Crystal structure and piezoelectric properties of lead-free epitaxial (K,Na)NbO₃ thin films grown on Si substrates

Epitaxial growth of lead-free (K,Na)NbO₃ (KNN) thin films on (001)SrRuO₃/Pt/ZrO₂/Si substrates was achieved by using RF magnetron sputtering. X-ray diffraction measurements revealed that lattice constants (a₁ = 0.3987 nm, a₂ = 0.3959 nm, a₃ = 0.4011 nm) of epitaxial KNN thin films with a pseudo cubic system were close to KNN single crystals. Vertical piezoelectric force microscopy observation indicated that the spontaneous polarization P_s with a downward direction was dominant, and the epitaxial KNN thin films were naturally polarized. The epitaxial KNN thin films exhibited low relative dielectric constant (ε_r ∼ 267). In addition, piezoelectric coefficients |e_31,f| showed a constant value of about 6.5 C m⁻² with the increase in applied positive voltages. Relatively high converse |e_31,f| values were obtained at low applied voltages due to an intrinsic piezoelectric effect. Therefore, the epitaxial KNN thin films might enable piezoelectric microelectromechanical systems driven at low applied voltages.