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.<>