Auger electron spectroscopy, transmission electron microscopy, and scanning electron microscopy studies of Nb/Al/Nb Josephson junction structures

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Abstract
Al films deposited on Nb can be oxidized and used to make Josephson junction devices. We studied the structure of Al films deposited under ‘‘warm’’ (estimated to be near 200 °C) and ‘‘cold’’ (near room temperature) conditions because the cold films produced better Josephson junction devices. For the warm case, the Al film was composed of islands with open channels between them, which we attribute to a high mobility of the Al atoms that lowers the island nucleation density. The Nb surface was extremely flat, which we ascribe to the high surface atom mobility at the higher deposition temperature. The cold Al film was of uniform thickness which can be explained by a high island nucleation density. The cold Nb films had an undulating surface, caused by the lack of surface atom mobility during deposition. There was no evidence of Al-Nb interdiffusion, even after postdeposition heating to 300 °C. Auger spectroscopy, transmission electron microscopy, and scanning electron microscopy were needed to obtain these definitive conclusions.