Intrinsic and substrate-dependent properties of polycrystalline barium titanate thin films.

摘要

Ferroelectric materials exhibit a range of phenomena of considerable fundamental interest, which will soon impact many emerging technologies. However, polycrystalline material experiences a suppression of the polar state for fine grain sizes, typically deep in the submicron range. The variation of the ferroelectric properties with grain size can be explored with study of the anneal temperature dependence. Thin films deposited on a heterogenous substrate can also experience different temperature dependent effects including thermal mismatch strain. In addition, the substrate affects the nucleation process of deposited material. Accordingly, study of the grain size effect in a ferroelectric film should be made in conjunction with analysis of substrate effects. This would enable a deeper understanding of what determines the ferroelectric behavior in polycrystalline films.; In this work, barium titanate films are deposited by RF magnetron sputtering on platinum coated silicon, sapphire, and magnesium oxide. On each substrate, microstructural analysis reveals cubic symmetry for process temperatures below 950°C. This indicates destabilizing influences on the tetragonal, polar phase are predominant for these conditions. Analysis of different preferred orientations in the barium titanate, for deposition on various substrates, indicates largely isotropic dielectric properties. The mechanism for the suppressed polar phase is believed to be related to how the small grain size influences the mechanical and electrical boundary conditions for individual grains.; For anneals above 950°C, an increasing degree of tetragonal distortion is evident in the films. This is correlated with a sharp increase in the dielectric constant (up to 1000), polarization hysteresis (up to 3 muC/cm2), and temperature dependence. The apparent phase transition temperature increases with increasing anneal temperature until converging near the bulk Curie temperature. These observations suggest that the increased grain size for the higher anneal temperatures (>950°C) allows the polar phase to be more stable. Under such conditions, films on MgO show the strongest dielectric properties, believed due to the {lcub}100{rcub} type preferred orientation. Films on sapphire show evidence for thermal mismatch strain, including changes in the lattice constant, non-linear electric field dependence, and downward shifts in the temperature dependence of the dielectric constant. Comparison of intrinsic (grain size related) and substrate dependent film properties reveals the intrinsic properties to dominate for those conditions leading to a strongly suppressed ferroelectric phase. However when the polar phase is sufficiently stable at room temperature, both the orientation as determined by the substrate, and perturbations from the substrate related to strain, influence the polar phase.