Korea Advanced Nano Fab Center

ABSTRACT Monolithic III–V multijunction photovoltaics are promising candidates for both space power systems and solar‐to‐chemical energy conversion, yet their deployment in flexible, chemically aggressive, and radiation‐rich environments requires simultaneous control of epitaxial quality, conformability, and barrier properties. Here, we present a flexible InGaP/GaAs/InGaAs triple‐junction platform that addresses these requirements within a single device architecture. The optimized epitaxial stack and its growth process enable almost fully relaxed subcells with low dislocation density, supporting power conversion efficiencies above 33% under AM1.5G and ∼31% under AM0, even after transferring the epitaxial stack to a thin plastic substrate to improve mechanical compliance. To ensure environmental robustness, the flexible triple‐junction cells are encapsulated with an ultrathin glass sheet that provides strong protection against aqueous and strongly alkaline conditions, as well as against high‐energy proton and electron irradiation. By exploiting the high output voltage and durability of this platform, bias‐free electrocatalysis with formate as the predominant product is demonstrated using a simple electrocatalyst configuration. These results establish a unified III–V multijunction photovoltaic architecture that couples space‐relevant efficiency, flexibility, and radiation hardness with chemical durability suitable for unassisted solar‐to‐chemical conversion, offering a versatile route toward next‐generation terrestrial and space energy systems.

High-pressure annealing (HPA) has emerged as a promising post-fabrication technique for enhancing carrier transport and reliability in semiconductor devices, yet its application to heterojunction-based III–V transistors remains limited. In this study, we explore the implementation of HPA in InGaAs/InAlAs metamorphic high-electron-mobility transistors to understand ambient-dependent effects in heterostructure interfaces. Representative annealing ambients, such as high-pressure nitrogen (HPNA) and deuterium (HPDA), were selected to assess the impact of ambient species on electrical performance. Through comprehensive characterization, including I–V and pulsed measurements and transmission-line method analysis, we systematically examined how HPA conditions influence trap behavior, channel resistance (Rch), and frequency response. HPNA can lead to reproducible improvements in DC performance, with a 6.20% increase in on-state current and a 5.76% enhancement in transconductance, alongside a 17.8% reduction in channel resistance, while maintaining a stable subthreshold swing. In contrast, HPDA-treated devices exhibited increased Rch and enhanced trap-related transient responses, offering insights into trap generation mechanisms in complex III–V interfaces. These results provide a valuable reference for the design of ambient-controlled annealing strategies and highlight the importance of tailoring HPA processes for the reliable integration of heterojunction-based transistors.

Background/Objectives: This study compared fat reduction efficacy of cholic acid-based formulation (MT921) and deoxycholic acid (DCA), as well as skin adverse reactions (ADR), in mini pigs, mice, and rats. DCA is the active pharmaceutical ingredient found in several fat-dissolving injectables, such as Kybella®, V-OLET®, and Bellacholine®. Methods: In one study, single subcutaneous (s.c.) injections of 1.5% MT921 and 1% DCA were administered to the back of a mini pig at different sites and time points to ascertain histopathological events. In another study, three mini pigs received six repeated s.c. injections of 1.5% MT921 and 1% DCA at 4-week intervals, and changes in subcutaneous fat volume were monitored by magnetic resonance imaging (MRI), along with visual examination for ADRs. Additional ADRs were assessed in rodents, such as ulcerative dermatitis (UD) following MT921 and DCA s.c. injections in ICR/CD1 mice, and footpad edema after intraplantar injections in SD rats. Results: In mini pigs, 1.5% MT921 and 1% DCA induced comparable localized fat necrosis, accompanied by inflammatory cell influx and fibrosis. Also, repeated injections of 1.5% MT921 and 1% DCA induced comparable fat volume reduction in outer subcutaneous layer, though changes in middle subcutaneous layer was unaffected. Notably, MT921 evoked milder ADR based on lower incidence of hematoma and absence of nodules in mini pigs, less severe UD in mice, and reduced edema in rats. Conclusions: Local injections of 1.5% MT921 demonstrated fat-reduction efficacy comparable to 1% DCA while eliciting fewer and milder ADRs, supporting MT921 as a promising alternative lipolytic agent.

Beam shaping in nanophotonic systems remains a challenge due to the reliance on complex heuristic optimization procedures. In this work, we experimentally demonstrate a novel approach to topological beam shaping using Jackiw-Rebbi states in metasurfaces. By fabricating thin-film dielectric structures with engineered Dirac-mass distributions, we create domain walls that allow precise control over beam profiles. We observe the emergence of Jackiw-Rebbi states and confirm their localized characteristics. Notably, we achieve a flat-top beam profile by carefully tailoring the Dirac-mass distribution, highlighting the potential of this method for customized beam shaping. This experimental realization establishes our approach as a new mechanism for beam control, rooted in topological physics, and offers an efficient strategy for nanophotonic design.

GaN-based devices for RF and high-power applications use multi-finger configuration to achieve desired high output power. The self-heating effects and reliability of single-finger GaN transistors have been reported, however, a holistic understanding of thermal effects within commercially viable multi-finger configurations, as well as the ramifications of these dynamics on device reliability, is yet to be fully realized. In this work, we (i) examine the difference in the thermal mechanism between single-finger and multi-finger GaN-on-Si HEMTs by high-resolution thermoreflectance microscopy, (ii) find that the reliability degradation of multi-finger GaN-on-Si HEMTs diverges from their single-finger counterparts, (iii) uncover the reliability degradation mechanism specific to multi-finger GaN-on-Si HEMTs, (iv) propose and fabricate a novel non-uniform multi-finger configuration to solve the thermal crosstalk issue, (v) and prove the improved device reliability potential of the novel multi-finger configuration. The extensive insights into multi-finger devices, including thermal effects and novel reliability degradation mechanisms would help eradicate performance and reliability barriers caused by thermal effects in various power devices as well as GaN devices.