Romain Bachelet

Atomically flat SrO-terminated SrTiO3(001) substrate

We show that atomically flat single SrO-terminated SrTiO3(001) substrates can be obtained through simple high-temperature treatment. Amplitude-modulation atomic force microscopy with phase-lag analysis and x-ray photoelectron spectroscopy, have been used to demonstrate that the ratio between the two chemical terminations can be tailored by choosing the annealing time. Moreover, the progressive SrO surface enrichment (up to 100%) is accompanied by a self-assembly process which results in the spatial separation at the nanoscale of both chemical terminations. We further demonstrate that this opens a interesting avenue for selective chemical reaction and growth of oxide nanostructures.

Ultra-flat BaTiO3 epitaxial films on Si(001) with large out-of-plane polarization

Ferroelectric BaTiO3 is rarely used in monolithic Si devices due to the low quality of BaTiO3 films on Si, as polycrystallinity, degradation of bottom Pt electrodes, low polarization, and high roughness. Here, we overcome these limitations by using a buffer structure that combines yttria-stabilized zirconia, CeO2, and conducting LaNiO3. BaTiO3 films on the multilayered buffer, with total thickness of the buffer below 100 nm, are epitaxial, display remnant polarization of 6–10 μC/cm2, and have roughness of a few A. These unprecedented properties pave the way to integrate ferroelectric BaTiO3 into Si platforms.

Domain matching epitaxy of ferrimagnetic CoFe2O4 thin films on Sc2O3/Si(111)

Ferrimagnetic spinel CoFe2O4 (CFO) films are integrated with Si(111) using Sc2O3 buffer layers. The huge lattice mismatch (17%) between CFO and Sc2O3 is accommodated by domain matching, and CFO grows epitaxially with (111) out-of-plane orientation and coexistence of A- and B-type in-plane crystal variants. CFO films have low roughness of 4 A and saturation magnetization of about 300 emu/cm3. These properties make CFO films on Sc2O3-buffered Si(111) comparable to those grown on oxide single crystals and thus extend the possibilities of using spinel oxides in electronic devices.

Strain-driven noncollinear magnetic ordering in orthorhombic epitaxial YMnO3 thin films

We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. These results, together with the measured anisotropy of the magnetic response, indicate that the unexpected observation of ferromagnetism in orthorhombic o-RMnO3 (R=Y,Ho,Tb, etc.) films originates from strain-driven breaking of the fully compensated magnetic ordering by pushing magnetic moments away from the antiferromagnetic [010] axis. We show that the resulting canting angle and the subsequent ferromagnetic response, gradually increase (up to ∼1.2°) by compression of the unit cell. We will discuss the relevance of these findings, in connection to the magnetoelectric response of orthorhombic manganites.

Step formation, faceting, and bunching in atomically flat SrTiO3 (110) surfaces

We demonstrate that single-terminated atomically flat surfaces of polar SrTiO3 (110) can be obtained by annealing in air at temperatures above 1000°C. We show that a morphological transition occurs from disordered step edges to faceted step edges in the range of 1000–1100°C with appropriate annealing conditions. The influence of miscut angles (polar and azimuthal) on the surface nanostructure, i.e., step bunching and faceting of step edges, is described. We show how the azimuthal angle has a critical role for the step-edge faceting and the step-bunching formation.

Large out-of-plane ferroelectric polarization in flat epitaxial BaTiO3 on CoFe2O4 heterostructures

Engineering interfaces in perovskite/spinel heterostructures is challenged by structural differences. We have used kinetic growth limitations to fabricate flat BaTiO3/CoFe2O4 (BTO/CFO) epitaxial bilayers on La2/3Sr1/3MnO3/SrTiO3(001). In situ analysis of lattice strain during growth has revealed that BTO grows relaxed on highly lattice-mismatched CFO, thus suppressing tensile epitaxial stress effects. As a result, BTO is ferroelectric along the out-of-plane direction with bulk-like polarization. These results show that very high lattice mismatch in heteroepitaxy can be an opportunity rather than a limitation to integrate dissimilar materials with optimized functional properties.

Persistent two-dimensional growth of (110) manganite films

We have deposited La2/3Sr1/3MnO3(110) thin films by pulsed laser deposition, changing the ratio of surface diffusivity to deposition flux (D/F) by adjusting substrate temperature and laser repetition rate. We show that persistent two-dimensional layer-by-layer growth, at least up to 30 nm, can be achieved by exploiting diffusion-limited growth (small D/F ratio), giving rise to atomically-flat epitaxial films. The results are of interest in spintronic applications such as tunnel magnetoresistance devices, and the approach presented here can be extended to other functional films of high surface energy.

Ferroelectric Pb(Zr,Ti)O3 epitaxial layers on GaAs

Ferroelectric epitaxial Pb(Zr,Ti)O3 (PZT) layers were grown by pulsed laser deposition on SrTiO3/GaAs templates fabricated by molecular beam epitaxy. The templates present an excellent structural quality and the SrTiO3/GaAs is abrupt at the atomic scale. The PZT layers contain a- and c-domains, as shown by X-Ray diffraction analyses. Piezoforce microscopy experiments and macroscopic electrical characterizations indicate that PZT is ferroelectric. A relative dielectric permittivity of 164 is extracted from these measurements.

Self-patterned oxide nanostructures grown by post-deposition thermal annealing on stepped surfaces

We report here on the self-patterned growth of nanostructured epitaxial oxide thin films using a bottom-up approach. Different stepped (0001) sapphire surfaces have been used as templates to grow different nanostructured thin films of yttria-stabilized zirconia elaborated by a chemical solution deposition technique, namely sol–gel dip-coating. For such films, a morphological instability occurs during post-deposition thermal annealing that forces the film to split into isolated islands. Taking advantage of this effect, three distinct types of nanostructures have been produced depending on the step morphology of the substrate surface, and depending on the relative magnitude of the mean island diameter (D) and the mean terrace length (L). Flat-top islands, dome-shaped islands, tilted flat-top islands or self-organized arrays of nano-islands can be generated. Atomic force microscopy was used to observe nanostructures and quantitative characterizations of crystallographic and nanostructural aspects were carried out by high-resolution x-ray diffraction reciprocal space mapping.

CoFe2O4/buffer layer ultrathin heterostructures on Si(001)

Epitaxial films of ferromagnetic CoFe2O4 (CFO) were grown by pulsed laser deposition on Si(001) buffered with ultrathin yttria-stabilized zirconia (YSZ) layers in a single process. Reflection high-energy electron diffraction was used to monitor in real time the crystallization of YSZ, allowing the fabrication of epitaxial YSZ buffers with thickness of about 2 nm. CFO films, with thicknesses in the 2–50 nm range were subsequently deposited. The magnetization of the CFO films is close to the bulk value. The ultrathin CFO/YSZ heterostructures have very flat morphology (0.1 nm roughness) and thin interfacial SiOx layer (about 2 nm thick) making them suitable for integration in tunnel (e.g., spin injection) devices.