Scalable, Tuneable Josephson Junctions and SQUIDs based on CVD Graphene

IntroductionSince the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions with graphene as the weak link (SGS junctions) can have their I-V properties easily tuned by the... [ view full abstract ]

Introduction

Since the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions with graphene as the weak link (SGS junctions) can have their I-V properties easily tuned by the gate voltage. Most of the previous works on SGS junctions and SQUIDs were based on mechanically exfoliated graphene, which is not compatible with large scale production. Here we show that SGS junctions and dc SQUIDs can be easily fabricated on CVD graphene and exhibit good electronic properties. Such scalable and tuneable SGS junctions are especially useful for applications involving arrays of Josephson junctions, such as single flux quantum devices and superconducting qubits, where the properties of the junctions can be tuned to be identical.

Methods

The SGS junctions and SQUIDs were fabricated on CVD graphene transferred to silicon substrates. The Nb electrodes were deposited by conventional EBL, sputtering and lift-off processes. The devices were measured in a ³He cryostat with a base temperature of 300 mK.

Results

To reduce the normal state resistance and to increase the critical current, the SGS junctions we fabricated were relatively short (50~450 nm) and wide (10~80 µm). The junctions show good I-V properties and could work in wide temperature range from 0.3 K to 1.5 K without hysteresis (Figure 1). The junctions show obvious self-interference pattern under a perpendicular magnetic field, indicating that the distribution of the supercurrent is quite uniform. The critical current can be effectively tuned by the gate voltage up to an order of magnitude (Figure 2). The dc SQUIDs made up of those junctions can have their critical current tuned by both the magnetic field and the gate voltage (Figure 3).

Discussion

We have fabricated and measured SGS junctions with different lengths, and we found that the shortest junction (50 nm long) worked in the ballistic regime. We had evidence from both the I_c-T relation and the R_n-V_g relation. Operating devices with clean junctions in the ballistic regime could lead to performance improvements such as lower intrinsic noise.