Effect of Lattice Strain and Improvement of the Piezoelectric Properties of (K,Na)NbO3 Lead-Free Film

Abstract

(K,Na)NbO3 (KNN) films with very high transverse piezoelectric coefficient d31, which attained values comparable to those of Pb(Zr,Ti)O3 (PZT) films for the first time, were successfully deposited on Pt/Ti/SiO2/Si φ 4-in. substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structure with a <001> preferred orientation. Furthermore, we focused on the effect of lattice strain on d31 of KNN to clarify the relationship between the piezoelectric properties and structural parameters apart from orientation. We found that -d31 increases with decreasing lattice strain c/a ratio of KNN when the in-plane lattice parameter a increases and the out-of-plane lattice parameter c decreases. By controlling the lattice strain c/a ratio strictly and maintaining a homogeneous strain, we achieved a high d31 (∼-100 pm/V) that can be uniformized on φ 4-in. substrates and with a standard deviation that decreases to about 4 pm/V.