As conventional CMOS technology approaches its scaling limits, alternative memory and logic device concepts are being actively pursued. Among these, resistive and ferroelectric switching mechanisms have emerged as promising candidates for non-volatile memory technologies due to their potential for high density, low power consumption, and compatibility with existing semiconductor processes. In particular, hafnium oxide-based materials stand out for their ability to support both valence change memory and ferroelectric switching phenomena at low thickness. This thesis investigates the coexistence and independent operation of resistive and ferroelectric switching in epitaxial Hf0.5Zr0.5O2 (HZO) thin films grown on La0.8Sr0.2MnO3 (LSMO)-bufferedSrTiO3. The crystalline model system is found to demonstrate both robust ferroelectricity and filamentary-type resistive switching within the same device, without the need for electroforming or external current compliance. Devices of this system exhibit reproducible polarization hysteresis loops upon AC bias, while resistive switching cycles can be initiated by quasi-static voltage sweeps. The resistive switching can be terminated through a standard RESET operation, returning to a pristine-like high-resistance state in which subsequent ferroelectric measurements can be performed. These findings demonstrate that the two switching modes are fundamentally decoupled and can operate in parallel in the same device under different electrical conditions. X-ray photoemission electron microscopy and hard X-ray photoelectron spectroscopy are employed to characterize the spatial and electro-chemical nature of the switching mechanisms. The localized filament responsible for resistive switching is directly visualized, with associated valence changes identified at the HZO/electrode interface. Filament formation at a site of enhanced oxygen vacancy mobility is suggested, as such structures are identified as inherent to the system. In the ferroelectric switching regime, depth-dependent spectroscopy reveals subtle electro-chemical changes associated with oxygen vacancy migration across the thickness of the HZO layer under common switching conditions. Oxygen vacancies accumulate preferentially at the LSMO/HZO interface, superimposed by polarization direction-dependent redistribution and accompanied by reversible oxygen exchange with the LSMO electrode. Quantitative analysis confirms that the oxygen vacancy concentrations involved in ferroelectric switching are substantially lower than those observed during filamentary switching. The dual-mode functionality established in this thesis, within which filamentary and ferroelectric switching mechanisms can coexist and be individually controlled within a single HZO-based device, highlights the pivotal role of oxygen vacancy dynamics, electrode interface engineering, and crystalline quality. It opens new paths for memory applications that can utilize the different strengths of both switching mechanisms and offers a unique platform for the study of oxygen vacancy dynamics and interface effects in hafnium-based systems. Additionally, the integration of single-crystalline ferroelectric HZO as a free-standing membrane is explored, demonstrating phase stability across different substrates and under mechanical stress. It provides a foundation for future investigations, opening up possibilities for the integration of single-crystalline films into flexible electronics and CMOS-compatible architectures.
Judith Rebecca Knabe (Wed,) studied this question.