Abstract
The downscaling of the capacitance equivalent oxide thickness (CET) of a gate dielectric film with a high dielectric constant, such as atomic layer deposited (ALD) HfO2, is a fundamental challenge in achieving high-performance graphene-based transistors with a low gate leakage current. Here, we assess the application of various surface modification methods on monolayer graphene sheets grown by chemical vapour deposition to obtain a uniform and pinhole-free ALD HfO2 film with a substantially small CET at a wafer scale. The effects of various surface modifications, such as N-methyl-2-pyrrolidone treatment and introduction of sputtered ZnO and e-beam-evaporated Hf seed layers on monolayer graphene, and the subsequent HfO2 film formation under identical ALD process parameters were systematically evaluated. The nucleation layer provided by the Hf seed layer (which transforms to the HfO2 layer during ALD) resulted in the uniform and conformal deposition of the HfO2 film without damaging the graphene, which is suitable for downscaling the CET. After verifying the feasibility of scaling down the HfO2 thickness to achieve a CET of ~1.5 nm from an array of top-gated metal-oxide-graphene field-effect transistors, we fabricated graphene heterojunction tunnelling transistors with a record-low subthreshold swing value of <60 mV/dec on an 8″ glass wafer.
Highlights
Of seed layers (organic[11,15,16] and metal layers17–22), have been suggested
To demonstrate the influence of the organic solvent residue on graphene, NMP was used for the wet treatment, which was performed on a 2D MoS2 sheet with surface conditions similar to graphene[23]
ALD HfO2 films were grown at 200 °C on chemical vapour deposition (CVD) graphene monolayers after various surface passivation protocols, such as NMP treatment, sputtering of a ZnO film, and e-beam-evaporation of a Hf film
Summary
Of seed layers (organic[11,15,16] and metal (metal-oxide) layers17–22), have been suggested These techniques have afforded the conformal and uniform deposition of various high-k dielectric films (mostly Al2O3 and HfO2), which are acceptable for device fabrication. We report the scaling of the ALD HfO2 film thickness on CVD-grown monolayer graphene and demonstrate the fabrication of high-performance GSM-TFETs with a substantially small CET at the wafer scale. Through a systematic comparison of the effects of these surface treatments on the subsequent growth of thermal ALD HfO2 films by various characterization techniques, we selected the most promising process of preparing high-quality HfO2 films on the monolayer graphene Based on this screening process, we obtained scaled HfO2 films of excellent gate dielectric quality with a CET of ~1.5 nm in the MOG-FET structure. GSM-TFETs with the scaled HfO2 gate dielectric films showing SS values less than ~60 mV/dec were fabricated on an 8′′ wafer
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