In-situ plasmon-enhanced characterization of optically accessible ultra-thin film ferroelectric devices
This PhD thesis presents novel nanoscale characterisation techniques and probes for optoelectronic analysis of ultra-thin film ferroelectric (FE) devices, which overcome the drawbacks of contemporary investigation methods such as their destructive nature, lack of precision, or in certain techniques the requirement for a vacuum environment. We show the use of a innovative nanoparticle-on-mirror (NPoM) technique that uses gold nanoparticles as electrical contacts, providing both nano-sized electrodes and substantial optical enhancement in the biased region. This allows for the first accurate and non-invasive simultaneous optical and electrical evaluation of nanomaterials. This thesis majorly concentrates on Hafnium oxide based ferroelectric and memristive materials due to their significant advantages in power consumption, high read-write speed and a well-established CMOS-compatibility compared to other non-volatile memories (NVMs). This thesis also introduces a unique characterization approach called scanning plasmon-enhanced microscopy, which addresses many issues in the current plasmon-enhanced optical characterization of ultra-thin films and two-dimensional materials. Chapter 1, Introduction, delineates the motivation for this study and provides a concise overview of the Nanoparticle-on-mirror approach used for nanoscale characterisation in this research. Chapter 2, Ferroelectricity and FeRAMs, presents the theory of ferroelectricity, emphasizing on Hafnium Oxide as a ferroelectric material. Chapter 3, Experimental Methods, describes the optical and electrical setup utilized in experiments. Scanning plasmon-enhanced microscopy for simultaneous opto-electrical characterization, a scanning plasmonic imaging method using an optically transparent and conductive plasmonic nanoprobe that conducts through a single gold nanoparticle is introduced and verified on MoS2. In chapter 4, Evidence of vacancy migration assisted phase change in FE memories, this study shows the optical tracking of oxygen migration induced phase change in ultra-thin film (≤5 nm) ferroelectric devices. While Hafnia-based films have achieved ferroelectricity at single-digit nanometre thicknesses, they exhibit nonideal behaviours, specifically wake-up and fatigue effects. Using plasmon-enhanced spectroscopy, we provide the first real-time in-operando nanoscale direct tracking of oxygen vacancy migration and phase changing explaining the underlying physics behind these phenomenon. In chapter 5, Redistribution of oxygen vacancies in ferroelectric HZO memories, this study reports a novel in-operando approach to control both wake-up and fatigue device dynamics. Via clever design of short ad-hoc square electrical pulses limited in time and amplitude, we can both speed-up or delay wake-up, and reduce both fatigue or leakage current in the ultra-thin films, which is a key factors for enhancing the performance of memory devices. In Appendix A, In-operando Raman tracking resistive switching devices, we demonstrate with the NPoM technique, direct observation of migration of metallic ions or the intercalation of ions into the lattice of binary oxide memristive materials. This alters the local electronic environment and, as a result, the material's resistive state. Because the NPoM method is flexible, it can be used to study a lot of different materials and switching events. This makes it an important tool for understanding and improving the performance of next-generation memory devices. In Appendix B, Scanning plasmon-enhanced microscopy results on co-doped Hafnia based thin-films are presented. Appendix C, A substrates critical role: Maximising Raman in heterostructure thin-films, offers valuable resources for researchers seeking optimal substrates for Raman characterization in heterostructure thin-film devices.
Electronic Devices, Optical spectrosccopy, Non-volatile ferroelectric memories, In-situ opto-electronic
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