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Doped HfO2 thin films: Engineering and Origins of its ferroelectric properties https://doi.org/10.17863/CAM.126664 Kim, Ji Soo 0000000307458754 Driscoll, Judith With growing interest in energy efficient memory and computing devices, non-volatile and hysteretic switching phenomena have garnered intensive interest from electronics industry. One of those physical phenomena of interest is ferroelectricity, capitalising on reversibly switchable polarisation with application of electric field. Especially, with the discovery of ferroelectricity in scalable and CMOS-compatible HfO2 thin film in which its polarisation amplifies with reduced thickness below 10-nm, ultrathin ferroelectric HfO2 thin films are subject to be harnessed for next-generation non-volatile memory (NVM) or neuromorphic computing devices. Therefore, this thesis explores the nanostructure, device stack engineering and electrochemical changes which occur upon ferroelectric polarisation in epitaxial doped-HfO2 systems. This thesis aims to elucidate a more fundamental understandings about ferroelectricity in polar HfO2 systems. In order to control coercive field (Ec) (one of the ferroelectric properties of interest which dictates operation voltage in a device), 10 nm-thick hafnium zirconium oxide (HZO) thin films were deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates with varied laser fluence. Detailed structural characterisation revealed stabilisation of a rhombohedrally distorted orthorhombic (r-d o-) phase. Ferroelectric measurements demonstrated that Ec in HZO is reduced from 3.3 MV/cm to 2.7 MV/cm (∼20% of reduction) by reducing laser fluence from 1.3 J/cm2 to 0.5 J/cm2. Elevated laser fluence induces additional (11-1)-oriented grains along with various structural defects, which cause domain pinning; hence increase Ec in HZO. In comparison to simple HZO, a new composition, 5 at .% Sm-doped HZO (HZSO) system is explored as ultrathin (∼2 nm) underlayer for HZO to facilitate oxygen insertion/extraction at the LSMO|HZSO interface. With its enhanced oxygen ionic conductivity with acceptor doping, HZSO not only exhibits reduced Ec compared to HZO (3.3 MV/cm to 2.5 MV/cm, ∼25% of reduction) owing to reduced energy barrier from doping induced oxygen vacancies (V•• O ), but also enhances endurance by an order of magnitude (5×106 to 5×107). From hard x-ray photoelectron spectroscopy (HAXPES), an absence of oxygen redistribution in HZSO upon polarisation switching is observed, which corroborates with its enhanced ionic conductivity due to structural V•• O from acceptor doping. This approach presents a new route to tuning the ferroelectric properties of HfO2 for applications requiring lower Ec and endurance. The co-occurring electrochemical changes with polarisation switching is explored in HZO and newly reported 2 at.% La and Ta co-doped HfO2 (HLTO). With the help of HAXPES and angle-resolved XPS, the depth profile of electrochemical changes of Hf/Zr, i.e. redox, is characterised. HZO exhibits pervasive redox throughout the layer with polarisation switching, while HLTO does not present any evidence of redox. HLTO demonstrated heavily localised electrochemical changes limited to the topmost surface while exhibiting ∼50% higher polarisation value (Pr) compared to HZO. Lack of correlation between Pr and redox reactions in those two doped-HfO2 systems verifies an intrinsic polar nature of HfO2-based thin films (which has been debated, but not clearly proven in the literature), presenting deeper understandings about the origin of polarisation in HfO2. University of Cambridge 2025-09-29 Thesis Doctoral Doctor of Philosophy (PhD) eng Engineering and Physical Sciences Research Council, Samsung Advanced Institute of Technology, Department of Materials Science and Metallurgy (University of Cambridge) https://www.repository.cam.ac.uk/handle/1810/397561 https://www.repository.cam.ac.uk/bitstreams/dd1cbf24-46d5-4e40-8a82-6dc2db0603ca/download 100f9f99a6343e63591ad980448b4d02 https://www.repository.cam.ac.uk/bitstreams/4bbb7e94-c118-469a-b838-dc5f4ae368ef/download 87eda9de84448d1f82354d60eee3eb5f http://purl.org/NET/rdflicense/allrightsreserved Ferroelectric Hafnium oxide Oxide thin film Epitaxy