Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation

Abstract: Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111), versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 hours in ambient, followed by a ~10^3 decrease in FET current on/off ratio and mobility after 48 hours. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ~10^3 and mobilities of ~100 cm2/(V*s) for over two weeks in ambient. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications.

• The paper demonstrates that ALD of aluminum oxide effectively passivates black phosphorus FETs against oxygen and moisture induced degradation.
• Microscopy and spectroscopy analyses reveal that substrate properties critically influence degradation rates, with hydrophobic surfaces accelerating damage.
• The ALD encapsulation preserved device performance, maintaining a mobility of ~44 cm²/V·s and an on/off ratio above ~250 after one week of exposure.

Evaluation of Effective Passivation Techniques for Black Phosphorus Field-Effect Transistors

The paper entitled "Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation" focuses on the challenges associated with the stability of black phosphorus (BP) when exposed to ambient conditions and explores passivation methods to mitigate its degradation. Black phosphorus, a semiconducting and layered material with promising electronic and optoelectronic properties, has gained significant attention due to its high carrier mobility and feasible monolayer construction from bulk crystals. However, the inherent instability of BP under ambient conditions compromises both its structural and electronic integrity, posing a significant hurdle to its application in advanced technologies.

Key Findings

The authors performed a comprehensive analysis using a range of microscopy and spectroscopy techniques such as AFM, EFM, TEM, XPS, and FTIR to investigate the degradation process of exfoliated BP. The findings indicate that exposure to oxygen and water in the atmosphere accelerates the conversion of BP into oxidized phosphorus compounds. This degradation leads to a marked increase in roughness and formation of degradation-induced bubbles, considerably affecting the performance of BP field-effect transistors (FETs). A noteworthy observation is that ambient degradation occurs more rapidly on hydrophobic substrates (e.g., OTS/SiO₂ and H-Si(111)) than on hydrophilic surfaces like SiO₂.

Passivation Strategy

The paper demonstrates that the application of atomic layer deposited (ALD) aluminum oxide (AlOₓ) overlayers is an effective and scalable solution to protect BP from ambient degradation. This encapsulation technique ensures preservation of the electronic properties of BP FETs over extended periods, maintaining high mobility and on/off ratios. ALD encapsulated BP retains its electronic function with a mobility of ~44 cm²/V·s and an on/off ratio above ~250 even after one week of ambient exposure. The encapsulated BP devices also display less threshold voltage drift, signify reduced adsorption from atmospheric dopants.

Implications and Future Directions

The research underscores the potential of ALD-derived encapsulation layers not only in safeguarding BP FETs but also potentially other 2D materials susceptible to environmental degradation. The findings contribute to the broader understanding of BP's surface chemistry and the critical role of substrate wetting properties in degradation kinetics. The potential to leverage ALD passivation techniques could bolster active research efforts in integrating BP into reliable electronic and optoelectronic applications.

Future research might address optimizing the deposition process or exploring newer encapsulating materials that further tailor electronic properties while providing passivation. Moreover, further investigations into the interaction dynamics between BP and encapsulating materials at the nanoscale would be beneficial in fully unraveling the prospective applications of passivated phosphorene.

In summary, this paper provides a thorough examination of BP's ambient degradation issues and presents a viable passivation strategy via ALD encapsulation, thus offering substantial contributions towards practical advancements in black phosphorus-based technologies.