Authors: M. Albrecht | Oliver Bierwagen | Claudia Draxl | Kornelius Tetzner | Holger von Wenckstern
Source: physica status solidi (b) (article, 2025-07-29)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Advances in Gallium Oxide—from Fundamental Materials Science to Device Implementation Ultra-wide bandgap (UWBG) semiconductors have attracted considerable interest for high-voltage, high-frequency, and radiation-tolerant applications. Among them, monoclinic β-Ga2O3 stands out through its 4.8 eV bandgap, record Baliga figure of merit, and the availability of melt-grown native substrates—advantages that place it alongside, and in some metrics ahead of, GaN and SiC. However, practical application still depends on solving challenges in bulk and epitaxial growth, defect control, dielectric reliability, and thermal management. The contributions assembled in this special issue address these challenges from complementary experimental and theoretical perspectives. They were selected from the Fifth International Workshop on Gallium Oxide and Related Materials (IWGO 2024, Berlin, 26–31 May 2024), the latest in a biennial series launched in Kyoto (2015) and subsequently held in Parma (2017), Columbus (2019), and Nagano (2022). IWGO 2024 drew 219 participants from 24 countries and received 241 abstracts—more than double the number at the inaugural meeting, underscoring the field's rapid expansion. Three keynote addresses, 19 invited lectures, 65 contributed talks, and over 150 posters covered a wide range of subjects, including bulk and epitaxial crystal growth, point-defect spectroscopy, charge-transport physics, device fabrication and processing, and multiscale computational modeling. This Special Issue comprises one topical review and 18 peer-reviewed articles grouped into five thematic areas: Papers on bulk crystal growth cover melt-based techniques like Czochralski (CZ) and vertical gradient freeze (VGF) for β-(InxGa1-x)2O3,[1] scalable vertical Bridgman growth of 6-inch β-Ga2O3 crystals.[2] Alloying and phase engineering highlights bandgap tuning in α-(Ga, Fe)2O3 alloys grown by mist CVD,[3] challenges in β-(InxGa1-x)2O3 growth,[1] such as indium segregation and metastable phase control, and a solution-based approach to the deposition of Al-doped (β-Ga2O3:Al) alongside its physical properties and Schottky contact properties.[4] These advances expand material capabilities for power electronics and UV optoelectronics while addressing synthesis hurdles for tailored oxide semiconductors. Papers in this area address advanced methods to assess structural, electrical, and optical properties of Ga2O3. Structural analysis addresses “YOLO”-based machine learning for rapid dislocation density estimation in α-Ga2O3 via weak-beam transmission electron microscopy (TEM),[5] optical characterization (bandgap, photoluminescence) of α-, β-, and κ-Ga2O3 polymorphs for optoelectronic benchmarking,[6] and thermoelectric evaluation through Seebeck coefficient-carrier density correlations in β-Ga2O3.[7] These techniques enable precise material quality assessment, polymorph-specific property mapping, and performance optimization for power and energy-harvesting devices. Dopant behavior studies include Ge diffusion governed by gallium vacancies (VGa),[8] metastable dark conductivity in carbon-doped samples,[9] and computational insights into nitrogen's limited p-type potential as substitutional/interstitial defects.[10] Defect characterization employs high-frequency EPR/ENDOR spectroscopy to probe VGa2− atomic structures[11] and positron annihilation spectroscopy to map irradiation-induced vacancy evolution.[12] The papers related to device fabrication and application focus on the advances in β-Ga2O3 device engineering. Process development includes high-aspect-ratio nano-fin fabrication via Inductively Coupled Plasma–Reactive Ion Etching (ICP-RIE) etching,[13] the use of sacrificial Ni layer to facilitate exfoliation of thick Ga2O3 layers on top,[14] and mist CVD-grown NiO/β-Ga2O3 heterostructures for p–n junctions.[15] Device performance covers vertical p-NiO/β-Ga2O3 diodes for sensitive X-ray detection,[16] reliability testing of Schottky/heterojunction diodes in DC–DC converters,[17] and a review of vertical Ga2O3 diodes addressing fabrication, optimization, and thermal management.[18] Together, these efforts advance β-Ga2O3 integration into high-power and radiation-sensing technologies. The workshop also addressed novel material platforms beyond mere β-Ga2O3. GeO2 research focuses on rutile-phase (r-GeO2) growth and doping strategies to harness its ultra-wide bandgap (UWBG) potential for high-voltage devices.[19] Heterostructures investigate NiO/β-Ga2O3 interfaces, optimizing interfacial properties for enhanced p–n junctions in power electronics and deep-UV optoelectronics. These systems extend the frontiers of oxide semiconductor technology, targeting next-generation energy-efficient and high-frequency applications. Despite marked progress, several critical questions require further work. Taken together, these papers represent significant advancements in the fields of substrate technology, defect spectroscopy, modeling accuracy, and prototype device performance, while delineating the remaining scientific and engineering barriers to commercialization. We thank all authors, reviewers, and IWGO 2024 participants for their contributions and trust that the results presented here will accelerate the development of reliable, high-performance Ga2O3-based electronics. Gallium oxide ; Gallium ; Materials science ; Nanotechnology ; Oxide ; Engineering physics ; Engineering ; Metallurgy Ga2O3 and related materials