| Summary: | In this work, Ga<sub>2</sub>O<sub>3</sub> films were deposited on sapphire substrates using a plasma-enhanced atomic layer deposition system with trimethylgallium precursor and oxygen (O<sub>2</sub>) plasma. To improve the quality of Ga<sub>2</sub>O<sub>3</sub> films, they were annealed in an O<sub>2</sub> ambient furnace system for 15 min at 700, 800, and 900 °C, respectively. The performance improvement was verified from the measurement results of X-ray diffraction, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. The optical bandgap energy of the Ga<sub>2</sub>O<sub>3</sub> films decreased with an increase of annealing temperatures. Metal-semiconductor-metal ultraviolet C photodetectors (MSM UVC-PDs) with various Ga<sub>2</sub>O<sub>3</sub> active layers were fabricated and studied in this work. The cut-off wavelength of the MSM UVC-PDs with the Ga<sub>2</sub>O<sub>3</sub> active layers annealed at 800 °C was 250 nm. Compared with the performance of the MSM UVC-PDs with the as-grown Ga<sub>2</sub>O<sub>3</sub> active layers, the MSM UVC-PDs with the 800 °C-annealed Ga<sub>2</sub>O<sub>3</sub> active layers under a bias voltage of 5 V exhibited better performances including photoresponsivity of 22.19 A/W, UV/visible rejection ratio of 5.98 × 10<sup>4</sup>, and detectivity of 8.74 × 10<sup>12</sup> cmHz<sup>1/2</sup>W<sup>−1</sup>.
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