A study of microstructure of Nb/AlOx-Al/Nb Josephson junctions by cross-sectional transmission electron microscopy yielded much information regarding the junction barrier region. Both thick Nb and several-nanometer Al form polycrystalline films with columnar structures. Nb is oriented to the (110) plane, and Al is (111). The 200 nm lower Nb has a wavy surface with ∼5 nm smoothness, but its surface is planarized by several nanometers Al deposited on it. Thus AlOx with a smoothness under 1 nm can be formed on Al. The upper Nb has a good crystalline structure even just above the AlOx barrier.
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Imamura et al. (1991) studied this question.
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