Wafer-scale CVD graphene-based Josephson field-effect transistors with local top-gate tunability

Critical current (I_c) tunability with an electrostatic gate in superconductor-graphene-superconductor (SGS) junctions is essential for superconducting electronics based on Josephson field-effect transistor (JoFET) [1]. Previously, CVD graphene JoFETs were demonstrated using the global Si-wafer back-gate, see e.g. [2]. Here, we present tunable SGS junctions encapsulated with atomic layer deposition (ALD) grown Al₂O₃ dielectric and lithography-defined local top gates.

Starting with 6” CVD graphene on Si/SiO₂ wafers provided by Graphenea, our graphene devices are encapsulated with Al₂O₃, the top contacts are then evaporated using Ti/Al, the gate dielectric is grown using ALD, and finally the top gate is evaporated with Ti/Al. Using process optimization, we achieve contact resistances down to 300 Ω∙µm.

The resulting device arrays fabricated on 6” wafers consist of SGS junctions with length from 150 nm to 350 nm and width from 10 µm to 50 µm. Cooling down JoFETs with different dimensions in a dilution refrigerator, we observed repeatable superconducting proximity effect in all tested devices. We show the I_c tunability with top gate and analyze the temperature dependence of I_c. Our results constitute an important milestone toward scalable superconducting electronics based on graphene JoFETs [3].

[1] F. Wen et al., IEEE TED, vol. 66, no.12, pp. 5367-5374 (2019)

[2] T. Li et al., IEEE Trans. Appl. Supercond. vol. 29, no. 5, pp. 1-4 (2019)

[3] A. Generalov, K. Viisanen et al., in preparation (2023)