Ch. Muller

Sr-doped PbZr1−xTixO3 ceramic: structural study and field-induced reorientation of ferroelectric domains

Abstract Sr-doped PZT ceramic, with a composition in the morphotropic phase boundary, was synthesised by solid state reaction. In a first step, from room temperature X-ray powder diffraction data, the structures of the tetragonal (space group P4mm, Z=1, at=4.040(1) A and ct=4.123(1) A) and rhombohedral (space group R3m, Z=3, ahex=5.755(4) A and chex=7.089(1) A) phases have been refined using a Rietveld-type method. Chemical occupancies, cation displacements and spontaneous polarisation were derived for the two coexisting phases. Moreover, to investigate the ferroelectric domains reorientation under a static electrical field, the ceramic was poled and then analysed by X-ray powder diffraction. The effect of the electrical field on the macroscopic polarisation of the ceramic was studied using the ratios of selected Bragg reflections intensities. An electrical field of 1 kV mm−1 necessary to trigger the domains reorientation in the two phases was determined and the percentage of domains switching has been estimated from simple models. On the other hand, to appreciate the degree of orientation, diffraction patterns were simulated using a preferred orientation function. Finally, the depoling process of the ceramic has been studied from successive thermal treatments.

Neutron diffraction study of the relaxor-ferroelectric phase transition in disordered Pb(Sc1/2Nb1/2)O3

The crystal structure of chemically disordered lead scandium niobate, Pb(Sc1/2Nb1/2)O3, is studied by the high-resolution neutron powder diffraction method. The data, collected over the temperature range 2-653?K, are analysed using Rietveld refinement. From 653?K to 378?K, the symmetry of PSN is found to be cubic (space group Pmm) with positional disorder on the lead and oxygen crystallographic sites. For temperatures close to the ferroelectric phase transition (~378?K), the formation of polar domains in the paraelectric phase is deduced from the observation of the diffuse scattering around Bragg peaks. For the ferroelectric phase, between 368?K and down to 2?K, the symmetry of PSN is determined to be trigonal (with space group R3m) and a positional disorder on the Pb site is evidenced perpendicular to the ferroelectric axis.

Microstructure and resistance switching in NiO binary oxide films obtained from Ni oxidation

Oxide Resistive Random Access Memories (OxRRAM) are discussed for future high density non volatile memory chips. NiO and other simple binary transition metal oxides (such as TiO2, HfO2 or ZrO2) have recently attracted much attention. In most cases, polycrystalline oxide films are deposited by reactive sputtering on conductive substrates to form bi-stable Metal/Resistive oxide/Metal (MRM) structures. In this paper, an alternative way is explored to obtain NiO films from the controlled oxidation of a Ni metallic film. Different thermal treatments were evaluated to oxidize the metallic film with conditions preventing the complete consumption of Ni film used as bottom electrode. Process parameters of Rapid Thermal Annealing (RTA) route were adjusted to achieve controlled oxidation. Microstructural and electrical analyzes were performed to apprehend the influence of the process parameters on the switching behavior. Reproducible resistive switching phenomena have been demonstrated in Pt/NiO/Ni structures with threshold voltage varying from 2 to 5 V depending on oxidizing conditions.

Polarization fatigue in PbZr0.45Ti0.55O3-based capacitors studied from high resolution synchrotron x-ray diffraction

High resolution synchrotron x-ray diffraction experiments were performed on (111)-oriented PbZr0.45Ti0.55O3-based capacitors with a composition in the morphotropic region. Diffraction analyzes were done after bipolar pulses were applied and removed, representing several places in the cyclic switching. Microstructural changes were evidenced from relative diffracted intensities variations of several Bragg reflections and a correlation with the evolution of the ferroelectric responses has been established. First, a peculiar microstructural evolution was observed during the first 3×104 switching cycles and was attributed to the so-called “wake-up” effect. On the other hand, the onset of the fatigue phenomenon was accompanied by significant variations on integrated diffraction intensities. Several mechanisms are proposed and discussed to explain such variations. Finally, the ferroelectric responses were analyzed after x-ray diffraction experiments and compared with those measured before exposure. A detailed an...

Temperature-dependent neutron powder diffraction evidence for splitting of the cationic sites in ferroelectric PbHf0.4Ti0.6O3.

Temperature-dependent neutron powder diffraction experiments (diffractometer 3T2-LLB, Saclay, France, lambda = 1.227 Å) have been performed on the perovskite-like lead hafnate titanate PbHf(0.4)Ti(0.6)O(3). This compound belongs to the solid solution denoted PHT, which derives from the well known ferroelectric PZT series. It exhibits a ferroelectric-to-paraelectric phase transition around 620 K, between the low-temperature tetragonal phase and the high-temperature cubic phase. The tetragonal structure of the ferroelectric phase has been refined at 10 and 300 K using a Rietveld-type method: space group P4mm with Z = 1; a(t) = 3.999 (1), c(t) = 4.120 (1) Å, c/a = 1.030, V = 65.89 Å(3) at 10 K; a(t) = 4.012 (1) and c(t) = 4.100 (1) Å, c/a = 1.022, V = 65.99 Å(3) at 300 K. The cubic structure of the paraelectric phase has also been refined at 720 K: space group Pm3;m, Z = 1, a(c) = 4.046 (1) Å, V = 66.23 Å(3). Cation displacements and oxygen-octahedra elongations have been observed as a function of temperature. Evidence for peculiar behaviour associated with the relative shifts of the Hf and Ti atoms (thought until now to be on the same crystallographic site) was found through an anomaly of the mean-square atomic displacements of the Hf/Ti pseudo-nucleus. The PDF Nos for PbHf(0.4)Ti(0.6)O(3) are 48-49-9 and 48-49-10.

Degradation and recovery of polarization under synchrotron x rays in SrBi2Ta2O9 ferroelectric capacitors

Elementary Pt∕SrBi2Ta2O9∕Pt ferroelectric capacitors have been structurally characterized by x-ray diffraction using highly brilliant synchrotron radiation. A microstructural analysis of the stacked layers was performed from the collection of high-quality one-dimensional and two-dimensional diffraction patterns. During x-ray diffraction experiments, peculiar electrical behaviors under irradiation were evidenced. Indeed, depending upon their initial state (poled or nonpoled), the capacitors have exhibited drastic changes in their electrical characteristics after or under irradiation, both “fatiguelike” (polarization reduction) and∕or “imprintlike” (voltage shift) phenomena being observed. Using a sample environment specially designed to measure in situ the evolutions of ferroelectric characteristics, the kinetics of both degradation and restoration of ferroelectric properties of the SrBi2Ta2O9-based capacitors under x-ray radiation have been analyzed. Reduction and recovery of switchable polarization have ...

Cationic disorder, microstructure and dielectric response of ferroelectric SBT ceramics

Polycrystalline samples of SrBi 2 Ta 2 O 9 (acronym SBT) have been prepared by means of solid-state reaction either using a classical route or by mechano-chemical activation. For each compound, a structural analysis of the ferroelectric orthorhombic phase (space group A2 1 am) has been performed from the fitting of neutron and/or X-ray powder diffraction data using the Rietveld method. A cationic disorder on Bi 3 and Sr 2 crystallographic sites has been revealed, the Sr atoms occupying the Bi sites and vice versa. From diffraction peak broadening analyses of high-resolution synchrotron X-ray diffraction data, it has been shown that the two grinding methods (manual or mechanical) induce local strains, the average apparent strain being three times larger for the mechanically ground sample. In order to link microstructure and ferroelectric properties, the dielectric constant has been measured as a function of the temperature. It appears that the position and the shape of the dielectric anomaly strongly depend upon the composition and the route used to elaborate the powders. More exactly, for the mechanically ground powder, the large apparent strain, probably correlated to the strong cation exchange revealed by the structural refinement, leads to a significantly enhanced dielectric response.

Sidewalls contribution in integrated three-dimensional Sr0.8Bi2.2Ta2O9-based ferroelectric capacitors

As compared to usual planar capacitors, three-dimensional (3D) ferroelectric capacitors display a large polarization increase due to the additional electrical contribution of the capacitor sidewalls. However, in 3D capacitors the polarization increase with respect to the planar geometry is lower than expected. To uncover the origin of this discrepancy, the microstructure of 3D Sr0.8Bi2.2Ta2O9-based (SBT) capacitors was studied. First, high-resolution synchrotron x-ray diffraction has found a unit cell expansion of the SBT phase in the capacitor sidewalls. From local chemical analyses, this lattice distortion has been correlated to composition variations in this region. These microstructural changes are due to the peculiar topography of the etched bottom electrode and to the variety of layers in contact with the SBT film, which can explain the nonoptimized polarization in 3D capacitors.

Composition control and ferroelectric properties of sidewalls in integrated three-dimensional SrBi2Ta2O9-based ferroelectric capacitors

The difficult scaling of ferroelectric random access memories with the complementary metal-oxide semiconductor technology roadmap requires integration of three-dimensional (3D) ferroelectric capacitors (FeCAP’s). In this work the unusual electrical behavior of 3D FeCAP sidewalls was studied by comparing the electrical properties of two-dimensional and 3D integrated FeCAP structures. We evidenced composition variations of the SrBi2Ta2O9 (SBT) film in the sidewalls with marked bismuth segregation during metal-organic chemical-vapor deposition (MOCVD) of the SBT film. The segregation was reduced after decreasing the deposition temperature from 440°C, whereby the Bi-rich phase in the sidewalls does not contribute to polarization, down to 405°C, whereby sidewall SBT contributes to polarization. After further optimization of the MOCVD conditions at 405°C, the segregation is minimized and the ferroelectric contribution of the sidewall SBT is almost the same as the contribution of the planar SBT. As a result, 3D ...

Ferroelectric-paraelectric phase transition in PbHf0.2Ti0.8O3 studied by neutron powder diffraction

Neutron powder diffraction data, collected over the temperature range 10-810 K, have been analysed in order to study the ferroelectric-paraelectric transition in the ferroelectric compound PbHf0.2Ti0.8O3. This transition appears at 670 K between the low-temperature tetragonal phase and the high-temperature cubic phase. From high-resolution neutron powder diffraction data (3T2-LLB), the tetragonal structure of the ferroelectric phase has been refined at 10, 300 and 400 K using a Rietveld-type method: space group P4mm, Z = 1; at T = 10 K, at = 3.9299(4) A, ct = 4.1239(5) A and Vt = 63.689 A3; at T = 300 K, at = 3.9405(4) A, ct = 4.1038(5) A and Vt = 63.723 A3; and at T = 400 K, at = 3.9468(4) A, ct = 4.0901(5) A and Vt = 63.713 A3. In addition, a neutron powder thermodiffractometry experiment (D1B-ILL) has been performed to study in situ the temperature induced phase transition. From sequential Rietveld refinements, the thermal dependence of the cationic displacements has been analysed and a spontaneous polarization has been derived. From a generalized effective field theory, the first-order character of the phase transition has been established. Finally, the structural results obtained on the ferroelectric PbHf0.2Ti0.8O3 are discussed in reference to PbTiO3 and PbHf0.4TiO0.6O3 compounds.