Residue-free field-effect transistors enhance silicon tech

Under the leadership of Professor Lee Young Hee, a team of researchers from the Institute of Basic Science (IBS) in South Korea has unveiled a discovery that could improve the fabrication of field-effect transistors (FET). A high-performance field-effect transistor is essential for next-generation beyond-silicon-based semiconductor technologies. Current three-dimensional silicon technology suffers from degradation of FET performance when the device is miniaturised past sub-3 nm scales. To overcome this limit, researchers have studied one-atom-thick (~0.7 nm) 2D transition metal dichalcogenides (TMDs) as a FET platform over the last decade. However, their practical applications are limited due to the inability to demonstrate integration at the wafer scale.

A significant challenge is the residue that occurs during fabrication. Traditionally, polymethyl methacrylate (PMMA) is used as a supporting holder for device transfer. This material is known for leaving insulating residues on TMD surfaces, which often generate mechanical damage to the TMD sheet during transfer. As an alternative to PMMA, other polymers such as polydimethylsiloxane (PDMS), polyvinyl alcohol (PVA), polystyrene (PS), polycarbonate (PC), ethylene vinyl acetate (EVA), polyvinylpyrrolidone (PVP) and organic molecules including paraffin, cellulose acetate and naphthalene have all been proposed as a supporting holder. However, residues and mechanical damage are introduced during transfer, which leads to degradation of FET performances.

The researchers addressed this problem by harnessing polypropylene carbonate (PPC) for residue-free wet transfer. Using PPC eliminated residue and also allowed for the production of wafer-scale TMD using chemical vapour deposition. Previous attempts at manufacturing large-scale TMDs often resulted in wrinkles occurring during the transfer process. The weak binding affinity between the PPC and the TMD eliminated residues and wrinkles as well.

Ashok Mondal, first author of the study, said the PPC transfer method enabled researchers to fabricate centimetre-scale TMDs. “Previously, TMD was limited to being produced using a stamping method, which generates flakes that are only 30–40 μm in size,” Mondal said.

The researchers built a FET device using a semimetal Bi contact electrode with a monolayer of MoS₂, which was transferred by the PPC method. Less than 0.08% of PPC residue was found to remain on the MoS₂ layer. Due to the lack of interfacial residues, the device was found to have an ohmic contact resistance of RC ~78 Ω·µm, which is close to the quantum limit. An ultrahigh current on/off ratio of ~1011 at 15 K and a high on-current of ~1.4 mA/µm were also achieved using the h-BN substrate.

The FET device produced through wafer-scale production and transfer of CVD-grown TMD was found to have electrical properties that exceeded that of previously reported values. This technology could be easily implemented using the currently available integrated circuit manufacturing technology.

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