arXiv subjects

Yu-Chieh Ku

Publications and source records attributed to Yu-Chieh Ku.

5 recordsLinked to original sources

arXiv2026

Nitrogen doping induced metal-insulator transition with iso-symmetric character in rutile VO2

Metal-insulator transitions (MITs) in correlated oxides offer immense potential for next-generation Mottronic devices. However, their integration into practical applications is often hindered by the coupling of MITs with symmetry-lowering structural phase transitions, which limits switching speed and endurance. In this study, we engineered an iso-symmetric MIT on average in epitaxial rutile VO2 thin films via an in-situ nitrogen doping strategy. Nitrogen incorporation effectively suppresses V-V dimerization, enabling an iso-symmetric MIT, while preserving the original crystal symmetry. Furthermore, in-operando time-resolved optical reflectivity measurements revealed a shortened switching time in nitrogen-doped films, highlighting their enhanced performance. Our findings provide critical insights into the underlying mechanisms of MITs and introduce anion doping as a powerful tool for tailoring phase transitions in strongly correlated electron systems. This approach opens new avenues for the development of high-performance electronic and photonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Enhancing ferroelectric stability: Wide-range of adaptive control in epitaxial HfO2/ZrO2 superlattices

The metastability of the polar phase in HfO2, despite its excellent compatibility with the complementary metal-oxide-semiconductor process, remains a key obstacle for its industrial applications. Traditional stabilization approaches, such as doping, often induce crystal defects and impose constraints on the thickness of ferroelectric HfO2 thin films. These limitations render the ferroelectric properties vulnerable to degradation, particularly due to phase transitions under operational conditions. Here, we demonstrate robust ferroelectricity in high-quality epitaxial (HfO2)n/(ZrO2)n superlattices, which exhibit significantly enhanced ferroelectric stability across an extended thickness range. Optimized-period superlattices maintain stable ferroelectricity from up to 100 nm, excellent fatigue resistance exceeding 109 switching cycles, and a low coercive field of ~0.85 MV/cm. First-principles calculations reveal that the kinetic energy barrier of phase transition and interfacial formation energy are crucial factors in suppressing the formation of non-polar phases. This work establishes a versatile platform for exploring high-performance fluorite-structured superlattices and advances the integration of HfO2-based ferroelectrics into a broader range of applications.

cond-mat.mtrl-sci↗

arXiv2024

Large Enhancement of Properties in Strained Lead-free Multiferroic Solid Solutions with Strong Deviation from Vegard's Law

Efforts to combine the advantages of multiple systems to enhance functionlities through solid solution design present a great challenge due to the constraint imposed by the classical Vegard law. Here, we successfully navigate this trade off by leveraging the synergistic effect of chemical doping and strain engineering in solid solution system of BiFeO3 BaTiO3. Unlike bulks, a significant deviation from the Vegard law accompanying with enhanced multiferroism is observed in the strained solid solution epitaxial films, where we achieve a pronounced tetragonality, enhanced saturated magnetization, substantial polarization, high ferroelectric Curie temperature, all while maintaining impressively low leakage current. These characteristics surpass the properties of their parent BiFeO3 and BaTiO3 films. Moreover, the superior ferroelectricity has never been reported in corresponding bulks. These findings underscore the potential of strained BiFeO3 BaTiO3 films as lead-free, room-temperature multiferroics.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping

In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

cond-mat.mtrl-sci↗

arXiv2023

Hidden Hydroxides in KOH-Grown BaNiO3 Crystals: A Potential Link to Their Catalytic Behavior

The hexagonal perovskite BaNiO3, prepared via non-ceramic approaches, is known to act as a good catalyst for the oxygen-evolution reaction (OER) in alkaline media. Here we report our observation that BaNiO3 synthesized via KOH flux growth and high O2 pressure ceramic synthesis have different magnetic properties. We show that this is because the KOH flux-grown crystals made in open-air are actually a hydroxide-containing form of BaNiO3 that can be dried upon annealing in O2 flow. This work not only unveils a previously unknown aspect of the BaNiO3 OER catalyst and offers some insights into the underlying mechanism, but also suggests that hydroxide ions may be present in other hexagonal perovskite oxides prepared in wet conditions.

cond-mat.mtrl-sci↗