Effects of non-stoichiometry on the microstructure, oxygen vacancies, and electrical properties of KNN-based thin films

Highly piezoelectric lead-free thin films of (K_0.5Na_0.5)NbO₃ + x mol% Na(I) and K(I) (KNN, x = 0-50 mol%) were fabricated using a sol-gel method on Pt (111)/Ti/SiO₂/Si(100) substrates and the effects of additives on microstructure, oxygen vacancies, and electrical properties of the proposed samples were investigated. Excess quantities of Na(I) and K(I) were shown to decrease the formation of secondary phases and promote the effective growing of the grains. The addition of these elements was also shown to inhibit the formation of alkaline ion vacancies and decrease the leakage current density. Our findings indicate that the crystallinity and microstructure of the samples have a stronger effect than the oxygen vacancies on the piezoelectric properties of KNN films. Non-stoichiometric KNN films showed the highest remnant polarization (P_r = 11.2 μC/cm²), piezoelectric coefficient (d₃₃ = 40.23 pm/V), voltage coefficient (g₃₃ = 7.9 mm V/N), and lowest leakage current (∼3.46 × 10^-7 A/cm²) when the excess ratio was 40 mol% following annealing at 700°C. Our results also demonstrate that the transport mechanism of the films is governed by Ohmic behavior under low electric fields and the effects of Poole-Frenkel emission under a strong electric field.