Test structure of JJ arrays connected to two types of wirings: the standard type connects JJs with BAS and COU Nb layers and the CC type connects JJs with BAS and stacked CTL/COU layers. The Nb layers below COU are covered with and the CTL layer is exposed to air. So, palladium remained in the vias in the CTL layer.
Plots of the variations in , , and of CC-type junction array with respect to normal type junction array as a function of elapsed time after fabrication. While the difference in gradually increases up to about 20%, and remain almost constant.
Dependence of increases on junction size. The increases are much larger for the Pd on the COU electrodes than for Pd on the BAS electrodes. There is a tendency for the larger JJ sizes to have bigger increases.
Junction resistance changes as a function of elapsed time after Pd deposition. The junction resistances were measured at room temperature. Two samples with Pd on at least the COU electrodes show a gradual decrease in . The sample with Pd only on the BAS electrodes shows no decreases. Measurements at 4.2 K were performed for one sample at a time (arrowed in the figure). The was 115% that of the initial value and this is consistent with the of 87% measured at room temperature.
Current-voltage curves measured before and about 2 weeks after palladium coating. Conventionally layered array TEG (top row) showed 20% increases for half of the JJs. The array TEG with an extra Al layer (bottom row) showed no changes.
Gap voltage plot for I-V curves shown in Fig. 5. After Pd coating, the conventionally layered array TEG (top row) had a stepwise of about 1% increases corresponding to the 20% increase. The array TEG with an extra Al layer showed about a 1% gradual increase.
Schematic drawing of energy diagrams for two structures. The left one is a conventional structure and the right is a structure, where Nb(H) represents the hydrogen-included Nb. The work function is defined as the difference between the vacuum energy level and Fermi level of the electrodes, and the barrier height is that between the conduction band energy of the and the Fermi level of the electrode. The barrier height is determined by focusing on the two electrodes and the dielectric layer separating them. If the work function increases due to hydrogen inclusion, depicted as Nb(H), the barrier height increases, resulting in less electron tunneling. There is a contact potential difference between different metals, and the resistance due to it is much smaller than the tunnel resistance.
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