Covalent Heterostructures of Ultrathin Amorphous Carbon Nitride and Si for High-Performance Vertical Photodiodes

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carbon nitride silicon photodiodes heterostructures photodetectors

Carbon nitride (CN), as a two-dimensional n-type semiconductor material, exhibits great potential in light-driven energy conversion and environmental applications due to its excellent photocatalytic activity and stability. However, despite its outstanding performance in photocatalysis, the application of CN in optoelectronic devices, especially silicon (Si)-based optoelectronics, has been limited. The main reason lies in the lack of synthetic methods capable of producing large-scale, high-quality, uniform, and processable CN thin films. Existing synthesis methods, such as nanosheet dispersion coating, liquid-solid interface synthesis, and high-temperature annealing, have partially achieved the preparation of CN films, but they still fall short in terms of wafer-scale uniformity, surface roughness, and interfacial bonding strength with silicon. These issues lead to numerous defects at the CN/Si heterointerface, hindering carrier transport and thus limiting device performance improvement.

To address these challenges, researchers have proposed a novel synthesis method that successfully prepares large-area, ultrathin, and uniform amorphous carbon nitride (ACN) films on silicon through a two-step chemical vapor deposition (CVD) process followed by hydrogen atmosphere annealing. This method not only significantly improves the uniformity and surface roughness of CN films but also enhances the formation of nitrogen-silicon (N-Si) covalent bonds, achieving strong interfacial bonding between ACN and silicon. Based on this ACN/Si heterostructure, researchers have developed high-performance vertical photodiodes, demonstrating their potential applications in photodetection and imaging.

Source of the Paper

This research was jointly conducted by a team of scientists from the Institute for Basic Science (IBS), Seoul National University, Korea Institute of Science and Technology (KIST), and other institutions. The primary authors of the paper include Hyojin Seung, Jinsol Bok, and Ji Su Kim, with corresponding authors Changsoon Choi, Taeghwan Hyeon, and Dae-Hyeong Kim. The paper was published in April 2025 in the journal Nature Synthesis under the title Covalent Heterostructures of Ultrathin Amorphous Carbon Nitride and Si for High-Performance Vertical Photodiodes.

Research Process and Results

1. Synthesis of Ultrathin Amorphous Carbon Nitride

The research team proposed a two-step synthesis method for preparing ultrathin amorphous carbon nitride (ACN) on silicon.
Step 1: A dual-heating-zone chemical vapor deposition (CVD) technique was employed, where melamine precursor and p-type silicon (p-Si) wafers were placed in furnace zones set at 300°C and 550°C, respectively. Under an argon (Ar) atmosphere, melamine underwent sublimation and transportation, forming a bilayer structure of non-uniform polymeric carbon nitride (PCN) and underlying ACN on the bottom side of the silicon wafer.
Step 2: The as-grown film from the first step was annealed in a hydrogen atmosphere. The annealing process involved ramping the temperature from room temperature to 490°C and maintaining it for a certain period. Through this step, the PCN layer was selectively etched, ultimately forming an ultrathin ACN film with a thickness of only 1.8 nm, while significantly enhancing the N-Si covalent bonds between ACN and silicon.

2. Material Characterization and Analysis

The research team conducted detailed analysis of the synthesized materials using various characterization techniques:
- Transmission Electron Microscopy (TEM): Confirmed the ultrathin thickness (~1.8 nm) and uniformity of the ACN film.
- Atomic Force Microscopy (AFM): Measured the surface roughness of the film, showing that the ACN film exhibited extremely low surface roughness (Rq ≈ 0.156 nm).
- X-ray Photoelectron Spectroscopy (XPS): Analyzed the compositional changes during the annealing process, confirming the formation of N-Si covalent bonds.
- Photoluminescence Spectroscopy (PL): Observed the gradual disappearance of the PL peak of PCN with increasing annealing time, indicating enhanced interfacial bonding between ACN and silicon, which suppressed electron-hole pair recombination.

3. Fabrication and Performance Testing of ACN/Si Vertical Photodiodes

Based on the ACN/Si heterostructure, the research team fabricated vertical photodiodes and tested their performance:
- Rectification Ratio: The ACN/Si photodiode exhibited a rectification ratio as high as 3.8×10⁸ at a bias of 4 V, significantly higher than the control groups (PCN/Si and p-Si devices).
- Photodetection Performance: Under zero bias, the device demonstrated high sensitivity to 638 nm laser irradiation, with a linear dynamic range (LDR) exceeding 130 dB and a response time of 6.7 µs.
- Spectral Response: The device showed excellent response performance across a spectral range of 400 nm to 1000 nm, making it suitable for multispectral imaging.

4. Integration and Imaging Demonstration of Active-Matrix Image Sensor Array

The research team further integrated ACN/Si photodiodes with amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) to construct an 8×8 active-matrix image sensor array. This array demonstrated multispectral imaging capabilities from visible to near-infrared (NIR) light, showcasing its potential in image sensing applications.

Research Conclusions and Significance

Through an innovative synthesis method, this study successfully prepared large-area, ultrathin, and uniform amorphous carbon nitride films on silicon, achieving strong covalent interfacial bonding with silicon. Based on this ACN/Si heterostructure, researchers developed high-performance vertical photodiodes, demonstrating their application prospects in photodetection and imaging. This research not only provides new insights into the application of carbon nitride in optoelectronic devices but also lays the foundation for the development of future flexible electronics and curved image sensors.

Research Highlights

  1. Innovative Synthesis Method: Achieved the preparation of large-area, ultrathin, and uniform amorphous carbon nitride films through a two-step CVD process and hydrogen atmosphere annealing.
  2. Strong Interfacial Bonding: Significantly improved the interfacial bonding strength between ACN and silicon by enhancing N-Si covalent bonds.
  3. High-Performance Devices: The ACN/Si heterostructure-based vertical photodiodes exhibited high rectification ratio, high sensitivity, and fast response time.
  4. Multispectral Imaging Applications: Successfully constructed an active-matrix image sensor array by integrating a-IGZO TFTs, demonstrating its potential in multispectral imaging.

Additional Valuable Information

The study also provided detailed experimental methods and data support, including material characterization, device performance testing, and imaging demonstrations, offering important references for follow-up research. Additionally, the research team explored the potential applications of the ACN/Si heterostructure in flexible electronics and curved image sensors, pointing the way for future technological advancements.