J.-R. Duclère

Nanoscale study of the ferroelectric properties of SrBi2Nb2O9 thin films grown by pulsed laser deposition on epitaxial Pt electrodes using atomic force microscope

Abstract SrBi 2 Nb 2 O 9 (SBN) thin films deposited by laser ablation on epitaxial ( 1 0 0 )Pt and ( 1 1 0 )Pt have been studied using an atomic force microscope (AFM) in the so-called “piezoresponse” mode. Previous X-ray studies have shown that in the first case two different orientations coexist in the film: a predominant ( 0 0 1 ) orientation with a ( 1 1 5 ) orientation. AFM topographical images reveal the presence of two different kinds of grains of different shape corresponding to each orientation and AFM piezoresponse images are in agreement with the crystallographic orientation of the grains: only the expected ( 1 1 5 ) oriented grains show a piezoelectric contrast. Moreover, hysteresis loops are obtained over ( 1 1 5 ) grains and not over ( 0 0 1 ) regions. Although ( 1 1 5 ) grains can be in a monodomain state, they also show intragranular ferroelectric domains with nanometric sizes, which orientation can be reversed by applying a dc field between the AFM tip and the grounded conductive bottom electrode of the sample. In the second case, the use of a ( 1 1 0 )Pt electrode instead of a ( 1 0 0 )Pt electrode leads to preferentially ( 1 1 6 ) SBN oriented films, inducing far better ferroelectrics properties. In spite of a weak remnant polarization, the surface shows an homogeneous polarization when a 1 μm ×1 μm area is probed after the reversal of the polarization by the AFM tip.

Ferroelectric (116) SrBi2Nb2O9 thin films epitaxially grown by pulsed laser deposition on epitaxial (110) Pt/(110) SrTiO3 electrode

Epitaxial SrBi2Nb2O9 thin films have been grown by pulsed-laser deposition on (110)Pt bottom electrode epitaxially grown on (110) SrTiO3 by dc sputtering. X-ray φ-scans and electron channeling patterns reveal the epitaxial growth of the (116) SrBi2Nb2O9/(110)Pt bilayers. The hysteresis loop, dielectric response, and capacitance–voltage characteristics of this bilayer are presented. The remnant polarization was measured to be close to 5 μC/cm2, and the coercive field was calculated to be ∼120 kV/cm. The zero field permittivity was about 150 and the dielectric loss was ∼2%.

Epitaxial growth and ferroelectric properties of SrBi2Nb2O9(115) thin films grown by pulsed-laser deposition on epitaxial Pt(111) electrode

Epitaxial SrBi2Nb2O9 thin films have been grown by pulsed-laser deposition on Pt(111) bottom electrode epitaxially grown by dc sputtering on sapphire(0001). Four-circle x-ray diffraction reveals the epitaxial growth of the SrBi2Nb2O9(115)/Pt(111) bilayers. The influence of the Pt bottom electrode on the growth of SrBi2Nb2O9 films is discussed in terms of atomic matching at the SrBi2Nb2O9/Pt interface. The remanent polarization is close to 6u2002μC/cm2, with a coercive field of 140u2002kV/cm. The zero-field relative permittivity is about 132 and the dielectric loss less than to 2%. The decay in remanent polarization is only 16% after 2.109 switching cycles, confirming the fatigue resistance of the film.

Evidence of intergrowth in SrBi2Nb2O9 (SBN) thin films grown by PLD on (1 0 0)SrTiO3 in relation with the composition

Abstract Thin films of the fatigue-free ferroelectric material SrBi 2 Nb 2 O 9 (SBN) have been epitaxially grown by pulsed laser deposition on SrTiO 3 (1xa00xa00) substrates. These films are fully c -axis oriented and show in-plane orientation as shown by X-ray diffraction in θ –2 θ and ϕ -scan modes, as well as reflection high energy electron diffraction and electron channeling patterns. We have evidenced that the epitaxial growth is strongly affected by the Bi content of the target, and then of the film. A specific intergrowth mechanism is encountered in the case of Bi deficient films; it is based on the random stacking along the growth direction of layers with different c unit-cell constants. Cross-section high resolution transmission electron microscopy confirms this mechanism and shows that the two types of layers involved correspond essentially to the compounds with m =2 and 3 in the usual Aurivillius phases notation (Bi 2 O 2 ) 2+ (Sr m −1 Nb m O 3 m +1 ) 2− . By contrast, in near-stoichiometric films, a few stacking faults are observed whereas in both cases a quite perfect in-plane orientation is evidenced.

Structural characterization of thin films of the SrBi2Nb2O9 ferroelectric Aurivillius phase epitaxially grown on (110)SrTiO3

Aurivillius-phase SrBi2Nb2O9 (SBN) films grown by pulsed-laser deposition on (110)SrTiO3 are nearly (116) oriented as shown by their X-ray diffraction θ–2θ scans. A specific artifact, leading possibly to an erroneous deduction of a (115) and (116) mixture of orientations, has been highlighted. The films present a quite small mosaicity (ω-scan full width at half-maximum = 0.2–0.5°). Sharp electron channeling patterns (ECPs) with the expected twofold symmetry are the signature of film epitaxy. These ECPs often display the superimposition of two individual patterns, rotated by 180° with respect to each other: two families of oriented SBN crystallites, rotated in the same way, coexist in the films. Indeed, from symmetry considerations at the film–substrate interface, these two families are a priori equiprobable. Similarly, two peaks, 180° shifted, are observed for the 00u200910 reflection φ scans and fully confirm this model. However, their relative intensity, which gives access to the quantitative distribution of the two families, shows that in many cases the two families do not present the same relative weight, in good correlation with the qualitative ECP observations, indicating a subtle asymmetrization at the nucleation or growth stage. The in-plane orientation is defined as: [bar{1}10]F || [001]S and nearly [33bar{1}]F or [bar{3}bar{3}1]F || [bar{1}10]S (generating the two families). The film–substrate interfacial relationship is discussed, taking into account the possible facetting of the substrate surface and the occurrence of the (11bar{7}) SBN plane as twin boundary.

Epitaxial Regrowth of Ferroelectric Thin Films on Bottom Electrodes

Abstract The practical use of ferroelectric thin films in devices such as FeRAM requires a capacitance-like geometry and then a bottom electrode. Platinum is proposed based on its resistance to oxidation and unit-cell constants fitting quite closely the ones of typical substrates and ferroelectric materials: indeed, high quality, smooth epitaxial Pt layers with various orientations, namely (100), (110) and (111), were reproducibly obtained. Various examples of the epitaxial regrowth on such sublayers of ferroelectrics are shown, in the case of SrBi2Nb2O9 (SBN) and KTa1− x Nb x O3 (KTN). The observed orientations are compared to the ones obtained on bare bulk oxide substrates and oxide sublayers and discussed in terms of interface anchoring (Near Coincidence Site Lattice model), explaining the orientational competition frequently encountered in such metal-oxide heterostructures.