Substrate effects on the ferroelectric properties of fine-grained BaTiO3 films

Abstract

The development of ferroelectric properties in barium titanate (BaTiO3) polycrystalline films has been investigated as a function of substrate type. The films were deposited by physical vapor deposition (PVD) onto different, Pt-coated substrates (magnesium oxide, thermally oxidized silicon, and sapphire) and annealed at temperatures from 725 to 1050 °C. Grain sizes from 50 to 200 nm were produced, with structural and dielectric properties that showed a marked transition from nonferroelectric to ferroelectric behavior across this range. Anneal temperatures below 950 °C result in films with grain sizes less than 150 nm, and ferroelectric properties that are strongly suppressed, regardless of the substrate. Observations for this temperature range include low dielectric constant (ε), no polarization hysteresis, and no peaks in the temperature dependence of the dielectric constant. The onset of ferroelectric behavior occurs for anneal temperatures above 950 °C, coinciding with the appearance of strong substrate effects on the dielectric properties. Such BaTiO3 films on Pt/MgO show the largest dielectric constant, remnant polarization, and temperature dependence of ε, while films on Pt/sapphire exhibit the weakest of each of these properties. Films on Pt/barrier/Si exhibit dielectric properties with values in between that of films on Pt/MgO and Pt/sapphire. The dependence of BaTiO3 ferroelectric properties on the substrate is related to a combination of the BaTiO3 grain and size orientation, determined partly by the underlying Pt surface, and to the thermal expansion properties of the substrate. However, these factors only influence the BaTiO3 dielectric properties for those films that exhibit ferroelectric properties. In this way, the anneal temperature and substrate dependence of the films are linked.