Effects of underlayer roughness on Nb/AlO/sub x//Nb junction characteristics

The effects of underlayer roughness on Nb/AlO/sub x//Nb Josephson junction characteristics are clarified. MoN/sub x/ and SiO films with 0.1-2.7-nm surface roughness are used as underlayers for the junctions. MoN/sub x/ films with varying levels of roughness are prepared by sputter-etching. SiO films retain a smooth surface after sputtering etching. The existence of grain boundaries in the MoN/sub x/ films causes surface roughness. Subgap leakage current of junctions on MoN/sub x/ underlayers is higher than that of junctions on SiO underlayers, and it increases with the roughness of the underlayers. The roughness of the underlayer's surface is reflected in the Nb base electrode's surface; however, the roughness of the Nb surface is not directly reflected in the AlO/sub x/ surface. The AlO/sub x/ surface is smoother than the Nb surface. The cause of the leakage increase in the junctions on rough underlayers may be dispersion in the deposited-Al thickness.>