We have fabricated NbTiN tunnel junctions by introducing a Hf overlayer to a junction barrier layer. Two types of junctions consisting of NbTiN/Hf/Al-AlN x /NbTiN and NbTiN/Hf-HfO x /NbTiN were fabricated on thermally oxidized Si substrates. The electrical characteristics for junctions with various thickness of the Hf layer were evaluated at 4.2 K and compared. The results indicated that the diffusion of nitrogen from the base NbTiN layer into the normal layer was suppressed. In addition, the normal coherence length in the Hf layer was estimated to be about 13 nm.
We have fabricated NbTiN/Al-AlN x /NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum (SFQ) circuits operating around 10 K. NbTiN films with a critical temperature (T c ) of 14.2 K and a root-mean-square (RMS) 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 critical current (I c ) of the junctions. We obtained NbTiN junctions with a gap voltage (V g ) of 4.3 mV and an I c R n product of 2.6 mV at 4.2 K, where R n is junction resistance. 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 m × 10 m exhibited a small I c spread, and the standard deviation, 1-, in I c uniformity was less than 1.6% in the temperature range between 4 and 10 K.
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