Study of the Surface Morphology of Nb Films and the Microstructure of Nb/AlOx-Al/Nb Trilayers

Abstract

To optimize the current-voltage characteristics of Nb/AlOx-Al/Nb Josephson tunnel junctions, a uniform and well defined insulating barrier (AlOx) is required so that no leakage current occurs between the upper and lower Nb electrodes. We investigated the dependence of the surface morphology of dc magnetron-sputtered Nb thin films on deposition parameters using atomic force microscopy (AFM), the cross-sectional microstructure of the Nb/AlOx-Al/Nb trilayers using transmission electron microscopy (TEM), and anodization profiling. The surface roughness of the base Nb layer was found to affect the AlOx-Al layer and thus the quality of Nb/AlOx-Al/Nb trilayer. Diffusion of Al at the Al/Nb interface increases with increasing roughness of the base Nb layer, which increases the minimum Al thickness required to cover base-Nb. The importance of sufficient sample-cooling during the trilayer deposition was also confirmed by the TEM study and anodization profiling.

Influence of Nb film surface morphology on the sub-gap leakage characteristics of Nb/AlOx–Al/Nb Josephson junctions

“…The deposition variables for optimizing Nb surface morphology included the argon (Ar) pressure and flow rate, plasma power, film thickness and substrate cooling effect. Some of the results were reported in a previous paper [12]. Here, we only show the AFM images of the Nb surface morphologies due to the influence of Ar pressure and flow rate (figures 1 and 2).…”

“…Their films [10] became rougher again with increasing compressive stress after passing the stress-free threshold. Our films remain smooth with increasing compressive stress after the stress-free threshold (see figure 1 in [12]). Very smooth Nb films (150 nm) with R a 0.5 nm and P-V height 5 nm were obtained at a low Ar pressure of 0.38-0.85 Pa with an optimized flow rate and plasma power.…”

“…The recent developments in superconducting quantum bits [1][2][3][4] and detector technologies [5,6] based on Nb/AlO x -Al/Nb Josephson junctions require the junctions to have a very low leakage current in the sub-gap voltage region and high current uniformity across the junction area. The importance of the quality and surface morphology of the Nb thin film in determining the junction quality has been documented [7][8][9][10][11][12]. Some effort [8][9][10]12] has been directed towards optimizing film deposition techniques and conditions in order to obtain smooth film morphologies.…”

“…The importance of the quality and surface morphology of the Nb thin film in determining the junction quality has been documented [7][8][9][10][11][12]. Some effort [8][9][10]12] has been directed towards optimizing film deposition techniques and conditions in order to obtain smooth film morphologies. Thomas et al [11] reduced the base-Nb surface roughness by using multiple alternating Al and Nb layers and demonstrated much improved leakage characteristics.…”

“…In an earlier study, we investigated the influence of deposition parameters on the Nb surface morphology and the microstructure of the Nb/AlO x -Al/Nb trilayer [12]. In this work, we further investigate the role of Nb surface roughness on the sub-gap leakage of Nb/AlO x -Al/Nb junctions.…”