A fast-response colorimetric ultraviolet-C (UVC) sensor was demonstrated using a gallium oxide (Ga2O3) photocatalyst with small amounts of triethanolamine (TEOA) in methylene blue (MB) solutions and a conventional RGB photodetector. The color of the MB solution changed upon UVC exposure, which was observed using an in situ RGB photodetector. Thereby, the UVC exposure was numerically quantified as an MB reduction rate with the R value of the photodetector, which was linearly correlated with the measured spectral absorbance using a UV-Vis spectrophotometer. Small amount of TEOA in the MB solution served as a hole scavenger, which resulted in fast MB color changes due to the enhanced charge separation. However, excessive TEOA over 5 wt.% started to block the catalytical active site on the surface of Ga2O3, prohibiting the chemical reaction between the MB molecules and catalytic sites. The proposed colorimetric UVC sensor could monitor the detrimental UVC radiation with high responsivity at a low cost.

This paper describes the heteroepitaxial growth of high-quality alpha-phase gallium oxide (α-Ga2O3) on a sapphire substrate without a buffer layer via the hydride vapor phase epitaxy method. Here, compressive strain arising from the difference in lattice constants between the substrate and α-Ga2O3 is relieved near the interface, resulting in a high crystal quality of 32.72 arcsec (full width at half maximum value) in the high-resolution x-ray diffraction 2θ scan spectrum. Subsequently, the fabricated hetero α-Ga2O3-based photodetector with a metal–semiconductor–metal structure operating under ultraviolet radiation in the C-band (UVC) demonstrates a high UVC responsivity of 5 × 102 A W−1 and a high visible blindness of 8.14 × 104 at 235 nm. The photodetector utilizes photogenerated holes trapped near the interface of the metal electrode, inducing amplified electron current flow. The developed hetero α-Ga2O3-based UVC photodetector can be used to detect early signs of fire, flames, or corona discharge in visible light environments for social and industrial safety applications.

The electrical and optoelectronic performance of semiconductor devices are mainly affected by the presence of defects or crystal imperfections in the semiconductor. Oxygen vacancies are one of the most common defects and are known to serve as electron trap sites whose energy levels are below the conduction band (CB) edge for metal oxide semiconductors, including β-Ga2O3. In this study, the effects of plasma nitridation (PN) on polycrystalline β-Ga2O3 thin films are discussed. In detail, the electrical and optical properties of polycrystalline β-Ga2O3 thin films are compared at different PN treatment times. The results show that PN treatment on polycrystalline β-Ga2O3 thin films effectively diminish the electron trap sites. This PN treatment technology could improve the device performance of both electronics and optoelectronics.

The formation of n-Ga2O3/p-GaN heterojunctions has been studied intensively due to the lack of p-Ga2O3. Metalorganic chemical vapor deposition (MOCVD) is known to provide a high-quality heterojunction of n-Ga2O3/p-GaN compared to thermal oxidation, sputtering, or pulsed-laser deposition (PLD). In this work, single-crystalline β-Ga2O3 of {−201} is grown on a GaN (001)/Al2O3 substrate using MOCVD. An abrupt heterojunction without a noticeable interfacial layer is observed between β-Ga2O3 and GaN. However, due to the lattice mismatch between β-Ga2O3 (−201) and GaN (001), grain boundaries and grain defects originating from the Ga2O3/GaN interface continue in β-Ga2O3 in a diagonal direction. The epitaxial nature of the grown β-Ga2O3 on the GaN (001)/Al2O3 substrate causes the nanorod-shaped morphology in the growth direction of β-Ga2O3. This work marks a step toward the formation of a high-quality heterojunction of Ga2O3/GaN, which would serve as an essential building block for various devices, including optoelectronics and power electronics.