High electromechanical strain and enhanced temperature characteristics in lead-free (Na,Bi)TiO3–BaTiO3 thin films on Si substrates

<jats:title>Abstract</jats:title><jats:p>Here, we demonstrate the high electromechanical strain and enhanced temperature characteristics in the <jats:italic>c</jats:italic>-axis-oriented lead-free (Na,Bi)TiO<jats:sub>3</jats:sub>–BaTiO<jats:sub>3</jats:sub> (NBT–BT) polycrystalline thin film prepared on Si substrates by rf magnetron sputtering. The effective transverse piezoelectric coefficient, <jats:italic>e</jats:italic><jats:sub>31</jats:sub><jats:sup>*</jats:sup>, estimated from the electromechanical strain measured under high electric field, reaches a high level of −12.5 C/m<jats:sup>2</jats:sup>, and is comparable to those of conventional Pb(Zr,Ti)O<jats:sub>3</jats:sub> films. <jats:italic>In-situ</jats:italic> X-ray diffraction measurement and electron diffraction analysis revealed the electromechanical strain of the NBT–BT film to originate predominantly in elongation of the tetragonal (<jats:italic>P4bm</jats:italic>) crystal lattice in the <jats:italic>c</jats:italic>-axis direction. In addition to the large <jats:italic>e</jats:italic><jats:sub>31</jats:sub><jats:sup>*</jats:sup>, the NBT–BT film exhibits enhanced permittivity maximum temperature, <jats:italic>T</jats:italic><jats:sub>m</jats:sub>, of ~400 °C and no depolarization below <jats:italic>T</jats:italic><jats:sub>m</jats:sub>, as compared to bulk NBT–BT having <jats:italic>T</jats:italic><jats:sub>m</jats:sub> ≈ 300 °C and a depolarization temperature of ~100 °C. We conclude that the enhancement of temperature characteristics is associated with the distorted <jats:italic>P4bm</jats:italic> crystal lattice formed by deposition-induced stress and defects. We believe that the present study paves the way for practical applications of lead-free piezoelectric thin films in electromechanical devices.</jats:p>