Microstructure-dependent leakage-current properties of solution-derived (K0.5Na0.5)NbO3 thin films

In the present study we clearly demonstrate the relationships between the microstructural details of lead-free ferroelectric (K0.5Na0.5)NbO3 thin films, synthesized from alkoxide-based solutions, and their leakage-current behaviours. The approximately 250-nm-thick films deposited from the 10 and 5 mol% excess potassium-acetate solutions and annealed at 750 °C possessed coarse-grained, columnar or fine-grained, equiaxed microstructures, respectively. The latter exhibited a current density of 2.9 × 10−7 A/cm2 at 50 kV/cm, whereas the value increased by two orders of magnitude in the films with the columnar grains. Transmission electron microscopy investigations showed the presence of amorphous material that wetted the boundaries of the columnar grains. Using conductive atomic force microscopy we obtained direct evidence that the current starts to flow initially along the boundaries of the columnar grains, thus unambiguously demonstrating the leakage mechanism at the sub-micron level.