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 3.6° with respect to the substrate plane is revealed in the case of (100)-oriented SrTiO₃. For growth performed at very low pressure, NdTiO_3+δ 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.