MXene-GaN van der Waals metal-semiconductor junctions for high performance multiple quantum well photodetectors

Photodetectors are central to sensing, detection, data transport, and processing in the Internet of Things era, requiring high sensitivity, bandwidth, and low noise. Among device types, metal–semiconductor–metal (MSM) photodetectors are attractive for their high speed and simple fabrication, but often suffer from high dark currents due to defect-rich metal–semiconductor interfaces created during metal deposition, which cause Fermi level pinning and enhanced reverse tunneling. Opaque metal electrodes also reduce responsivity by reflecting incident light; transparent oxides are costly and brittle, while backside illumination complicates processing. MXenes, a family of 2D transition metal carbides/carbonitrides discovered in 2011, offer metallic conductivity, flexibility, hydrophilicity, optical transmittance, chemical stability, and tunable work function, enabling van der Waals (vdW) metal–semiconductor junctions that minimize interfacial defects and Fermi level pinning. This work proposes and demonstrates an InGaN/GaN multiple quantum well (MQW) MSM photodetector using MXene-GaN-MXene contacts on patterned sapphire substrate (PSS) to reduce GaN dislocation densities, aiming to lower dark current, improve responsivity and noise performance, and validate benefits for underwater optical detection and communication.

Conventional MSM PDs employ deposited metals forming Schottky contacts with semiconductors, but deposition can induce chemical disorder and interface defect states (e.g., N-vacancy-related deep donors in GaN), narrowing Schottky barriers and elevating dark current via thermionic-field emission. Strategies to mitigate dark current include using wide bandgap semiconductors to increase barrier heights, repelling injected carriers through confined gases, and reducing semiconductor density of states. However, MSMs generally exhibit inferior signal-to-noise ratios compared to PIN diodes, and top opaque metal electrodes reduce photon coupling; transparent electrodes (ITO, TIN, RuO, IrO) are costly/brittle, and backside illumination adds fabrication complexity. MXenes (Mn+1XnTx) possess tunable work functions (1.6–6.2 eV) via surface terminations (−O, −OH, −F), enabling tailored ohmic/Schottky contacts. Their 2D nature supports vdW junctions with bulk semiconductors, mitigating Fermi level pinning and reverse tunneling. MXenes have been explored in photodetectors, sensors, EMI shielding/absorption, photoelectrochemical catalysis, and optical transmission/modulation, motivating their use as electrodes for improved MSM PDs.

Device fabrication and materials: InGaN/GaN MQWs were grown on a patterned sapphire substrate (PSS) to promote epitaxial lateral overgrowth and reduce threading dislocations and basal stacking faults. Ti3C2Tx MXene single-layer nanosheets were synthesized by chemically etching Al from the Ti3AlC2 MAX phase and exfoliating. A polyvinyl chloride (PVC) electrostatic film served as a shadow mask; electrode patterns were carved into the mask. The aqueous MXene solution was drop-cast through the mask onto the MQW sample to form MXene electrodes, followed by mask removal, yielding MXene–GaN–MXene MSM structures via vdW junctions. A reference Cr/Au–GaN–Cr/Au MSM PD was fabricated using the same mask via conventional metal evaporation. Device structure and operation: The MSM PD comprises two back-to-back Schottky contacts. Under moderate bias, the cathode-side junction is reverse-biased, setting a larger depletion width and dominating dark current, which follows thermionic emission with barrier height Φb. Conventional Cr/Au deposition can create deep donor defects near the interface (e.g., N vacancies/complexes), narrowing the barrier and increasing reverse tunneling; vdW MXene–GaN interfaces reduce such defects and Fermi level pinning, targeting lower dark current. Characterization: I–V characteristics were measured in dark, ambient, and under illumination at λ = 405 nm, 450 nm, and 520 nm. Responsivity versus incident optical power was obtained over 0.3 µW to several mW. Transient photoresponse was measured under chopped illumination at +5 V to extract rise time (10–90%) and decay time (90–10%); the 3 dB bandwidth was estimated as f3dB ≈ 0.35/tr. Time-dependent photoresponse traces were recorded across wavelengths and powers to assess minimum detectable power and linear dynamic range (LDR). Noise spectral density Si(f) was measured up to 100 kHz in ambient and under 405 nm illumination at various biases, analyzed via Hooge’s empirical law Si ∝ I^β / f^α to extract α and β. Specific detectivity D* was calculated using measured responsivity and noise.

- MXene–GaN–MXene MSM PDs exhibit dark current reduced by three orders of magnitude at +5 V compared with Cr/Au–GaN–Cr/Au references; distinct photocurrent is evident whereas Cr/Au devices’ photocurrent is overwhelmed by dark current. - Maximum responsivity R ≈ 64.62 A/W at λ = 405 nm under Popt ≈ 0.3 µW; R decreases to ~0.07 A/W at Popt ≈ 3771.48 µW. Similar power-law behavior at 450 nm and 520 nm with fitted exponents −0.61 (405 nm), −0.62 (450 nm), and −0.56 (520 nm). - Transient response: rise time tr ≈ 300 µs and decay time td ≈ 402 µs for a large-area (~9.6 mm²), long-channel (~0.54 mm) device; estimated f3dB ≈ 0.35/tr ≈ 1167 Hz. Responses show fast and slow components attributed to free-carrier and trap-assisted processes. - Minimum detectable optical power: ~80 nW at 405 nm and 450 nm; ~14.11 µW at 520 nm due to lower InGaN/GaN QW absorption at this wavelength. - Linear dynamic range ≈ 40.6 dB, spanning >4 orders of magnitude. - Noise: 1/f noise dominates up to 100 kHz with α ≈ 0.98–1.02. MXene-contact devices show consistently lower noise spectral density than Cr/Au devices across biases and conditions, especially at low bias. Under illumination, MXene-device current increases by ~2 orders while noise increases by ~1 order. Extracted β ≈ ~2 (closer to 2) for MXene devices versus ~1.6 for Cr/Au, consistent with reduced Schottky barrier height fluctuations and volume trapping-limited 1/f noise in vdW junctions. - Specific detectivity D* up to ~1.9–1.93 × 10^12 Jones at 405 nm. - Patterned sapphire substrate aids localized photon extraction and photocurrent collection. - The MXene electrode approach outperforms Cr/Au MSM counterparts in responsivity, dark current, and noise, supporting suitability for underwater optical detection/communication; proof-of-concept UWOC and turbidity sensing systems are noted.

The study demonstrates that replacing conventional evaporated metal contacts with Ti3C2Tx MXene electrodes forming van der Waals junctions with GaN markedly improves MSM photodetector performance. By minimizing interfacial chemical disorder and defect states, the MXene–GaN interface alleviates Fermi level pinning and reduces Schottky barrier height fluctuations, suppressing reverse tunneling currents and 1/f noise. Consequently, dark current is reduced by three orders of magnitude at +5 V, enabling high responsivity and detectable photocurrent over a broad dynamic range. Noise analysis via Hooge’s law (α ≈ 1, β closer to 2) corroborates improved interface quality and volume trapping-limited behavior in MXene devices compared to Cr/Au counterparts (β ~1.6). The transient response reveals both fast and trap-mediated slow components, consistent with trap-related gain behavior observed in responsivity-power scaling. Use of a patterned sapphire substrate not only lowers GaN defect densities via ELOG but also enhances localized light extraction and photocurrent collection, suggesting avenues for spatially engineered detector arrays. Collectively, the findings validate the hypothesis that MXene vdW contacts with III–V semiconductors can deliver superior MSM photodetector sensitivity and noise performance, supporting applications such as underwater optical communication and sensing.

A high-performance InGaN/GaN MQW MSM photodetector using MXene–GaN–MXene van der Waals junctions was fabricated by a simple drop-casting process on patterned sapphire substrates. Compared to Cr/Au MSM references, the devices exhibit three orders lower dark current, high responsivity up to ~64.6 A/W, specific detectivity approaching 1.93 × 10^12 Jones, large linear dynamic range (~40.6 dB), and favorable transient response (~300 µs rise). Noise measurements confirm reduced 1/f noise and β values near 2, indicating stable barrier characteristics and low interface imperfection. The high-quality vdW interfaces also enable verification of patterned-substrate-assisted light extraction and photocurrent collection, positioning these devices for underwater optical detection/communication and sensing. Future work could include optimizing MXene terminations/work function for tailored Schottky barriers, scaling to focal plane arrays leveraging patterned substrates for localized response, exploring alternative MXene compositions, and engineering device geometry to enhance bandwidth while maintaining low noise.

- Sensitivity at 520 nm is lower, with a higher minimum detectable power (~14.11 µW) attributed to reduced InGaN/GaN MQW absorption at this wavelength. - The characterized device has a large area (~9.6 mm²) and long channel (~0.54 mm), yielding an estimated bandwidth of ~1.17 kHz; higher-speed operation may require geometry optimization (not explored here). - Performance and noise analyses are presented for three wavelengths (405, 450, 520 nm); broader spectral characterization is not provided in the excerpt. - Full affiliation details for all authors and some supplementary device/interface models are referenced but not included in the provided text.