Impact of Strain Engineering on Antiferroelectricity in NaNbO3 Thin Films

Thin films of NaNbO3 were grown on varioussubstratesto investigate the effect of epitaxial strain on their structuraland electrical properties. Reciprocal space maps confirmed the presenceof epitaxial strain from +0.8% to -1.2%. A bulk-like antipolarground state was detected via structural characterization for NaNbO3 thin films grown with strains ranging from a compressivestrain of 0.8% to small tensile strains, up to -0.2%. For largertensile strains on the other hand, no indication of antipolar displacementscan be detected, even beyond the relaxation of the film at largerthicknesses. Electrical characterization revealed a ferroelectrichysteresis loop for thin films under a strain of +0.8% to -0.2%,while the films under larger tensile strain showed no out-of-planepolarization component. However, the films with a compressive strainof 0.8% present a saturation polarization of up to 55 & mu;C & BULL;cm(-2), more than twice as large for films grown under conditionswith small strain, which is also larger than the highest values reportedfor bulk materials. Our results indicate the high potential for strainengineering in antiferroelectric materials, as the antipolar groundstate could be retained with compressive strain. The observed enhancementof the saturation polarization by strain allows for substantial increaseof energy density of the capacitors with antiferroelectric materials.