Macroscopic and nanoscale electrical properties of pulsed laser deposited (100) epitaxial lead-free Na0.5Bi0.5TiO3 thin films

Abstract

Epitaxial Na0.5Bi0.5TiO3 thin films presenting various thicknesses were grown by pulsed laser deposition on epitaxial (100) platinum bottom layers supported by (100)MgO single crystal substrates. X-ray diffraction data indicated that all Na0.5Bi0.5TiO3 layers are single-phased and that (100)-oriented Na0.5Bi0.5TiO3 (NBT) crystallites are extremely predominant. The thinner films (respectively 230 and 400 nm) display a quasiunique (100) orientation (close to 100%), whereas for the thickest film (610 nm), the proportion of (100)-oriented Na0.5Bi0.5TiO3 crystallites decreases to 85.50 vol %. Such variation is supposed to result from the degree of misorientation of the Pt layer. Further x-ray investigations revealed a pronounced asymmetry of the (100)NBT reflection. Such asymmetry is also observed in the (310)NBT reciprocal space maps. The analysis of the asymmetrical broadening of the reciprocal lattice point suggests a variation in the chemical composition across the samples thickness, in agreement with comparative Rutherford backscattering spectroscopy (RBS) data. In addition, x-ray diffraction φ-scans data indicate the systematic epitaxial growth of the (100)-oriented crystallites. The observation of the microstructure of Na0.5Bi0.5TiO3 films completely corroborates the x-ray diffraction information. Whereas the two thinnest films are characterized by the presence of only one type of grains: i.e., very fine and spherical grains (around 50–100 nm in size), the thickest film is characterized by the presence of two types of grains: the aforementioned one and some elongated and “factory roof”-like grains. Thus, we unambiguously attribute that the spherical grains correspond to (100)-oriented crystallites, whereas the “factory roof”-like grains are (110)-oriented. The room temperature macroscopic ferroelectric properties were measured only for the thickest film. A rather well-defined shape of the polarization-electric (P-E) field hysteresis loops was recorded, and a vertical drift of the loops was systematically observed. Recentering the hysteresis loops leads to a Pr value of 12.6 μC/cm2, associated to a coercive field of about 94 kV/cm. This P-E vertical drift originates from the very asymmetric conduction of the Pt/NBT/Pt capacitors at different polarities, as testified by the current density-electric field curves. Such drift can be caused by the existence of different barrier heights at the bottom and top Pt/Na0.5Bi0.5TiO3 interfaces. In addition, based on the combined RBS and x-ray data, we suggest that the chemical composition variation across the layer also impacts on the polarization vertical drift. Finally, the nanoscale electrical properties of the thinnest film have been characterized by both tunneling atomic force microscopy (TUNA) and piezoforce microscopy (PFM). The TUNA data revealed that leakage currents cannot be noticeably detected below 8 or 10 V, in negative or positive biases, respectively. The PFM data showed that most of the grains seem to be constituted of single ferroelectric domains. In addition, the recorded d33 piezoloops are strongly distorted, and systematically remain in the vertical positive side, in agreement with the vertical drift observed for the macroscopic ferroelectric data. The presence of self-polarization within our thinnest film is finally invoked, and supported by some piezohistogram, in order to justify the distorted shape of the loops as well as the supplementary horizontal shift.