Evidence for antipolar displacements in NaNbO3 thin films

Abstract

An antipolar phase is confirmed for NaNbO3 thin films grown by pulsed laser deposition on SrTiO3 (100) substrates. Reciprocal space maps and transmission electron microscopy reveal the presence of characteristic 1/4 superlattice reflections, indicative of the antipolar displacement of Na and Nb-ions. Furthermore, x-ray diffraction unveils the presence of two different orientations of the same phase for thin films beyond a critical thickness of about 60 nm. This orientation change with increasing thickness can be explained as an extraordinary strain compensation mechanism, changing magnitude and sign of the strain at the same time. The polarization vs electric field behavior exposes a characteristic thickness dependence, with the antiferroelectric phase stabilized for very thin films and a field induced ferroelectric hysteresis for a film of 310 nm having a maximum polarization of 26.5 μC [Formula: see text], which is among the highest values reported for NaNbO3 thin films grown on SrTiO3 (100).

Impact of Strain Engineering on Antiferroelectricity in NaNbO3 Thin Films

“…The latter was approximated as 0.3, based on similar perovskite materials . For the case of growth on SrTiO 3 (Figure b), instead of the strained and unstrained values for the lattice parameter, the strained lattice parameters of the different orientations are given, as no relaxation occurs …”

“…This minimizes the overall strain, which is induced into the film and allows to grow thicker films without a relaxation effect. For a more detailed discussion of this film–substrate combination, see our previous publication …”

“…The NaNbO 3 thin films investigated in this work were produced via pulsed laser deposition (PLD). Details of the fabrication can be found in a previous report . After initial optimization of the film properties on SrTiO 3 (100), the same deposition parameters were utilized to grow NaNbO 3 on the other substrates, (LaAlO 3 ) 0.3 (Sr 2 TaAlO 6 ) 0.7 (100) (LSAT), DyScO 3 (110) and GdScO 3 (110).…”

“…For the electrical characterization, a bottom electrode consisting of 25 nm thick LaNiO 3 was grown before the NaNbO 3 directly onto the substrate, also via PLD. While the out-of-plane lattice parameter of LaNiO 3 appears to vary for different films, this layer has been shown to be fully in-plane strained and matching the orientation of the substrate . A parallel plate capacitor structure was fabricated via photolithography and sputtering of gold contacts, resulting in well-defined circular top electrodes with varying diameter from 0.002 to 0.78 nm 2 and a thickness of 500 nm.…”

“…Strain engineering has been established in many scientific fields to tailor and optimize desired properties as well as create new functionalities in devices. − One major success is the improvement of ferroelectric (FE) properties, where percent levels of strain have the potential for a colossal increase in relevant quantities, such as saturation polarization and remanent polarization. , Thus, strain engineering is a promising approach to extend to further research fields, especially ones closely linked to the field of FE, such as antiferroelectricity (AFE). Research into AFE has been intensified in recent years due to a number of potential applications, primarily in short-term energy storage and pulse power applications. − However, only few publications directly address the influence of strain on AFE. , Thus, despite the close relation between AFE and FE, a thorough understanding has not yet been achieved concerning the effect of strain on AFE performance.…”