Effects of annealing atmosphere on the structural and electrical properties of (Na _0.5K _0.5)NbO ₃ thin films grown by RF magnetron sputtering

Amorphous (Na _0.5K _0.5)NbO ₃ (NKN) thin films were grown at 300 °C and subsequently annealed at 800 °C under Na ₂O, K ₂O and NKN atmospheres. When the annealing time was less than 50 min, K ₆Nb _10.88O ₃₀ and Na ₂Nb ₄O ₁₁ secondary phases were formed in all the films. Moreover, they were also found in films annealed at 750 °C for 180 min under NKN atmosphere, indicating that they were transient phases formed when the sintering time and temperature were not sufficient. For the film annealed for 50 min under Na ₂O atmosphere, an Na-excess (Na _{1- x}K _x)NbO ₃ (N _1-xK _xN) phase was formed, whereas a K-excess N _{1- x}K _xN phase was developed in the film annealed under K ₂O atmosphere. On the other hand, a homogeneous NKN phase was developed in the film annealed under NKN atmosphere and this was maintained after a long period (100 min) of annealing at 800 °C. A high leakage current density and a small dielectric constant ( _r) were observed for films annealed under Na ₂O and K ₂O atmospheres due to the evaporation of K ₂O and Na ₂O, respectively. Moreover, they exhibited a small remnant polarization (P _r) and a small coercive electric field (E _c). On the other hand, the film annealed under NKN atmosphere exhibited a very low leakage current density of 2.6 × 10 ^-9 A cm ^-2 at 0.2 MV cm ^-1 and had good ferroelectric and piezoelectric properties of _r = 620, P _r = 11.7 μC cm ^-2, E _c = 133.8 kV cm ^-1 and d ₃₃ = 74 pm V ^-2 at 50 kV cm ^-1.