Broadband WS₂ Avalanche Photodetector via a Monolithic p-i-n Lateral Homojunction

Two-dimensional transition-metal dichalcogenides (TMDs) enable low-power optoelectronic properties via strong light–matter interactions and high carrier mobility. Since conventional doping techniques are inappropriate for TMDs, p-doping is commonly achieved by charge transfer doping, such as forming tungsten oxide (WO_X) onto WS₂ by UV-ozone oxidation. Here, we report a broadband multilayer WS₂ avalanche photodetector via a monolithic p-i-n lateral homojunction, fabricated by area-specific p-doping through simple control of the UV-ozone oxidation time. We defined clear p-i-n regions on a single WS₂ flake with clean contacts and stable junction characteristics, shown by Kelvin probe force microscopy and photocurrent mapping. Across visible-to-near-IR illumination (530–940 nm), the device delivers high internal multiplication and high responsivity at sub-breakdown fields (<0.1 MV/cm) with low dark current and excellent reproducibility, enabling a wide dynamic range without avalanche-induced excess noise. Field-normalized analysis shows unity-gain enhancement by multiple orders with near-linear response. The synergy of this simple charge-transfer doping and a robust WS₂ homojunction yields a field-tunable, low-noise photodetector architecture, offering both a practical blueprint for scalable two-dimensional optoelectronics.