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Article Dans Une Revue Journal of Applied Physics Année : 2010

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

S. Députier
Aurelian Crunteanu

Résumé

Epitaxial Na0.5Bi0.5TiO3 thin films presenting various thicknesses were grown by pulsed laser deposition on epitaxial 100 platinum bottom layers supported by 100MgO 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 100NBT reflection. Such asymmetry is also observed in the 310NBT 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 -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 C/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.
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Dates et versions

hal-00470318 , version 1 (06-04-2010)

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M. Bousquet, Jean-René Duclere, Corinne Champeaux, Alexandre Boulle, Pascal Marchet, et al.. Macroscopic and nanoscale electrical properties of pulsed laser deposited (100) epitaxial lead-free Na0.5Bi0.5TiO3 thin films. Journal of Applied Physics, 2010, 107, pp.034102-1-034102-12. ⟨10.1063/1.3290956⟩. ⟨hal-00470318⟩
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