Saturday, January 4, 2025

Scalable ALD Process for High-Performance MoS₂ Films on Flexible Substrates Unlocks Advanced Electronics Applications

This study by researchers from the University of Southampton (UK), LMU Munich (Germany), and VTT Technical Research Centre of Finland, presents a scalable Atomic Layer Deposition (ALD) method to grow large-area, atomically thin molybdenum disulfide (MoS₂) films with high electrical performance, addressing a key challenge for Transition Metal Dichalcogenides (TMDCs) in commercial semiconductor applications. The ALD process enables precise control over film thickness, stoichiometry, and crystallinity, starting with a MoO₃ layer grown via ALD, followed by a sulfurization process to convert it into MoS₂. This two-step approach decouples film properties, ensuring uniform growth on substrates up to six inches in size. The MoS₂ films are then transferred to flexible substrates using a chemical-free transfer process, resulting in highly uniform films with low surface roughness. Field-effect transistors (FETs) fabricated with these MoS₂ films demonstrate impressive mobility values (up to 55 cm²/Vs), subthreshold slopes as low as 80 mV/dec, and on/off ratios of 10⁷, making them suitable for advanced flexible electronics.



The process begins with 6-inch p-type silicon wafers, onto which a 285 nm layer of thermal SiO₂ is grown at 1000°C using a tube furnace. To enhance the chemical termination of the surface oxide, the wafers are treated in a UV/O₃ reactor for 10 minutes. The subsequent step involves the deposition of MoO₃ via a thermal atomic layer deposition (ALD) process using a Cambridge Nanotech Savannah S200 system. The ALD process utilizes bis(tert-butylimido)bis(dimethylamido) molybdenum as the molybdenum precursor and ozone as the oxidant. By conducting 15 ALD cycles at 250°C, a uniform MoO₃ film with a thickness of 1.31 ± 0.13 nm is achieved across the entire 6-inch wafer, ensuring excellent consistency. This initial MoO₃ layer provides precise control over the number of MoS₂ layers that are subsequently formed, making it a critical step in the overall process.


The study further highlights the integration of MoS₂ in ferroelectric field-effect transistors (FeFETs), which show a memory window of 3 V at ±5 V operation and stable multi-state switching capabilities. These FeFETs, utilizing a thin P(VDF-TrFE) layer as a ferroelectric gate dielectric, offer superior performance compared to traditional flexible memory devices. Electrical measurements confirm the devices’ scalability and uniformity over a 5 × 5 mm² area, with minimal device-to-device variation. The ALD-grown MoS₂ films also retain high stability under repeated bias stress, demonstrating their potential for use in flexible memory and neuromorphic applications. This process provides a commercially viable pathway for integrating high-quality 2D materials into next-generation electronics.

Sources:
Aspiotis, N., Morgan, K., März, B. et al. Large-area synthesis of high electrical performance MoS2 by a commercially scalable atomic layer deposition process. npj 2D Mater Appl 7, 18 (2023). https://doi.org/10.1038/s41699-023-00379-z

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