The effect of surface termination on dielectric nonlinearity in potassium sodium niobate thin films

The impact of alkali-rich and alkali-depleted surface terminations on dielectric nonlinearity and ferroelectric properties was investigated in sputtered K_0.5Na_0.5NbO₃ (KNN) films. For the alkali depleted surface termination, a ∼3 nm thick amorphous interfacial layer was found near the top electrode. The presence of a non-ferroelectric interfacial layer reduces both the net remanent (P_r) and maximum polarization (P_max) while increasing the coercive field (E_c) in the KNN films. P_max decreased from 31.5 to 30.3 µC/cm², likely due to the reduced electric field in the bulk of the film. The influence of the interfacial layer on dielectric properties was evaluated using a capacitor in series model. After removing the effects of the interfacial layer, the Rayleigh coefficients ε_init and α increased by 11 and 47%, respectively. The interfacial layer has more impact on irreversible domain wall motion than on reversible contributions to the relative permittivity. It is believed that this occurs because a higher concentration of defects, such as (Formula presented.) or (Formula presented.), associated with the alkali-depleted interfacial layer generates internal fields that pin domain walls. Alkali depleted surface terminations also produce a higher pseudo-activation energy for polarization reversal compared to films with alkali rich terminations, with deeper potential wells in the material's energy landscape.