Alexandre Bayart

Ferroelectricity in La2Zr2O7 thin films with a frustrated pyrochlore-type structure

Thin films of lanthanum zirconate (La2Zr2O7) have been grown by a sol–gel route on (110)-oriented SrTiO3 substrates. Electrical measurements, locally performed by piezoresponse force microscopy, evidence unambiguously the ferroelectric state of the films at the nanoscale level. In the La2Zr2O7 bulk material, ferroelectricity is absent due to the centro-symmetric cubic pyrochlore structure. In thin films, the extensive study carried out by high resolution X-ray diffraction highlights a lowering of the cubic symmetry that may explain the emergence of ferroelectricity. This slight structural modification is interpreted as a geometrical frustration induced by the substrates strains during the film growth. In addition, pole figure experiments are used to give epitaxial relationships between the La2Zr2O7 film and the SrTiO3 substrate. Finally, high resolution transmission electron microscopy images obtained on a cross-section of the film are given.

Epitaxial growth and nanoscale electrical properties of Ce2Ti2O7 thin films

(00l) epitaxial Ce2Ti2O7 thin films with a layered perovskite/monoclinic structure were grown on (110)-oriented Nb-doped SrTiO3 substrates via pulsed laser deposition and a sol–gel method associated with spin-coating. Using the sol–gel method, the Ce2Ti2O7 films were obtained by annealing at 950 °C under a reductive Ar/H2 atmosphere. Employing the pulsed laser deposition technique, they were directly grown under vacuum (10−6 mbar) with a controlled re-oxidation during the cooling step. The pole figure measurements provide the in-plane crystallographic relationships between the film and substrate: [001]SrTiO3//[100]Ce2Ti2O7 and [1−10]SrTiO3//[010]Ce2Ti2O7. Piezoresponse force microscopy measurements highlight the local ferroelectric character of the films synthetized. The switching capability was more reliable for the film grown via pulsed laser deposition, which was explained by the lower mosaic spread. Higher local conductivity was also detected using conductive-atomic force microscopy of the physically deposited film and was attributed to its lower thickness. Such epitaxially deposited functional oxides may be considered as promising candidates for integration into advanced electronic devices.

Fabrication of La 2 Ti 2 O 7 nanostructures by focused Ga 3+ ion beam and characterization by piezoresponse force microscopy

(012)-oriented lead-free La₂Ti₂O₇ thin films with the monoclinic/perovskite layered structure have been grown by a solgel route on (100)-oriented doped Nb:SrTiO₃ substrates. On nanoscale, both poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains confirm the piezo-/ferro-electric behaviour of the films. Islands in the lateral range 300-500 nm have been fabricated by focused Ga³⁺ ion beam etching on platinum top electrode. As measured on piezoloops, electromechanical activity within the islands is shown to be similar to the one obtained for the virgin film; no piezoelectric degradation for La₂Ti₂O₇ islands is highlighted. These results confirm that La₂Ti₂O₇ is a highly resistant oxide to ion-beam irradiation. La₂Ti₂O₇ could be considered as a material of choice for the realization of lead-free piezoelectric nanostructures.

Microstructure and local electrical behavior in [(Nd2Ti2O7)4/(SrTiO3)n]10 (n = 4–8) superlattices

Artificial [(Nd2Ti2O7)4/(SrTiO3)n]10 superlattices (n = 4 and 8) were successfully epitaxially grown on SrTiO3 substrates by pulsed laser deposition using the in situ high energy electron diffraction reflection diagnostic. The crystallographic relationships between Nd2Ti2O7 (NTO) and SrTiO3 (STO) (layers and substrate) were: [100]NTO//[001]STO, [010]NTO//[10]STO, and (00l)NTO//(110)STO. Nanoscale current variation was detected on both superlattices, with the (NTO4/STO4)10 heterostructure showing a higher density. The (NTO4/STO4)10 sample did not show a piezoelectric response when measured by piezo-force microscopy (PFM), while ambiguous piezoactivity was observed on the (NTO4/STO8)10 superlattice. Scanning transmission electron microscopy energy dispersive spectroscopy analysis showed the diffusion of Nd3+ cations on Sr2+ sites in SrTiO3 structure into the multilayers, which was more pronounced when the value of n was lower. These particular nanoscale electrical behaviors, evidenced by electrical conducting channels and misleading PFM signals, were mainly attributed to the presence of oxygen vacancies in the SrTiO3 layers at higher concentrations near the interface and to the mixed valence state of the titanium (Ti3+/Ti4+). This work showed the strong influence of interface structure on nanoscale electrical phenomena in complex oxide superlattices.