Uni-Traveling-Carrier Photodiode Based on MoS2/GaN van der Waals Heterojunction for High-Speed Visible-Light Detection

Uni-traveling-carrier photodiodes (UTC-PDs), which utilize only electrons as the active carriers, have become indispensable in high-speed optoelectronics due to their unique capabilities, such as high saturation power and broad bandwidth. However, extending the operating wavelengths into the visible region for wider applications is challenging due to the lack of suitable wide-bandgap III-V semiconductor combinations with the necessary band alignment and lattice matching. Here, we show that a UTC-PD based on a van der Waals heterojunction composed of a 2D transition metal dichalcogenide, molybdenum disulfide (MoS2), as a photoabsorption layer and a gallium nitride (GaN) film as a carrier transport layer, offers a solution to this challenge. The fast vertical carrier transport across the heterointerface is enabled by the direct epitaxial growth of a MoS2 layer on a GaN film. Our device demonstrates a frequency response in the several-GHz range with a quantum efficiency on the order of 1% throughout the entire visible spectrum, highlighting the promise for high-speed visible optoelectronics.

💡 Research Summary

This paper presents a novel Uni‑Traveling‑Carrier photodiode (UTC‑PD) that operates across the entire visible spectrum by leveraging a van der Waals heterojunction between two‑dimensional molybdenum disulfide (MoS₂) and wide‑bandgap gallium nitride (GaN). Conventional UTC‑PDs, which employ only electrons as the active carriers, have demonstrated high saturation power and broad bandwidth in the infrared telecom band, but extending this concept to visible wavelengths has been hampered by the lack of suitable III‑V material pairs that provide the required type‑II band alignment and lattice compatibility. The authors address this gap by directly epitaxially growing few‑layer MoS₂ on a GaN film using metal‑organic chemical vapor deposition (MOCVD), achieving a lattice mismatch of only ~0.8 % and eliminating interfacial oxides that typically plague transferred 2D/III‑V stacks.

Structural verification through Raman spectroscopy (Δω = 22 cm⁻¹ indicating bilayer/trilayer MoS₂), high‑resolution HAADF‑STEM imaging, and Kelvin‑Probe Force Microscopy (KPFM) confirms a clean, epitaxial interface and a conduction‑band offset of ~0.26 eV, establishing a Type‑II alignment. In this configuration, photons absorbed in the MoS₂ layer generate electron‑hole pairs; holes are rapidly collected at the anode while electrons are injected across the band offset into the intrinsic GaN transport layer. The electrons then traverse the i‑GaN region under reverse bias, exploiting GaN’s high electron saturation velocity (~3 × 10⁷ cm s⁻¹) and low effective mass, which are the hallmarks of UTC operation.

The fabricated device comprises a 60 µm‑diameter mesa with an n‑GaN contact layer (3.6 µm, 4.2 × 10¹⁹ cm⁻³), an i‑GaN transport layer (0.3 µm, 10¹⁵‑10¹⁶ cm⁻³), and a 2‑4‑layer MoS₂ absorption layer (sheet carrier density ~8.5 × 10¹³ cm⁻²). Ohmic contact to MoS₂ is confirmed by transmission‑line‑model (TLM) measurements, and the rectifying I–V behavior originates from the MoS₂/GaN heterojunction rather than Schottky barriers. Hall measurements indicate n‑type MoS₂, likely due to sulfur vacancies.

Optical characterization shows distinct A, B, and C excitonic absorption peaks of MoS₂ spanning 400‑680 nm, while GaN contributes below 380 nm. Quantum efficiency (QE) reaches ~1.5 % at 405 nm and ~0.5 % at longer visible wavelengths, reflecting the absorption profile of MoS₂. Internal quantum efficiency (IQE), defined as QE divided by the absorption fraction, lies between 1 % and 5 %, improving with higher reverse bias, especially at longer wavelengths—an effect attributed to momentum‑space alignment and GaN polarization charges.

Frequency response measurements using intensity‑modulated laser light (465 nm) reveal a –3 dB bandwidth of approximately 5 GHz at 3 V reverse bias. The response follows a simple RC low‑pass model with a measured device capacitance of 0.8 pF and a 50 Ω load, indicating that the current bandwidth limitation is extrinsic (circuit parasitics) rather than intrinsic carrier transit time. The authors estimate that reducing the i‑GaN thickness or shrinking the active area could push the bandwidth into the tens of gigahertz regime.

Time‑domain photocurrent measurements under 100 µs pulses confirm that the device follows the incident light waveform, with rise/fall times limited only by the light source, demonstrating a response three to six orders of magnitude faster than previously reported MoS₂/GaN photodetectors, which suffered from microsecond‑scale trapping at imperfect interfaces.

In summary, the work demonstrates that a high‑quality epitaxial MoS₂/GaN van der Waals heterojunction can serve as an effective UTC‑PD for visible light, delivering GHz‑scale bandwidth and ~1 % quantum efficiency. The study highlights the importance of clean, lattice‑matched interfaces, Type‑II band alignment, and electron‑only transport for high‑speed photodetection. Future directions include exploring alternative 2D absorbers, optimizing transport‑layer thickness, and scaling down the device footprint to achieve even higher bandwidths and efficiencies, opening pathways for visible‑light Li‑Fi, inter‑satellite links, LiDAR, and optical coherence tomography applications.