Abstract

We fabricated MgB2/AlN/NbN trilayerJosephson junctions on c-plane sapphire substrates, and investigated the dependence of theMgB2 surfacemorphology on MgB2 growth temperature. AFM measurement showed that the grain size wasabout 150 nm and the surface roughness was about 10 nm when depositionwas done at the highest critical temperature. The critical temperature of theMgB2 layer after SIS junction fabrication remained the same as that for bareMgB2 film. The current–voltage characteristics of theMgB2/AlN/NbN junctions showed a very clear Josephson current and a gap structure. The critical current density wasover 1 kA cm−2, and the ratio of the sub-gap resistance to the normal resistance was 16.6 when the AlNinsulator thickness was 0.14 nm. The critical current was ideally modulated by applying amagnetic field, indicating that showed that the Josephson current flowed uniformly in thejunctions.

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