Synthesis and optimisation of Aurivillius phase thin films for next generation data storage
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Date
2025-12-11
Authors
Dutta, Debismita
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University College Cork
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Abstract
Aurivillius phase oxides are well-established room-temperature ferroelectrics, whose high Curie temperatures and intrinsic fatigue resistance make them attractive candidates for emerging energy-efficient non-volatile memory and neuromorphic device concepts. However their integration in vertically oriented device architectures is limited by three interconnected challenges: (i) their spontaneous polarisation is predominantly in-plane, which is misaligned with the out-of-plane electric fields used in standard capacitor geometries; (ii) there is no predictive framework linking complex three-dimensional defect motifs to measurable X-ray diffraction (XRD) signatures; and (iii) crystal twinning frequently emerges during thin-film growth, reducing domain coherence and hindering reliable switching.
This thesis addresses these challenges through controlled thin-film synthesis, advanced structural characterisation, and analytical model development. Supersaturation, tuned through the precursor delivery kinetics and oxygen partial pressure of Direct Liquid Injection Chemical Vapour Deposition (DLI-CVD), serves as the unifying experimental variable across the three chapters, governing whether Bi₄Ti₃O₁₂ thin films grow via two-dimensional nucleation, dislocation-mediated spiral formation, or diffusion-limited single-variant epitaxy. This work identifies for the first time that films exhibiting spiral morphologies displayed a measurably enhanced vertical polarisation, with the required switching voltage reduced from ±20 V to ±5 V. Correlative piezoresponse force microscopy, density functional theory, and electron microscopy reveal a previously unrecognised mechanism by which inclined out-of-phase boundary defects generated by spiral growth alter local symmetry and strain environments, thereby stabilising an out-of-plane polarisation in a material class normally constrained to in-plane orientation. To interpret the diffraction signatures of these non-planar defects, a new three-dimensional XRD model was developed. This framework extends the structure-factor formalism to incorporate both vertical registry offsets and angular boundary inclinations, enabling accurate simulation of the asymmetric peak splitting observed experimentally. The model provides a non-destructive estimation of defect density and orientation, establishing a predictive link between defect topology and reciprocal-space features.
Finally, the thesis demonstrates a previously unreported route to supressing crystal twinning. By regulating supersaturation during metal–organic chemical vapour deposition, the twinning typically observed in Aurivillius films on epitaxially matched substrates can be effectively eliminated. Lower supersaturation enhances adatom mobility and promotes single-variant epitaxy, yielding films with improved structural coherence and spatially uniform ferroelectric switching. This approach provides an accessible, growth-parameter-based route to producing twin-free layered ferroelectrics without substrate modification or post-processing. Collectively, these findings provide new insight into the layered crystallography of Aurivillius phases and the growth mechanisms that modify their ferroelectric response. The thesis delivers three distinct advances: (i) a previously unrecognised mechanism by which spiral growth enhances out-of-plane polarisation; (ii) a new three-dimensional XRD framework that predicts diffraction signatures arising from complex defect geometries; and (iii) a previously unreported supersaturation-controlled route for eliminating twin variants in layered ferroelectric thin films. Together, these advances contribute a materials-centric foundation for integrating layered ferroelectrics into vertically oriented memory and neuromorphic device platforms, where controlled polarisation, structural coherence, and engineered defect landscapes are essential for meeting key requirements for reliable operation at technologically relevant scales.
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Keywords
Ferroelectrics , Thin film engineering , Aurivillius materials , Layered ferroelectrics , Multiferroics , Chemical vapor deposition , Scanning probe microscopy , Defect engineering , Defect modelling , Growth spiral , Crystal twinning , Neuromorphic computing , Crystal growth and design
Citation
Dutta, D. 2025. Synthesis and optimisation of Aurivillius phase thin films for next generation data storage. PhD Thesis, University College Cork.