Flexible Ga₂O₃ photodetectors have attracted considerable interest owing to their potential use in the development of implantable, foldable, and wearable optoelectronics. In particular, β-phase Ga₂O₃ has been most widely investigated due to the highest thermodynamic stability. However, high-quality β-phase Ga₂O₃ 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 Ga₂O₃ (002) thin film on a flexible mica (001) substrate under 680 °C and develop a flexible κ-Ga₂O₃ thin film photodetector with ultrahigh performance. Epitaxial κ-Ga₂O₃ 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 × 10⁷, 4.08 × 10¹⁴ Jones, and 3.49 × 10⁵ %, respectively, for 250 nm incident light and a 20 V bias voltage. These values are record-high values reported to date for flexible Ga₂O₃ 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 Ga₂O₃ grown heteroepitaxially on a flexible mica substrate, especially its potential for use in future flexible solar-blind detection systems.

Schottky and Ohmic contacts are key matters affecting carrier transport in oxide semiconductor-based electrical and optical devices. For Ga ₂ O ₃ , the comparison of optoelectrical behaviors and the fundamental physical mechanism between these two contacts are not well known yet. In this work, β -Ga ₂ O ₃ thin films were grown via metal–organic chemical vapor deposition then deposited with symmetrical Ni/Au (Schottky) or Ti/Au (Ohmic) contacts. Optoelectrical measurements show that the Ohmic contacted device exhibits superior responsivities thanks to its higher photocurrents. Meanwhile, for the Schottky contacted device, firstly, it has a faster response speed, and secondly it exhibits larger photo-to-dark current ratios owing to their low dark current. Specifically, the voltage- and light intensity-dependent responsivity and detectivities of the Schottky and Ohmic contacted devices were measured and discussed under the consideration of different voltages and UV light intensities.

In this research, β-Ga₂O₃/NiO heterostructures were grown directly on CeO₂ buffered Hastelloy flexible substrates. With pulsed laser deposition under high temperatures, as-grown β-Ga₂O₃ and NiO thin films have a preferred out-of-plane orientation along the ⟨−201⟩ and ➎111➉ directions. This is due to the ideal epitaxial ability of the CeO₂ buffer layer, which serves as a perfect template for the epitaxial growth of single-oriented NiO and β-Ga₂O₃ by creating a constant gradient from CeO₂ (2.7 Å along ➎001➉) to NiO (2.9 Å along ➎110➉), and eventually to β-Ga₂O₃ (3.04 Å along ➎010➉). The Hastelloy substrates endow photodetectors with good deformability and mechanical robustness. Moreover, owing to the type-II band alignment of β-Ga₂O₃/NiO heterostructures, the photodetectors have a good photocurrent at zero bias under 284 nm of light illumination. In addition, the photocurrent is significantly higher than when using an analogous heterostructure (as described in some previous reports), because the β-Ga₂O₃ and NiO thin films are crystalized along a single orientation with fewer defects.