Fabrication of NbTiN/Al–AlNX/NbTiN Josephson Junctions for Superconducting Circuits Operating Around 10 K

We have fabricated NbTiN/Al-AlN
_X
/NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum circuits operating around 10 K. NbTiN films with a critical temperature T
_c
of 14.2 K and a root-mean-square value of their surface roughness of 0.45 nm were used as junction electrodes. For a junction barrier, radical-nitrided AlN
_X
was used to realize high uniformity in the critical current I
_c
of the junctions. We obtained NbTiN junctions with a gap voltage Vg of 4.3 mV and an I
_c
R
_n
. product of 2.6 mV at 4.2 K, where R. is the junction resistance. The critical current density J
_c
, at 4.2 K was obtained in the range between 0.2 and 6.2 kA/cm
²
by changing a nitridation time for the AlN
_X
barrier formation. A series array of 200 junctions with an area of 10 μm × 10 μm exhibited a small I', spread, and the standard deviation 1-σ in I
_c
uniformity was less than 1.6% in the temperature range between 4 K and 10 K.