Oxidized Ultra-thin Metallic Glass: Thermal-mechanical Behaviors and Microelectronic Applications

Abstract

Metallic glasses (MGs) are widely recognized for their disordered atomic structures, which distinguish them from their crystalline counterparts. The amorphous nature of MGs confers them outstanding physical and functional properties, including superb thermal formability, excellent mechanical strength and hardness, as well as exceptional electrochemical and soft magnetic properties. However, the structure and properties of MGs are highly sensitive to impurities, especially oxygen atoms introduced during the preparation and storage. Oxide inclusions in MGs act as additional nucleation sites, shifting the crystallization mode of MGs from homogeneous to heterogeneous, thereby degrading the glass-forming ability (GFA) and overall properties of MGs. This issue is particularly critical in metallic glass nanostructures, where the high surface-to-volume ratio increases susceptibility to oxygen contamination and amplifies the effects of oxidation. Therefore, considerable efforts have been made to eliminate the detrimental effects of oxidation and to leverage it for improving MG properties. For instance, the directional formation of crystallization-resistant metastable phases with the assistance of oxygen was utilized to improve the GFA. Another involves increasing structural fluctuation via oxygen-induced densely packed regions, thereby enhancing mechanical properties. However, these approaches require precise control of the oxygen content (0.1 at% precision) and are limited to minor oxygen doping (< 5 at%). In my PhD research, I aimed to explore the microstructure change of MGs in the cases of severe oxidation and even fully oxidation, construct the structure-property relationship for oxidized MGs, and finally explored the potential novel applications of oxidized MGs.

In chapter 2, I initiated my investigation into the effect of oxidation on the thermal stability of MGs. By employing the well-developed Polymer Surface Buckling Enabled Exfoliation (PSBEE) method, the freestanding ZrCu-MG-based nanomembrane was successfully fabricated. As a result of the interaction with the polymeric substrate during preparation, the nanomembranes develop a distinctive structure consisting of nanosized metallic-glasses encapsulated within an interconnected nano-amorphous-oxide network. Owning to the severe nano-confinement imposed by this oxide network, the nanomembrane exhibits superior thermal properties, including a near-room temperature glass transition temperature of 324 K and an expansive supercooled liquid region of 448 K, outperforming various MG and oxide glass systems. Through AFM indentation experiment, I found that the nanomembrane shows an ultralow elastic modulus of 47 GPa and viscous liquid-like flow at even room temperature, attributable to its high homologous temperature. My research revealed that the proper oxidization could not only improve the glass forming ability but also impart more intriguing properties and potential applications to MGs.

Beyond oxidation during preparation, post-annealing offers another efficient approach for introducing oxygen into MGs. Thermal oxidation induces significant structural and property changes in MGs, potentially enabling unprecedented applications. Motivated by this, in the third chapter of my research, I focus on the thermal oxidation of ultra-thin monoatomic metallic glass such as amorphous tantalum and hafnium nanomembranes. These nanomembranes were thoroughly oxidized by custom-developed low-temperature reactive oxygen thermal oxidation process. Detailed structural and chemical analysis revealed that oxidation led to a homogenized density distribution and a reduction in electrical defects throughout the nanomembrane. Consequently, the oxidized oxides (OGs) exhibit superior dielectric properties, including low leakage current (10^-8 A/cm² at 1 MV/cm), ultrahigh breakdown electric field (> 14 MV/cm) and low dielectric loss (< 0.01), highlighting its potential as high-performance dielectric materials for microelectronics.

In chapter 4, I proceeded to explore the application of oxidized MG nanomembrane in microelectronic devices, specifically as gate insulators in thin film transistors (TFTs). After functional layer deposition and patterning, I successfully fabricated TFT devices incorporating oxidized MG nanomembranes as gate dielectric. These TFTs showcase outstanding output and transfer characteristics, including a superb on/off ratio of 10⁹, a steep subthreshold swing of 67 mV dec⁻¹ and neglectable hysteresis of 90 mV, which could attribute to the efficient gate control ability of the oxidized MG and high-quality interface between the oxidized MG and with the Indium Gallium Zinc Oxide (IGZO) channel layer. These distinct electronic properties make our TFT an ideal candidate for cutting-edge devices including 2D transistors and capacitorless DRAMs.

Date of Award	26 Feb 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yong YANG (Supervisor)