Improving MoS₂ Photodetectors with Hydrogen Plasma Treatment and Al₂O₃/HfO₂ Passivation

Molybdenum disulfide (MoS₂) is two-dimensional material from the transition metal dichalcogenides family that has been gaining a lot of attention lately because it holds incredible promise for cutting-edge technology like photodetectors, transistors, and solar cells. It has some standout features, including a tunable bandgap (ranging from 1.2 to 1.9 eV), a strong ability to interact with light, and impressive carrier mobility. These qualities make MoS₂ an exciting option for building compact, high-performance semiconductors. Among its many potential applications, photodetectors stand out because they can detect light across a wide range of wavelengths, from visible to near-infrared. Despite all its potential, though, MoS₂-based devices face challenges that limit their real-world use. Probably, the biggest limitation comes from how sensitive MoS₂ is to its environment. It reacts easily with moisture and oxygen in the air, which causes it to degrade over time. This can lead to reduced stability and shorter device lifespans. On top of that, defects in its crystal structure, especially sulfur vacancies, create another layer of problems. These vacancies mess with how charges move through the material, lowering its efficiency. While defect-engineering techniques like oxygen or argon plasma treatments have been explored to tackle this issue, they often do more harm than good by damaging the material’s structure or creating new defects. Clearly, there is a need for a smarter, less destructive solution. New research paper published in Journal of Materials Chemistry A  and conducted by Yulin Li, Yajun Tian, Lingjie Bao, Dr. Haoran Cheng and Associate Professor Qijin Cheng from the School of Electronic Science and Engineering at Xiamen University developed a new method where they combined hydrogen plasma treatment with alternating passivation layers of aluminum oxide (Al₂O₃) and hafnium oxide (HfO₂). Hydrogen plasma, known for being gentle but effective, helped create controlled sulfur vacancies to enhance charge movement without damaging the material. Meanwhile, the alternating Al₂O₃ and HfO₂ layers provided protection against environmental damage. Together, these layers brought the stability of Al₂O₃ and the strong dielectric properties of HfO₂ to the table, creating a durable, high-performing device.

The researchers started by creating high-quality monolayer MoS₂ using a chemical vapor deposition process. They took great care to fine-tune the synthesis to ensure the material is uniform and structurally accurate. The resulting MoS₂ films had a smooth, continuous surface—ideal for advanced applications. To enhance the material’s performance in photodetectors, they used a remote hydrogen plasma treatment. This process introduced controlled sulfur vacancies, which are known to improve charge flow by providing better pathways for electrons and adjusting the energy band structure of MoS₂. The authors  used atomic force microscopy and Raman spectroscopy and found that a 15-minute plasma treatment was just right. It created enough sulfur vacancies to boost charge transport without causing significant damage to the material’s structure. To tackle the big challenge of making MoS₂ stable in real-world environments, the researchers came up with an innovative passivation technique. They added alternating layers of Al₂O₃ and  HfO₂ to the surface using atomic layer deposition (ALD). These layers, totaling 20 nanometers thick, combined the strong dielectric properties of HfO₂ with the chemical and thermal stability of Al₂O₃. This dual-layer system acted as a protective barrier against environmental threats like oxygen and moisture. At the same time, it introduced fixed positive charges, improving the electrostatic doping of the MoS₂. Detailed analysis with X-ray photoelectron spectroscopy confirmed that the passivation effectively removed harmful adsorbed molecules while keeping the material’s structure intact, which is key for long-lasting stability. The team tested the performance of their devices, which were built as metal-semiconductor-metal (MSM) photodetectors. The results were impressive. Under the best conditions, the photodetector’s responsivity soared to 567 A/W at a bias of -5 V—far surpassing untreated devices. The device’s specific detectivity, a measure of how well it can detect weak light, reached 1.12 × 10¹⁰ Jones, showing it could pick up even the faintest signals. Remarkably, after two months in an open environment, the photodetector maintained about 95% of its original performance, proving how effective the Al₂O₃/HfO₂ passivation was at preventing degradation. One interesting observation was that the response time of the photodetectors increased slightly after passivation. This was due to electron trapping and slow release caused by the oxide layers, which extended the recombination time of charge carriers. While this was a small drawback, the gains in responsivity and stability more than made up for it.

In conclusion, the work led by Associate Professor Qijin Cheng and the team successfully resolved major limitations in MoS₂-based photodetectors technology. The new strategy improves device performance and ensures long-term reliability without causing damage to the material’s structure. The impact of this research goes far beyond just MoS₂ photodetectors. It offers a solid framework for enhancing the properties of other two-dimensional materials using similar techniques. The excellent jump in responsivity makes it important in advanced imaging, optical communications, and environmental monitoring.  Moreover, we think one of the standout aspects of this study is how it highlights the benefits of controlled defect engineering. Carefully introducing sulfur vacancies significantly improved material performance without compromising structural integrity. This opens up exciting possibilities for exploring similar low-damage plasma treatments in other transition metal dichalcogenides and 2D materials to tweak their electronic and optical properties. From an industrial point of view, the methods used here are a game-changer because atomic layer deposition and hydrogen plasma treatment are already well-established techniques in manufacturing which makes their approach both scalable and easy to integrate into current production lines. This simplicity, paired with the impressive results, positions this research as a key step toward making next-generation optoelectronic devices a reality.