Self-Powered High-Performance WS₂ Visible-Light Photodetector Based on a Seamless Lateral p-i-n Homojunction

Transition metal dichalcogenides (TMDCs) have emerged as promising materials for nanometer-scale devices, exhibiting attractive properties such as high carrier mobility, low surface trap density, and strong light-matter interaction. Their potential applications in advanced field-effect transistors (FETs) and optoelectronic devices have sparked significant interest in semiconductor research. Manipulating the doping characteristics of TMDCs is crucial for controlling device properties and constructing complex structures. However, conventional doping techniques, including diffusion or ion implantation, have limited applicability to these materials. Consequently, p-doping via charge transfer, involving the interaction between a semiconductor and an electron-withdrawing agent, has gained widespread utilization. For tungsten-based TMDC materials such as WS₂, exposure to ultraviolet-ozone (UV-O₃) can induce the formation of sub-stoichiometric oxide WO_X (X &lt; 3) with high electron affinity, resulting in effective p-doping. While existing research on p-n junction diodes has primarily focused on type-II-based heterojunction diodes, the performance of these devices is often limited by defects and nonuniformity at junction interfaces arising during the fabrication process. To overcome these limitations, high-performance two-dimensional p-n devices with homojunction have been developed as a promising alternative to eliminate defects by achieving atomically clean interfaces. Among these, p-type/intrinsic/n-type (p-i-n) junctions have been extensively utilized to attain high-speed response by effectively reducing junction capacitance by incorporating a wider depletion region.<br/>This work first investigates the manipulation of p-doping in multilayer WS₂ (ML-WS₂) through the formation of WO_X using incremental UV-O₃ treatment times. Mechanically exfoliated ML-WS₂ flakes were dry-transferred onto a Ni plate and oxidized via UV-O₃ treatment with a flow rate of 8 L/min at 100 °C in 30 min intervals. The extent of p-doping by WO_X formation was examined at each step by measuring work function changes using Kelvin probe force microscopy until saturation was observed to optimize the p-doping of ML-WS₂ flakes. A lateral homojunction p-i-n photodetector was fabricated based on the optimized UV-O₃ oxidation process. A ML-WS₂ flake was dry-transferred onto a Ni (20 nm) electrode pre-deposited on a p⁺⁺-Si/SiO₂ substrate for the p-type region contact and a Ti/Au (20/80 nm) electrode for the n-type region contact was formed after the transfer, where both electrodes were defined by electron-beam lithography and deposited using an electron-beam evaporator. The intrinsic region width was set to 3 μm. Thermal annealing was performed in high vacuum (~10^-6 Torr) at 200 °C for 2 h to enhance the metal-WS₂ contacts. The p-type region of the ML-WS₂, located directly above the Ni electrode, was defined by electron-beam lithography and underwent UV-O₃ treatment for 3 h. After rinsing off the resist, the exposed ML-WS₂ comprising both p-type and intrinsic regions was subjected to an additional 1 h UV-O₃ treatment.<br/>The fabricated p-i-n photodetector exhibited impressive performance with self-powered operation. It demonstrated a high responsivity of 496.30 mA/W under 530 nm light (power density of 12.33 mW/cm²), with fast rise and decay times of 4.5 and 4.4 ms, respectively. Notably, the device showed an excellent rejection ratio of 221.56 under 530 nm and 780 nm light with a responsivity of 2.24 mA/W at 12.4 mW/cm², indicating the absence of defect levels responding to longer wavelengths. This work demonstrates that controllable p-doping of ML-WS₂ can be realized by manipulating the UV-O₃ treatment time without introducing severe defects and it is a promising approach for creating seamless lateral homojunction p-i-n diodes for optoelectronic applications.