High Resolution (S)TEM Analysis of the Chemical Solution processed PbZrO₃ Thin Films—Defect Occurrence on Different Scales

Lead zirconate PbZrO₃ (PZO) belongs to the perovskite structural type and is well-known as an archetypal antiferroelectric material, however, it is expected to demonstrate a complex picture of a polarization behaviour at the nanoscale. The emergence of ferroelectricity and the possible co-existence of FE-AFE ordering has been predicted using first-principles density functional theory (DFT) for the case of size/dimensional confinement [1]. The reduction of film thickness to achieve this has been attempted in PbZrO₃ with the expectation that the substantial changes in electrical and mechanical boundary conditions would tilt the energy balance towards the FE phase.<br/><br/>For this purpose, continuous PZO thin films ranging from ~120nm to ~650nm of thickness were grown via repeated chemical solution deposition (CSD) of organic precursors on Pt/Ti/SiO₂/Si wafers with the subsequent drying, pyrolysis, and crystallization. According to the outcome of X-ray diffraction (XRD), phase pure PbZrO₃ thin films with the [001]_O orientation were successfully obtained. The proposed layer-by-layer synthesis method poses high flexibility and the scalability potential, however, the spatial continuity of crystallites within the films and their chemical homogeneity has to be thoughtfully analyzed to confirm the suitability of the exact processing conditions. To assess this, the spatial continuity has been analysed with high-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS), focusing on the microscopic structural changes within the films.<br/><br/>Lamellae from representative areas of samples were prepared using gallium focused ion beam (Ga-FIB) with subsequent argon ion milling to remove residual gallium and amorphous layers, and to reach the suitable lamellae thicknesses. Thickness was estimated using electron energy loss spectroscopy (EELS) t/λ mapping, the ratio of elastic and inelastic electron scattering.<br/><br/>STEM-EDS and STEM-EELS analyses have been performed using a ThermoFisher Talos F200X and Titan-G2 microscopes in QUB and the Advanced Microscopy Laboratory (AML) respectively. High spatial-precision imaging at atomic resolution was performed using the Nion UltraSTEM 200 instrument at the AML. In order to reduce the beam damage and sample drift effects, low-dose multi-frame non-rigid registration approaches were employed [2].<br/><br/>To assess and verify the proposed explanations of the features observed in the STEM images, in addition to the routine FFT-based analysis, the ab-initio simulations were performed to create the idealized image of the proposed model structures at the experimental conditions in use.<br/><br/>It has been confirmed that the obtained films demonstrate high structural and compositional continuity with a minor amount of nanometer-sized inclusions and defects. The results obtained enable further investigation to advance the fundamental understanding of antiferroelectricity in PbZrO₃ thin films and nanostructures.<br/><br/>This work is supported by the US-Ireland NSF-SFI-EPSRC tripartite grant (SFI grant number SFI/21/US/3785, NSF grant numbers DMR-2219476 (GT) and 2219477 (USF)), the SFI grant AMBER2 12/RC/2278_P2, and the SFI grant URF/RI/191637<br/><br/>References:<br/>[1] N. Maity <i>et al</i>, arXiv:2402.14176v1 (2024).<br/>[2] L. Jones <i>et al</i>, Microscopy <b>67</b> suppl_1 (2018) pp. i98-i113.