In the past two years, we focused on the stabilization and advanced characterization of polar textures in BaTiO3-based systems and on their integration into devices.
We achieved the realization of nanostructures or nanoscale heterostructures on silicon using three different approaches:
(1) Nanostructures (nanodisks) fabricated by a top-down approach
(2) Nanostructures (nanoislands) fabricated by a bottom-up approaches
(3) Superlattices
Nanodisks fabricated by Ne ion milling of epitaxial BaTiO3 thin films on Si: Nanodisks of diameter ~100 nm are shown to host non-trivial polar patterns . Three distinct patterns are observed for the vertical component of the polarization, which correspond to those predicted by theory. Moreover, controllable multilevel polarization states are achieved, which is of particular interest for analogue memory applications.
Nanoislands, with a lateral size as small as of 30-60 nm, embedded in a BaTiO3 continuous film, were synthesized by a modified molecular beam epitaxy (MBE) route. The domains are of center-down convergent type.Compared to a cylindrical shape, the conical shape of the nanoislands (like a funnel) gives rise to a lateral swirling polarization component, which confers chirality. Reversible electrical switching of these topological polar domains is demonstrated. One future challenge is to succeed in stabilizing nanoislands with a single chirality and to “switch” the chirality.
We achieved the first realization of periodic ordered vortices and of polar skyrmion-like bubbles in BaTiO3/SrTiO3 superlattices on silicon. The polar domains are ordered along the <110>BaTiO3 directions in contrast to PbTiO3/SrTiO3 systems, for which the polar states are order along <100>PbTiO3 directions. Strain plays a fundamental role in the stabilization of polar textures in in BaTiO3/SrTiO3 superlattices on silicon.
As part of the tasks for achieving objective 3, we developed the epitaxial growth on SrTiO3-buffered Si of epitaxial La0.7Sr0.3MnO₃ by MBE and epitaxial SrRuO3 by sputtering. LSMO/BaTiO3/LSMO/SrTiO3/Si and LSMO/(BaTiO3/SrTiO3)n/LSMO/ SrTiO3/Si devices exhibit well-saturated ferroelectric hysteresis loops with a coercive voltage Vc of 0.5 V and a remanent polarization Pr of 12 μC cm-2. The large leakage currents in BaTiO3 appear to be a major obstacle for device development. We therefore focused on how to reduce the leakage currents. We demonstrated a huge reduction in leakage currents, by up to seven orders of magnitude, for doping with 5-8 mol% Mn. With this breakthrough, we then used this doping for the growth of Ba(Ti, Mn)O3/SrTiO3 superlattices fabricated on p++ doped Si substrates and achieved closed and fully saturated P-V hysteresis loops (without the need for a conducting oxide bottom electrode!).
We collaborated with the group of Prof. Christophe Gatel / Dr. Martin Hÿtch at CEMES-CNRS in Toulouse, France, for operando (in situ biased) dark field electron holography to study Hf0.5Zr0.5O2 (HZO) ferroelectric tunnel junctions. We have achieved the direct measurement of local electric fields, the visualization of ferroelectric domain switching (on a large field of view of ~ 300 nm) and the quantification of interfacial charges. These results have been obtained thanks to the development of a novel methodology by this group. For the very first time, it has been possible to visualize unambigusouly ferroelectric switching in a ferroelectric film by dark field electron holography.
