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

Abstract

The growth conditions required to obtain high-quality layered-perovskite Nd2Ti2O7 thin films with a monoclinic structure on both (100)- and (110)-oriented SrTiO3 substrates by pulsed laser deposition are investigated. To synthesize the expected ferroelectric crystalline phase (and optimize the crystallization), two major critical parameters have to be carefully controlled, i.e. the oxygen pressure during deposition and the substrate temperature. Combining X-ray diffraction and transmission electron microscopy for structural characterizations, a twinned structure with (00l) orientation is evidenced for the films grown on (110)-oriented SrTiO3 while a twinned structure with a (012) orientation slightly tilted from 3.6u with respect to the substrate plane is revealed in the case of (100)-oriented SrTiO3. For growth performed at very low pressure, NdTiO3+delta thin films are obtained while polymorphic non-ferroelectric Nd2Ti2O7 samples with an orthorhombic structure are found at higher pressures. Piezoresponse force microscopy experiments evidence nanoscale ferroelectricity in the layered-perovskite monoclinic Nd2Ti2O7 films deposited on both substrates. Domain switching properties are shown to be more reliable for films grown onto (110)-SrTiO3 substrates, demonstrating these are more suitable as a functional material for applications in the field of micro-/nano-electronics. On the other hand, no ferroelectricity is probed in the polymorphic Nd2Ti2O7 thin films, as expected in a centro-symmetric structure

Optical properties of Ln2Ti2O7 (with Ln = La to Lu) thin films grown on (110)-SrTiO3 substrates by pulsed laser deposition

“…Structural investigations : As pointed out previously, the growth parameters required to obtain ferroelectric LnTO thins films with Ln=La, Pr, Nd were determined in previous work [7] , [46]. We highlighted the presence of a competition between a monoclinic ferroelectric phase (called -phase) and a polymorphic phase with orthorhombic structure with Ammm space group (called -phase) identified by S. Havelia et al [48].…”

“…To calculate the optical indices, it is necessary to know the thickness of the synthetized films. In the case of layered-perovskite films, the thickness has already been measured at 60 nm in a previous study [7] performed by STEM-HAADF (Scanning Transmission Electron Microscopy -High Angle Annular Dark Field). In this paper, we carried out the same study on the pyrochlore phase.…”

“…For a ratio greater than 1.78 (i.e. Ln=La to Nd), oxide crystallizes in a layered-perovskite with monoclinic structure and have piezoelectric [6], [7] and ferroelectric properties [8], [9] with a high Curie temperature [10], [11]. For a ratio smaller than 1.78 (i.e.…”

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

“…Further details regarding the synthesis parameters are detailed elsewhere. 9 For the STO target, commercial polycrystalline STO (disk diameter, 2.54 cm; a cubic ¼ 3.905Å) from crystal, Germany was used.…”

“…4,19 Recently, we successfully grew perovskite-layered ferroelectric NTO thin lms through both a sol-gel route and pulsed laser deposition (PLD) on SrTiO 3 (STO) substrates. 8,9,20 Furthermore, the introduction of insulating STO layers within ferroelectric layers to form articial superlattices has been shown to be promising for modulating the physical properties of thin lms. [21][22][23][24] For example, improved ferroelectric properties have been reported for BaTiO 3 (BTO)/STO superlattices, [25][26][27] while for BiFeO 3 (BFO)/ STO superlattices, a reduced leakage current was observed in the symmetric superlattices.…”