IWGO 2026 Session IWGO-MoM1: Plenary Session I

Ultra-wide bandgap (UWBG) oxide materials attract a considerable scientific and technological attention in the research community to boost the development of next generation power devices capable of switching high voltages. The development chain includes single crystal growth, substrates preparation, homo- and heteroepitaxial film growth, device fabrication in different architectures, study of physical properties of obtained materials, and characterization of the devices. This requires a multidisciplinary approach interconnecting science and technology.

During this talk, I will overview melt growth methods applied for β-Ga₂O₃ and β-(Al_xGa_1-x)₂O₃ single crystals [1-3], their structural quality, and wafers. I will also highlight epitaxial film growth of β-Ga₂O₃ and β-(Al_yGa_1-y)₂O₃ on β-(Al_xGa_1-x)₂O₃ substrates by Metal-Organic Vapor Phase Epitaxy (MOVPE) [4, 5]. Ultra-high purity β-Ga₂O₃ single crystals grown by the Czochralski method and their features will be described as well. Additionally, doping of Czochralski-grown β-(Al_xGa_1-x)₂O₃ single crystals with different 4+ elements will reveal their impact on electrical conductivity at high Al content (>20 mol.%).

The discussion will also cover rutile-GeO₂ (r-GeO₂) single crystals grown by a modified Top-Seeded Solution Growth (TSSG) with prepared epi-ready wafers [6, 7].

Finally, a comparison of basic physical properties of the discussed β-Ga₂O₃, β-(Al_xGa_1-x)₂O₃, and r-GeO₂ single crystals and wafers might be useful for further development directions.

Acknowledgements:The work was financially supported by the following grants from the Bundesministerium für Bildung und Forschung (BMBF) nos. 03VP03712 and 16ES1084K, German Research Foundation (DFG) nos. 491040331 and 555506919, Senatsausschuss Wettbewerb (SAW) no. K417/2021, and ONRG no. N629092412105.

[1] Z. Galazka;J. Appl. Phys. 131 (2022) 031103.

[2] Z. Galazka; “Growth of bulk β-Ga₂O₃ single crystals” in “Comprehensive Semiconductor Science and Technology 2nd Ed.”, Ed. R. Fornari, Elsevier (2025).

[3] Z. Galazka; IEEE Trans. Semicond. Manuf. 38 (2025) 796-802.

[4] S. Bin Anooz et al.; J. Phys. D: Appl. Phys. 59 (2026) 015308.

[5] S. Bin Anooz et al.; J. Vac. Sci. Technol. A 44 (2026) 032702.

[6] Z. Galazka et al.; J. Appl. Phys. 133 (2023) 035702.

[7] Z. Galazka et al.; Adv. Mater. Interfaces (2024) 2400122.

The control of charge carrier concentration in (ultra-)wide bandgap oxide semiconductors usually involves the substitutional incorporation of foreign dopant atoms during growth or post growth by ion implantation or dopant in-diffusion. Above-room temperature growth or annealing processes are required for these approaches and often the dopant effect gets partially compensated by the formation of native point defects.

This talk will discuss a completely different approach based on two different room-temperature surface plasma treatments that induce free carrier systems in undoped (ultra-)wide bandgap oxide semiconductors.

In the first part, the creation of near-surface electron systems in n-type oxides, such as In₂O₃, SnO₂, and different polymorphs of Ga₂O₃ are discussed in detail. For example, the resulting electron systems in undoped κ-Ga₂O₃ exhibited an unprecedentedly high electron mobility above 25 cm²/Vs at sheet/volume carrier concentrations around 2x10¹⁴ cm^-2/4x10¹⁹ cm^-3, and a similar carrier system was induced in a semi-insulating Mg-doped β-Ga₂O₃ substrate. The thermal stability of these electron systems is discussed and a native-point-defect based doping mechanism is proposed. That the discussed feature can also be a bug, is further highlighted by the unintentional creation of an electron system during sputter deposition of a dielectric SiO₂ layer on a homoepitaxial β-Ga₂O₃ layer.

The creation and thermal stability of a surface hole system in the p-type oxide NiO is demonstrated in the second part and shown to be related to surface charge transfer doping.

The presented approaches may provide an easy way of scouting the charge-carrier-transport potential of novel oxide semiconductors before elaborating substitutional doping, and will be tested against further topical and novel oxide semiconductors.

The presented body of work has been accumulated over more than ten years and we acknowledge all contributors, including, Marko Perestjuk, Ivana Lapsanska, Carmine Borelli, Andrea Ardenghi, Theresa Berthold, Melanie Budde, Marcel Himmerlich, Christian Golz, Abbes Tahraoui.