Cracking, texture evolution and electrical properties of ferroelectric thin films.

Abstract

Wet chemical methods of sol-gel and metallo-organic decomposition processes were used to prepare BaTiO$\sb3$ and Pb(Zr,Ti)O$\sb3$ (PZT) ferroelectric thin films. These films were pyrolyzed under a variety of heating rates, temperatures, and atmospheres to obtain different textures and microstructures. Thermal analysis, X-ray diffraction, dielectric properties and critical conditions of cracking have been evaluated to obtain an improved understanding of the mechanisms of pyrolysis, texture evolution and dielectric response in thin films. Two distinct cracking mechanisms which operate in different temperature regimes have been identified. Drying cracks form at lower temperatures when the maximum evaporation/decomposition rate is experienced, while oxidation cracks form at higher temperatures when the decomposition is nearly complete. The critical thickness, crack spacing, and heating rate follow unique scaling laws predicted by fracture mechanics taking into account the viscoelastic origin of pyrolysis stresses. The mechanisms of texture selection at the PZT/Pt/Si substrate interface have been identified. An intermetallic phase Pt$\sb{5-7}$Pb(111) that is epitaxial with Pt(111) precedes the formation of PZT(111). A layer compound PbO (001) that forms at lower temperatures provides a lattice-matching buffer for PZT(100). Fast heating at high temperature favors (111) texture, while intermediate temperature pyrolysis prior to high temperature annealing promotes (100) texture. The optimal textures at different compositions are phase-dependent. Zr-rich(111) rhombohedral films and Ti-rich(100) tetragonal films have higher dielectric constant and polarization because their textures match the preferred polarization in the respective ferroelectric phases. At the morphotropic phase boundary where both tetragonal and rhombohedral symmetry, and hence, multiple polarization directions, are allowed, single textured films have a lower dielectric constant than the mixed textured films. The effects of chemistry of precursors (alkoxides and carboxylates) can be rationalized by their different carbon content and decomposition temperature in the precursor. These characteristics influence the crack formation, texture evolution, microstructure development and dielectric properties in a predictable manner, following our mechanistic understanding of these aspects. This is despite their very different conversion ratio from organics to ceramics.