Bicrystal Si(BiSi) substrates for grain boundary (GB) Josephson junctions (GBJJs) have been fabricated by a direct bonding technique using a hot press method. The fracture strength and structure of the bonding interfaces were investigated to obtain substrates suitable for the junctions. It was found that an increase in the pressure of the hot press improves the reproducibility of the GBJJs. YBa2Cu3O7 - y GBJJs were successfully fabricated on Bi-Si substrates with a misorientation angle of 15 ° bonded under a pressure of 90 kgf cm-2 at 1200 °C in a vacuum of ≈10-3 Pa. These junctions showed typical I-V curves described by the RSJ model. The Shapiro steps induced by millimetre wave irradiation of 101 GHz were clearly observed in the I-V curves up to 3 mV, corresponding to at least 1.5 THz (= 2e V h, where e is the unit charge, V the voltage and h Planck's constant).
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