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β-Ga2O3 is a wide bandgap semiconductor material that is promising for many fields such as gas sensors, UV detectors, and high-power electronics. Until now, most epitaxial β-Ga2O3 thin films could only be realized on six-fold symmetric single crystal substrates including sapphire (0001), 3C-SiC (001), and native β-Ga2O3. In this report, we demonstrate the epitaxial growth of β-Ga2O3 (−201) thin films on non-six-fold symmetric substrates, i.e., the CeO2 (001) substrate. Different from the conventional six-fold symmetric sapphire substrates, the four-fold symmetric cubic phase CeO2 (001) induces the formation of two sets of hexagonal-like atom frameworks with a mutual rotation angle of 90° in the β-Ga2O3 (−201) plane. This is due to the small lattice mismatch between the β-Ga2O3 (−201) plane and the CeO2 (001) plane in two directions: CeO2 [100]//β-Ga2O3 [010] and CeO2 [010]//β-Ga2O3 [010]. Besides, the valence band offset (VBO) and the conduction band offset (CBO) at the β-Ga2O3/CeO2 heterojunction are examined using high-resolution X-ray photoelectron spectroscopy (HR-XPS) and are estimated to be 1.63 eV and 0.18 eV, respectively, suggesting a type-II heterostructure. The obtained epitaxial β-Ga2O3 thin films are fabricated into photodetectors (PDs), which show key photoelectrical characteristics that are similar to those of PDs using the conventional sapphire substrate. The results indicate the epitaxial β-Ga2O3 thin films on CeO2 have a high crystallization quality, and thus are capable of producing various essential devices. Moreover, the epitaxy between β-Ga2O3 (−201) and CeO2 (001) demonstrated in this work can pave the way for constructing heterostructures between β-Ga2O3 and other cubic-phase functional materials, such as p-type semiconductors, piezoelectric semiconductors, and superconductors. [ABSTRACT FROM AUTHOR]

Ultrawide-bandgap gallium oxide (Ga 2 O 3 ) holds immense potential for crucial applications such as solar-blind photonics and high-power electronics. Although several Ga 2 O 3 polymorphs, i.e., α, β, γ, δ, ε, and κ phases, have been identified, the band alignments between these phases have been largely overlooked due to epitaxy challenges and inadvertent neglect. Despite having similar stoichiometry, heterojunctions involving different phases may exhibit band offsets. Here, β-Ga 2 O 3 /κ-Ga 2 O 3 -stacked "phase heterojunction" is demonstrated experimentally. This phase heterojunction has a sharp and well-defined interface, and subsequent measurements reveal an unbeknown type-II band alignment with significant valence/conduction band offsets of ≈0.65 eV/0.71 eV. This alignment is promising for self-powered deep ultraviolet (DUV) signal detection, necessitating an internal electric field near the junction and matching the absorption properties for effective electron-hole separation. The fabricated phase heterojunction photodetector displays a responsivity of three orders of magnitude higher at 17.8 mA W -1 , with improved response times (rise time ≈0.21 s, decay time ≈0.53 s) under DUV illumination and without external bias in comparison to the bare β-Ga 2 O 3 and κ-Ga 2 O 3 photodetectors, confirming the strong interfacial electrical field. This study provides profound insight into Ga 2 O 3 /Ga 2 O 3 heterojunction interfaces with different polymorphs, allowing the use of phase heterojunctions to advance electronic device applications., (© 2025 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

Recent advancements in power electronics have been driven by Ga 2 O 3 -based ultrawide bandgap (UWBG) semiconductor devices, enabling efficient high-current switching. However, integrating Ga 2 O 3 power devices with essential silicon CMOS logic circuits for advanced control poses fabrication challenges. Researchers have introduced Ga 2 O 3 -based NMOS and pseudo-CMOS circuits for integration, but these circuits may either consume more power or increase the design complexity. Hence, this article proposes Ga 2 O 3 -based CMOS realized using heterogeneous 3D-stacked bilayer ambipolar transistors. These ambipolar transistors consist of HfO 2 /NiO/Ga 2 O 3 /NiO/HfO 2 heterostructures that are wrapped around by the Ti/Au gate electrode, resulting in record high electron and hole current on/off ratios of 10 9 and 10 7 . The threshold voltage, subthreshold swing, and current density measured from 100 ambipolar devices (across 5 batches) are around -7.99 ± 0.92 V (p-channel) and 7.81 ± 0.81 V (n-channel), 0.59 ± 0.07 V/dec (p-channel) and 0.61 ± 0.06 V/dec (n-channel), and 0.99 ± 0.26 mA/mm (p-channel) and 58.23 ± 12.99 mA/mm (n-channel), respectively. All the 100 ambipolar devices showed decent long-term stability over a period of 200 days, exhibiting reliable electrical performance. The threshold voltage shift (Δ V TH ) after negative bias stressing for a period of 3500 s is around 11.52 V (p-channel) and 10.21 V (n-channel), respectively. Notably, the n-channels exhibit ∼2 orders higher on/off ratio than the best Ga 2 O 3 unipolar transistors at 300 °C. Moreover, the polarities of ambipolar transistors are reconfigurable into p- or n-MOS, which are integrated to demonstrate CMOS inverter, NOR, and NAND logic gates. The switching periods from "0" to "1" and from "1" to "0" of NOR are 0.12 and 0.17 μs, and those of NAND are 0.16 and 0.13 μs. This work lays the foundation of oxide-semiconductor-based CMOS for future integrated electronics.

Transferable Ga 2 O 3 thin film membrane is desirable for vertical and flexible solar-blind photonics and high-power electronics applications. However, Ga 2 O 3 epitaxially grown on rigid substrates such as sapphire, Si, and SiC hinders its exfoliation due to the strong covalent bond between Ga 2 O 3 and substrates, determining its lateral device configuration and also hardly reaching the ever-increasing demand for wearable and foldable applications. Mica substrate, which has an atomic-level flat surface and high-temperature tolerance, could be a good candidate for the van der Waals (vdW) epitaxy of crystalline Ga 2 O 3 membrane. Beyond that, benefiting from the weak vdW bond between Ga 2 O 3 and mica substrate, in this work, the Ga 2 O 3 membrane is exfoliated and transferred to arbitrary flexible and adhesive tape, allowing for the vertical and flexible electronic configuration. This straightforward exfoliation method is verified to be consistent and reproducible by the transfer and characterization of thick (∼380 nm)/thin (∼95 nm) κ-phase Ga 2 O 3 and conductive n-type β-Ga 2 O 3 . Vertical photodetectors are fabricated based on the exfoliated Ga 2 O 3 membrane, denoting the peak response at ∼250 nm. Through the integration of Ti/Au Ohmic contact and Ni/Ag Schottky contact electrode, the vertical photodetector exhibits self-powered photodetection behavior with a responsivity of 17 mA/W under zero bias. The vdW-bond-assisted exfoliation of the Ga 2 O 3 membrane demonstrated here could provide enormous opportunities in the pursuit of vertical and flexible Ga 2 O 3 electronics.

Flexible Ga 2 O 3 photodetectors have attracted considerable interest owing to their potential use in the development of implantable, foldable, and wearable optoelectronics. In particular, β-phase Ga 2 O 3 has been most widely investigated due to the highest thermodynamic stability. However, high-quality β-phase Ga 2 O 3 relies on the ultrahigh crystallization temperature (usually ≥750 °C), beyond the thermal tolerance of most flexible substrates. In this work, we epitaxially grow a high-quality metastable κ-phase Ga 2 O 3 (002) thin film on a flexible mica (001) substrate under 680 °C and develop a flexible κ-Ga 2 O 3 thin film photodetector with ultrahigh performance. Epitaxial κ-Ga 2 O 3 and the mica substrate are maintained to be thermally stable up to 750 °C, suggesting their potential for harsh environment applications. The responsivity, on/off ratio, detectivity, and external quantum efficiency of the fabricated photodetector are 703 A/W, 1.66 × 10 7 , 4.08 × 10 14 Jones, and 3.49 × 10 5 %, respectively, for 250 nm incident light and a 20 V bias voltage. These values are record-high values reported to date for flexible Ga 2 O 3 photodetectors. Furthermore, the flexible photodetector shows robust flexibility for bending radii of 1, 2, and 3 cm. More importantly, it shows strong mechanical stability against 10,000 bending test cycles. These results reveal the significance of high-quality κ-phase Ga 2 O 3 grown heteroepitaxially on a flexible mica substrate, especially its potential for use in future flexible solar-blind detection systems.