To study the effect of stress concentration at the nanoscale, we performed fracture experiments on single edge notched thin film specimens inside the transmission electron microscope. Even at about 4 GPa stress at the notch tip, the specimens failed far away from the notch at places with no apparent stress concentration. The in situ electron microscopy showed evidence of little or no plastic deformation at the notch tip. We propose that the apparent notch insensitivity arises from the breakdown of the classical fracture mechanics at the nanoscale, where materials fail by reaching a uniform rupture stress and not due to stress concentration.

Ultra-wide bandgap β-gallium oxide (Ga₂O₃) devices are of considerable interest with potential applications in both power electronics and radio frequency devices. However, current Ga₂O₃ device technologies are limited by the material's low intrinsic electron mobility and thermal conductivity. The former problem can be addressed by employing modulation-doped β-(Al_xGa_1−x)₂O₃/Ga₂O₃ heterostructures in the device architecture. In this work, (Al_xGa_1−x)₂O₃/Ga₂O₃ modulation-doped field effect transistors (MODFETs) have been investigated from a thermal perspective. Thermoreflectance thermal imaging was used to characterize the self-heating of the MODFETs. The (Al_0.18Ga_0.82)₂O₃ thermal conductivity (3.1–3.6 W/mK) was determined using a frequency domain thermoreflectance technique. Electro-thermal modeling was used to discern the effect of design parameters such as substrate orientation and channel length on the device self-heating behavior. Various thermal management schemes were evaluated using the electro-thermal device model. From an electro-thermal co-design perspective, the improvement in electrical performance followed by the mitigation of self-heating was also studied. For example, by employing a Ga₂O₃-on-SiC composite wafer, which was fabricated in this work, a 50% increase in power handling capability can be achieved as compared to a homoepitaxial device. Furthermore, flip-chip heterointegration and double-sided cooling approaches can lead to more than 2× improvement in the power handling capability. Using an augmented double-sided cooling design that includes nanocrystalline diamond passivation, a 5× improvement in the power handling capability can be accomplished, indicating the potential of the technology upon implementation of a suitable thermal management scheme.

Gallium oxide (Ga2O3) has been the subject of extensive research over the past decade due to its potential in next-generation power electronics and solar-blind ultraviolet (UV) photodetectors. While Ga2O3 exhibits promising material characteristics for applications in harsh environments, its commercial viability remains under debate, particularly when compared to materials such as aluminum nitride (AlN) and diamond, which possess superior intrinsic properties. This perspective addresses the critical challenges that currently impede the widespread commercialization of Ga2O3-based devices. These challenges include a relatively immature technology base, the difficulty in achieving stable p-type conductivity, inherently low thermal conductivity, the presence of crystallographic defects (nano- and micro-voids), and elevated fabrication costs, all of which negatively impact device reliability and scalability. Mitigation strategies, such as heterojunction implementation, the development of thermal management solutions such as wafer bonding, and defect passivation approaches, are also under investigation. The near-term feasibility of commercially viable Ga2O3-based power electronic devices is a central focus of this discussion. The current status is that Ga2O3 development is far advanced relative to either diamond or especially AlN power electronics but is hampered by lack of a broad base of substrate vendors and a compelling vision for device implementations that provide sufficient improvement over SiC power devices. There are strong geographic differences in device focus, with China prioritizing implementation in grid applications while the United States/Europe appear to consider Ga2O3 devices more for defense and aerospace applications.