Fabrication of NbTiN Josephson Junctions With Thermally Oxidized Hf Tunnel Barriers

We fabricated NbTiN Josephson junctions (JJs) with HfO
_x
barriers on thermally oxidized Si substrates. Hf overlayers with different resistivities were used for the HfO
_x
barrier formation. The Hf overlayer with low resistivity produced junctions with high quality parameters. The critical current density (J
_c
) was obtained in the range of 0.01-0.4 kA/cm
²
by changing the thermal oxidation conditions of the Hf layer. The critical current uniformity was estimated at a maximum-to-minimum spread of ±10.1% for 85 of the 100 JJs. We evaluated the specific capacitance, which was approximately 110 fF/μm
²
for the J
_c
ranging from 0.1 to 0.4 kA/cm
²
. The quasiparticle tunnel characteristics were improved by using the NbN electrodes instead of the NbTiN electrodes.