finger electrode case for a film under tensile stress. Thus, switching and the related sensing technique has to be considered accordingly in applications. Presence of a bottom electrode for the case of the finger electrode capacitor also has a substantial impact on electrical properties. The hysteresis and domain characteristics of the two capacitor geometries as a function of interface conditions and defects are discussed for BaTiO3 strained on compressive and tensile single crystal substrates.