title: Impact of Epitaxial Strain on Antiferroelectricity in NaNbO₃ and AgNbO₃ Thin Films
Creators: name: Schneider, Thorsten
Publisher: name: UNSPECIFIED
Year: 2024

description: Antiferroelectric materials offer a tantalising prospect for enhancing modern capacitors by introducing novel functionalities and significantly increasing the energy density. Although initial steps have been taken towards commercialisation, progress is impeded by a limited understanding of the antiferroelectric effect and a scarcity of materials that exhibit this unique property. In this study, antiferroelectric perovskites are synthesised as epitaxial thin films to explore the nature of the antiferroelectric effect when impacted by strain, as well as the influence of epitaxy on the balance of the antiferroelectric and ferroelectric phases.
Thin films of antiferroelectric materials, specifically NaNbO₃ and AgNbO₃, are fabricated through pulsed laser deposition. Challenges similar to those encountered in bulk synthesis are identified, concerning the preservation of stoichiometric cation ratios during thin film growth. These are overcome by adaption of growth parameters for NaNbO₃, while silver deficiency in AgNbO₃ films is addressed by provision of a silver reservoir. This reservoir supplies silver ions to rectify the off-stoichiometric transfer during the pulsed laser deposition process.
The successful growth of high-quality thin films is subsequently extended to various
substrates, introducing a range of strains, from compressive to tensile, into the films. This allows to explore the formation of the ground state for both materials, and to determine the stability region of the antiferroelectric phase as a function of the induced strain. For both materials, the presence of a bulk-like antiferroelectric ground state is observed in films grown under compressive strain, whereas films under tensile strain do not exhibit the expected structural symmetry characteristic of antiferroelectric materials.
To evaluate the stability of this phase, electrical characterisation in a capacitor structure is performed. Interestingly, while an antipolar ground state is maintained in thin films grown under compressive strain, the dielectric behaviour of these NaNbO₃
thin films resembles that of a ferroelectric material rather than an antiferroelectric one. This shows that no additional stabilisation of the antiferroelectric phase is observed due to the induced strain. Nonetheless, the saturation polarisation of thin films grown under compressive strain increases to over 200%, offering the promise of a significant boost in energy density for antiferroelectric capacitors.
In summary, this research demonstrates the potential of strain to enhance antiferroelectric capacitors. Thin films grown under compressive strain maintain the antiferroelectric characteristics of the bulk material, with a substantial enhancement in saturation polarisation of the ferroelectric phase. By combining this research with the thoroughly investigated doping approach to further stabilise the antiferroelectric phase, a substantial improvement in the energy density of antiferroelectric capacitors is anticipated.