Alain Pignolet

NANO-phase SBT-family ferroelectric memories

Abstract Recent studies have produced 0.1 × 0.1 μm ferroelectric cells in both bismuth titanate and strontium bismuth tantalate, thus taking thin-film ferroelectric memories into the regime of nanoscale (100 nm or less) devices. A review is presented of deposition, switching, and leakage current in these devices, which are small enough to permit 1 Gbit memories on a standard Si chip.

Photovoltaic properties of Bi2FeCrO6 epitaxial thin films

We report a large photovoltaic (PV) effect in multiferroic Bi2FeCrO6 (BFCO) films under monochromatic illumination at 635 nm with an intensity of 1.5 mW cm−2. These multiferroic films exhibit a large photocurrent at zero bias voltage and an open-circuit voltage of about 0.6 V. A high PV power conversion efficiency of about 6% for red light is achieved and attributed to a high degree of B-site cationic ordering between Fe and Cr sublattices, the tuning of which is likely to play a key role in further improvements of the PV properties in BFCO.

Switching properties of self-assembled ferroelectric memory cells

In this letter, we report on the switching properties of an ordered system of Bi4Ti3O12 ferroelectric memory cells of an average lateral size of 0.18 μm formed via a self-assembling process. The ferroelectricity of these cells has been measured microscopically and it has been demonstrated that an individual cell of 0.18 μm size is switching. Switching of single nanoelectrode cells was achieved via scanning force microscopy working in piezoresponse mode.

Raman scattering from PbTiO3 thin films prepared on silicon substrates by radio frequency sputtering and thermal treatment

Raman scattering has been carried out on PbTiO3 thin films prepared on platinum‐coated (100) silicon by radio‐frequency (rf)‐magnetron sputtering without substrate heating and a post‐deposition thermal treatment. The Raman spectra obtained from the thin film are characteristic of powder Raman spectra: In comparison with the single crystal spectra, the intensity of the background is relatively high at low frequencies and the Raman lines are broad. The lattice phonon modes corresponding to the observed lines are identified by comparison with the data on single crystals and powder. The Raman frequencies for the thin film remarkably shift to low frequencies compared with single‐crystal data. It is shown that the phenomenon of the frequency shifts is similar with the hydrostatic pressure effect on single crystals of PbTiO3. The result indicates that the thin films are composed of grains that are stressed depending on the grain size by neighboring grains of different orientations when they are split up into fer...

Self-patterning nano-electrodes on ferroelectric thin films for gigabit memory applications

In the present work, we report self-assembling bismuth-containing nano-electroded cells of layered perovskite ferroelectric thin films that are about 200 nm in size, that is 50 times smaller than the smallest cell reported to date. Heteroepitaxial Bi-rich Bi4Ti3O12 films were grown by pulsed laser deposition (PLD) on top of epitaxial conductive La0.5Sr0.5CoO3 (LSCO) layers equally deposited by PLD. The epitaxial LSCO has been grown on top of an epitaxial CeO2 yttrium-stabilized zirconia (YSZ) stack, itself deposited by PLD on Si(100) substrates. As a consequence of the high substrate temperature during the epitaxial deposition of the Bi4Ti3O12 layer, the excess Bi segregates, migrates to the surface where it forms a self-organized array of epitaxial mesas which possess metallic-like electrical characteristics.

Contact resonances in voltage-modulated force microscopy

A study of the frequency dependence of the signal in piezoresponse scanning force microscopy of ferroelectric materials has been performed. It is found that, for soft cantilevers, the signal is governed by the cantilever elastic properties. Both ferroelectric-electromechanical and electrostatic interaction contributions to the overall signal were found to depend on the frequency of the testing voltage. Indications for optimal measurement regimes are given.

Piezoresponse Scanning Force Microscopy: What Quantitative Information Can We Really Get Out of Piezoresponse Measurements on Ferroelectric Thin Films

Abstract Voltage modulated scanning force microscopy in contact mode or piezoresponse scanning force microscopy is now turning into an established technique for imaging ferroelectric domains in ferroelectric thin films. The quantities measured, however, are the amplitude and phase of a locally induced piezoelectric strain, and not the ferroelectric polarization itself. As all ferroelectrics possess piezoelectric properties, the domain structure visualized does correspond to that of the ferroelectric domains whose polarization is partly normal to the film surface, but the amplitude of the signal is actually not proportional to the magnitude of the normal component of the polarization. Likewise, the shear mode of scanning force microscopy allows the imaging of domains with a polarization component in the plane of the film. Methods to relate the amplitude of the piezoresponse signal to the magnitude of the ferroelectric polarization taking into account the anisotropic nature of the piezoelectric coefficients are described and discussed for the most common ferroelectric materials.

Structural and electrical anisotropy of (001)-, (116)-, and (103)-oriented epitaxial SrBi2Ta2O9 thin films on SrTiO3 substrates grown by pulsed laser deposition

Epitaxial SrBi2Ta2O9 (SBT) thin films with well-defined (001), (116), and (103) orientations have been grown by pulsed laser deposition on (001)-, (011)-, and (111)-oriented Nb-doped SrTiO3 substrates. X-ray diffraction pole figure and φ-scan measurements revealed that the three-dimensional epitaxial orientation relation SBT(001)‖SrTiO3(001), and SBT[110]‖SrTiO3[100] is valid for all cases of SBT thin films on SrTiO3 substrates, irrespective of their orientations. Atomic force microscopy images of the c-axis-oriented SBT revealed polyhedron-shaped grains showing spiral growth around screw dislocations. The terrace steps of the c-axis-oriented SBT films were integral multiples of a quarter of the lattice parameter c of SBT (∼0.6 nm). The grains of (103)-oriented SBT films were arranged in a triple-domain configuration consistent with the symmetry of the SrTiO3(111) substrate. The measured remanent polarization (2Pr) and coercive field (2Ec) of (116)-oriented SBT films were 9.6 μC/cm2 and 168 kV/cm, respec...

Growth, structure, and properties of epitaxial thin films of first-principles predicted multiferroic Bi2FeCrO6

The authors report the structural and physical properties of epitaxial Bi2FeCrO6 thin films on epitaxial SrRuO3 grown on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The 300nm thick films exhibit both ferroelectricity and magnetism at room temperature with a maximum dielectric polarization of 2.8μC∕cm2 at Emax=82kV∕cm and a saturated magnetization of 20emu∕cm3 (corresponding to ∼0.26μB per rhombohedral unit cell), with coercive fields below 100Oe. The results confirm the predictions made using ab initio calculations about the existence of multiferroic properties in Bi2FeCrO6.

Bi4Ti3O12 ferroelectric thin film ultraviolet detectors

Bi4Ti3O12 (BiT) ferroelectric thin films were used as photoconductive or photovoltaic ultraviolet detectors. The maximum of the spectral distribution lies around 370 or 390 nm, depending on whether the incident light is continuous or modulated. The maximum current responsivity obtained for the films annealed at 700 °C is about 0.01 A/W. The specific detectivity is 5.5×108 cm W−1 Hz1/2 at a modulation frequency of 20 Hz. The frequency characteristics of the photovoltaic signal measured in modulated light suggests a 1/f frequency dependence of this signal.