The increasing demand for photodetectors (PDs) in the Internet of Things (IoT) necessitates high sensitivity, high bandwidth, low thermal loss, and reduced electromagnetic interference. Metal-semiconductor-metal (MSM) photodetectors, known for their high response speed and ease of fabrication, often suffer from high dark currents due to defects at the metal-semiconductor interface. Traditional methods to mitigate this, such as using wide bandgap semiconductors or confined carrier gases, have limitations. Opaque metal electrodes also reduce responsivity by reflecting incident light. Transparent electrodes are expensive and brittle. Backside illumination, while improving responsivity, increases fabrication complexity. MXenes, a new class of two-dimensional (2D) materials, offer a potential solution. Their metallic conductivity, flexibility, hydrophilicity, good transmittance, and chemical stability, combined with tunable work function, make them suitable for forming ohmic or Schottky contacts with various semiconductors. Crucially, MXene-semiconductor van der Waals junctions are free of the chemical disorder and defect states that plague traditional metal-semiconductor interfaces, reducing reverse tunneling currents and the Fermi level pinning effect. Previous research has explored MXenes in various applications, including photodetectors, sensors, and electromagnetic interference shielding. This paper presents an InGaN/GaN multiple quantum well (MQW) photodetector using a MXene-GaN-MXene structure grown on a patterned sapphire substrate (PSS) to reduce defects. A facile drop casting method was used for fabrication. The researchers expected that the resulting device would exhibit high responsivity and low dark current, suitable for underwater detection and communication. The use of a patterned sapphire substrate was expected to improve light extraction and photocurrent collection.

The introduction section extensively reviews existing photodetector technologies, highlighting the advantages and disadvantages of different types, such as Schottky barrier detectors, MSM detectors, PIN photodiodes, APDs, photoconductors, and phototransistors. The limitations of MSM photodetectors, specifically the high dark currents resulting from defects at the metal-semiconductor interface, are emphasized. Various attempts to solve this problem, including using wide bandgap semiconductors and confined carrier gases, are discussed. The limitations of traditional metal electrodes and the potential advantages of transparent electrodes are also mentioned. The introduction also provides background information on MXenes, outlining their properties and potential applications. The use of patterned sapphire substrates to improve GaN growth quality is also noted.

The researchers fabricated a MXene-GaN-MXene MQW photodetector using a facile drop casting method. Single-layer Ti3C2Tx MXene nanosheets were synthesized by chemically etching the Al layers of the Ti3AlC2 MAX phase. A polyvinyl chloride (PVC) electrostatic film served as a shadow mask for electrode patterning. The MXene solution was drop-casted onto an InGaN/GaN MQW sample grown on a PSS. A Cr/Au-GaN-Cr/Au MSM photodetector was fabricated as a control using conventional methods. The device structure and energy band diagram are illustrated, highlighting the Schottky junctions at the interfaces. The dark current is explained using the Richardson-Dushman equation, emphasizing the role of the Schottky barrier height. The paper contrasts the conventional metal-semiconductor interface, prone to defects and Fermi level pinning, with the MXene-GaN van der Waals junction, which is expected to have fewer defects and thus lower dark current. The experimental characterization involved measuring I-V characteristics, responsivity, transient photocurrent response, and noise spectral density under various illumination conditions. Specific equations (e.g., for 3dB bandwidth and 1/f noise) are utilized to analyze the experimental data.

The MXene-GaN-MXene photodetector exhibited significantly lower dark current (three orders of magnitude lower) than the Cr/Au-GaN-Cr/Au control device. The responsivity reached 64.6 A/W at 405 nm with 0.3 µW incident power but decreased with increasing optical power, exhibiting a power exponent around -0.6. The rise and decay times were relatively short (300 µs and 402 µs, respectively), yielding an estimated 3 dB bandwidth of 1167 Hz. The device showed a linear dynamic range of 40.6 dB, spanning four orders of magnitude. Noise spectral density measurements revealed that 1/f noise dominated up to 100 kHz. The MXene-based device consistently showed lower noise than the control device, particularly at low bias voltages. The analysis of the Hooge empirical law showed that the noise parameter β was closer to 2 for the MXene device, suggesting volume trapping-limited 1/f noise, which is indicative of superior interface quality compared to the control device with β closer to 1.6.

The superior performance of the MXene-GaN-MXene photodetector is attributed to the high-quality MXene-GaN van der Waals interfaces, which are free of the chemical disorder and defects that plague conventional metal-semiconductor junctions. This reduction in defects leads to significantly lower dark current and improved noise characteristics. The high responsivity demonstrates the effectiveness of MXene electrodes and the patterned sapphire substrate in enhancing light absorption and charge collection. The large linear dynamic range suggests the device's suitability for a wide range of applications. The results highlight the potential of combining MXenes with conventional III-V materials for developing high-performance photodetectors.

This research successfully demonstrated a high-performance InGaN/GaN MQW photodetector using MXene-GaN-MXene van der Waals junctions. The device exhibited significantly reduced dark current, high responsivity, short response times, and low noise, exceeding the performance of its Cr/Au counterpart. The simple fabrication method and excellent performance make this approach promising for various applications, particularly in underwater optical detection and communication. Future work could explore different MXene compositions and surface terminations to further optimize device performance and investigate other III-V semiconductor combinations.

The study primarily focused on the performance of the photodetector under specific illumination conditions. Further research is needed to investigate the device's performance under different wavelengths, temperatures, and operating conditions. The impact of long-term stability needs to be assessed. While the noise characteristics were analyzed, a more detailed investigation into the underlying noise sources could provide additional insights. The relatively large device area may influence response speed; miniaturization studies would be beneficial.