Effects of two-stage post-annealing process on microstructure and electrical properties of sol-gel derived non-stoichiometric NKN thin films

Highly (100)-oriented lead-free Na _0.5 K _0.5 NbO ₃ (NKN) + 40 mol% Na(I) and K(I) thin films are fabricated on Pt/Ti/SiO ₂ /Si substrates via a sol-gel processing method. A two-stage post-annealing process consisting of rapid thermal annealing (RTA) at various temperatures in the range of 650 ∼ 850 °C followed by tube furnace (TF) treatment at a temperature of 700 °C is then employed to modify the microstructure and chemical bonds of the NKN films in an attempt to improve their dielectric and ferroelectric properties. It is shown that the optimal values of the dielectric constant (ε = 658 at 100 kHz), dielectric loss (tanδ = 0.113 at 100 kHz), remnant polarization (2P _r = 17.1 μC/cm ² at 1 kHz), and coercive field (2E _c = 372 kV/cm at 1 kHz) are obtained when using an RTA temperature of 750 °C. The superior electrical properties of the NKN film are the results mainly of an improved crystallization, a higher film density and a denser grain structure. It is shown that the ferroelectric properties of the NKN film are maintained for more than 25000 cycles in thermal environments of less than 150 °C. Hence, it is inferred that NKN has good thermal stability and endurance for ferroelectric devices.