Microstructure and Nanoscale Piezoelectric/Ferroelectric Properties in La₂Ti₂O₇ Thin Films Grown on (110)‐Oriented Doped Nb:SrTiO₃ Substrates

Abstract

Abstract(00l)‐Oriented La2Ti2O7 (LTO) thin films with monoclinic perovskite‐layer structure [a = 7.806(2) Å, b = 5.552(3) Å, c = 13.015(5) Å, β = 98.62(2)°] have been grown by a sol–gel route on conducting (110)‐oriented doped Nb:SrTiO3 (STO) substrates. The narrow rocking curves (0.24° width for 004LTO peak) demonstrate the sharp mosaïcity of the films. Using high‐resolution X‐ray diffraction (HR‐XRD), epitaxial relationships between the LTO, and the STO substrate are given. In addition, HR‐XRD evidences the existence of (212)‐oriented crystallites 1.5° disoriented with respect to the plane of the substrate. We confirm, by DFT calculations, that the polarization vector lies in the b‐axis of the LTO cell and consequently, the existence of these (212)‐oriented crystallites enables to explain the origin of the various contrasts observed both on the in‐plane and out‐of‐plane images when collected by piezoresponse force microscopy. Finally, both successful poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains, unambiguously confirm the ferroelectric state of the films at the nanoscale level. Once again, this study demonstrates that a clear understanding of nanoscale piezoelectric/ferroelectric phenomena in oriented thin films passes through a carefully structural analysis as performed by HR‐XRD.

Microstructural investigations and nanoscale ferroelectric properties in lead-free Nd2Ti2O7 thin films grown on SrTiO3 substrates by pulsed laser deposition

“…Note that previous works on NTO thin films grown by a sol-gel technique on similar (100)-and (110)-STO substrates showed R rms values of about 17 and 9 nm, respectively. 35,38 To conclude, the growth of the NTO thin films on (110)-or (100)-oriented STO substrates by a laser ablation technique follows the same microstructural behavior as LTO thin films prepared by a solgel technique which are presented in previous work. 22,35 For lower pressures (10 25 mbar), we observed the formation of a NdTiO 3+d (Nd n Ti n O 3n+2 with n = ') phase.…”

Phase transition of Eu2Ti2O7 under high pressure and a new ferroelectric phase with perovskite‐like layered structure

“…The original area shows no preferential direction for ferroelectric polarization, with randomly mixed small red and yellow areas. The domain is in a needle shape with an average length of 120 nm and width of 30 nm, which is similar to the domains in La 2 Ti 2 O 7 and PL‐structure Sm 2 Ti 2 O 7 . Then we apply DC +200 V on a tape area on the sample, all the polarizations switched into 1 direction shown in red color (marked by “+”), whereas the polarization switched back shown in yellow color (marked by “−”) if we apply DC −200 V. We observed fully uniform red and yellow areas, and these results clearly evidence poled domains with downward and upward polarization and that the polarization process is completely reversible.…”

Ion‐Beam Etching on Nanostructured La2Ti2O7 Piezoelectric Thin Films

“…Such value for d zz was expected taking into account both the polarization vector direction in the film and the piezoelectric coefficient for LTO single crystal. Indeed, for layered‐perovskite LTO films, the polarization vector is exclusively along the b ‐axis, and the d 22 piezoelectric coefficient in single crystal is 16 pm/V, whereas the d 21 and d 23 piezoelectric coefficients are 3 and 6 pm/V, respectively, as reported by Nanamatsu et al . Therefore, as the polar axis lies in an intermediate direction between the normal and the plane of the substrate, as expected for (012)‐oriented crystallites in the LTO cell (in fact, the angle between the b ‐axis of LTO and the surface plane is 52.6°), it is not surprising the d zz value measured is lower than 16 pm/V.…”