On-chip photonic integrated circuits (PICs) are crucial for advancements in optical computing, communications, and sensing. While materials like silicon, silicon nitride, and lithium niobate offer advantages, monolithic integration of waveguides and photodetectors within a single material is challenging. Hetero-integration of absorptive materials presents difficulties, particularly the high cost and complex processing of germanium or III-V semiconductors, and challenges in wafer bonding. Two-dimensional (2D) materials offer an attractive alternative due to their unique electronic and optical properties and lack of dangling bonds, eliminating lattice mismatch issues. Previous chip-integrated 2D material photodetectors, often using metal-semiconductor-metal (M-S-M) configurations, suffered from high dark currents and low responsivity at high speeds. This research focuses on overcoming these limitations by integrating van der Waals PN heterojunctions of 2D materials onto optical waveguides to achieve chip-integrated photodetectors with low dark current, high responsivity, and fast speed. The use of a PN heterojunction, formed by stacking p-type black phosphorus (BP) and n-type molybdenum ditelluride (MoTe2), offers a simple fabrication method compared to traditional PN junction techniques, enabling electrostatic doping to further enhance performance. The silicon nitride waveguide provides efficient light coupling, and the vertical built-in electric field within the heterojunction facilitates efficient charge carrier separation and collection, leading to improved photodetection performance.

The literature review highlights the use of various materials for PICs, including silicon, silicon nitride, and lithium niobate, along with the challenges associated with integrating waveguides and photodetectors using a single material. The review discusses hetero-integration approaches using bulk materials like germanium and III-V semiconductors, acknowledging the associated high costs and processing complexity. It also mentions wafer bonding techniques and their limitations. The emergence of 2D materials as a promising alternative for photodetector integration is discussed, citing examples of graphene and molybdenum disulfide integrated onto silicon nitride waveguides. However, the limitations of these previous approaches, particularly the high dark current and low bandwidth of graphene and M-S-M structures in other 2D materials, are noted. The review emphasizes the need for a photodetector with a combination of low dark current, high responsivity, and fast response speed, setting the stage for the proposed van der Waals PN heterojunction approach.

The study employed a van der Waals PN heterojunction photodetector fabricated by mechanically stacking p-doped black phosphorus (BP) and n-doped molybdenum ditelluride (MoTe2) on a silicon nitride waveguide. Hexagonal boron nitride (h-BN) layers were used for surface passivation and encapsulation of the BP/MoTe2 heterostructure. The drain and source electrodes were transferred directly onto the BP and MoTe2 flakes, respectively. A Mach-Zehnder interferometer (MZI) based on the silicon nitride waveguide was used to quantify the light absorption by the BP/MoTe2 heterostructure. The optical transmission spectra were measured by coupling a wavelength-tunable laser into the MZI. The absorption coefficient of the BP layer was determined from the changes in the transmission spectra before and after the integration of BP. Electrical characterizations were performed by applying a drain-source bias voltage and measuring the resulting current. Photoresponse measurements were conducted by illuminating the device with light at different wavelengths and measuring the generated photocurrent. Electrostatic doping was achieved by applying a gate voltage between the drain electrode and the doped silicon substrate. Raman spectroscopy and atomic force microscopy were used to characterize the heterostructure and the thickness of the 2D materials. The device fabrication involved electron beam lithography, plasma dry etching, and transfer printing techniques. Annealing in a forming gas environment ensured good contact between the 2D materials and metal electrodes.

The fabricated van der Waals PN heterojunction photodetector exhibited a remarkably low dark current of less than 7 nA under a 1 V bias, significantly lower than other waveguide-integrated BP photodetectors. The device demonstrated a high intrinsic responsivity of up to 577 mA/W, which increased to 709 mA/W with electrostatic doping. The response bandwidth was measured to be approximately 1.0 GHz. Uniform photodetection was observed over a wide spectral range (1500-1630 nm). The absorption coefficient of the BP layer was experimentally determined to be ~0.09815 dB/µm, consistent with theoretical predictions. The high light absorption efficiency of the BP layer was attributed to its narrow direct bandgap and the extended interaction length with the waveguide mode. The device also showed strong rectifying characteristics, with an ON/OFF current ratio of about 1.6 × 10³, and an ideality factor of 1.87. At zero bias, a significant photocurrent was generated due to the built-in electric field of the PN heterojunction, demonstrating the photovoltaic effect. An open-circuit voltage of 228 mV was measured at a BP absorption power of 12.79 µW, further showcasing the effectiveness of the heterojunction in separating photogenerated carriers.

The results demonstrate the success of integrating van der Waals PN heterojunctions of 2D materials onto optical waveguides to create high-performance chip-integrated photodetectors. The combination of low dark current, high responsivity, and fast response speed surpasses the performance of previously reported 2D material photodetectors. The use of the PN heterojunction significantly improves the performance compared to M-S-M structures by enabling efficient separation of photogenerated carriers. The ability to tune the heterojunction's properties through electrostatic doping provides a pathway for further performance optimization. The high internal quantum efficiency suggests the potential for widespread applications in integrated photonic circuits. This work offers a promising route for developing next-generation on-chip photodetectors for various applications in silicon, lithium niobate, and polymer-based photonic integrated circuits.

This study successfully demonstrated a high-performance chip-integrated photodetector based on a van der Waals PN heterojunction of BP and MoTe2. The device exhibited superior performance in terms of low dark current, high responsivity, and fast response speed compared to existing 2D material-based photodetectors. The tunability of the heterojunction through electrostatic doping enhances its potential for further performance improvements. Future research could explore the integration of other 2D materials to expand the operational wavelength range and explore different waveguide platforms.

The study primarily focused on the performance of the photodetector within a specific wavelength range (1500-1630 nm). Future work could investigate the performance at other wavelengths. The long-term stability of the BP/MoTe2 heterostructure in ambient conditions needs further investigation, despite the use of h-BN encapsulation. The scalability of the fabrication process for large-scale production also warrants further exploration. While the device exhibited excellent performance, the exact influence of each material layer on the overall device characteristics (e.g., the contribution of MoTe2 to photocurrent generation) could be further elucidated through detailed modeling and simulation.