Electrical properties of (110) epitaxial lead-free ferroelectric Na_(0.5)Bi_(0.5)TiO_3 thin films grown by pulsed laser deposition: Macroscopic and nanoscale data

摘要

We report the electrical properties, measured both at the macroscopic and nanometric scales, of epitaxial (110)-Na_(0.5)Bi_(0.5)TiO_3 (NBT) thin films grown on (110)Pt/(110)SrTiO_3 by pulsed laser deposition (PLD). The influence of the A-site composition (Na and/or Bi excess) on both the structural/microstructural characteristics and the electrical properties is discussed. Whatever the composition of the NBT target, the final layers are systematically epitaxially grown, with NBT crystallites mainly (110)-oriented, and as well (100)-oriented for some minor proportion. Atomic force microscopy (AFM) images reveal the coexistence of two kinds of grains presenting different shapes: namely flat and elongated grains, corresponding to (100)- and (110)-oriented NBT crystallites, respectively. The macroscopic ferroelectric properties were measured at room temperature. A rather well-defined shape of the hysteresis loops was obtained: the incorporation of a Bi excess in the target clearly improves the saturation of the loops. The ferroelectric performances are a remanent polarization (P_r) value, ranging from 7 to 14 μC/cm~2, associated with a coercive field (E_c) in the range 68-85 kV/cm. In addition, at 10~5 Hz, the relative permittivity was about ε_r ～ 255-410 and the dielectric losses (tan δ) were ～6%-7%. Finally, the electrical properties at the local scale were investigated by coupling piezoresponse force microscopy (PFM) and tunneling AFM (TUNA) measurements. The collected data reveal that the two types of grains behave differently. The PFM amplitude signal of (110)-oriented grains is very contrasted and such grains are often divided in ferroelectric bi-domains of nanometric sizes, whereas the response of (100)-oriented grains is less contrasted and more homogeneous. The interpretation of the PFM signal is provided. The piezoloop recorded on a (110)NBT grain is strongly distorted and shifted along the vertical axis, in agreement with the vertical drift observed for macroscopic ferroelectric data. Finally, TUNA data clearly indicate that flat grains are leakier than elongated grains, highlighting the anisotropy of the electrical properties at the local scale.