Evidence for chemical annealing effects in indium oxide tunnel-junction barriers

Thermally grown native oxides of indium have the reputation of being rather poor-quality tunneling barriers when indium is used as the base electrode. We have developed a simple technique in which a freshly evaporated indium film is allowed to oxidize in the presence of acetic-acid vapor prior to the deposition of the lead counterelectrode. These easily reproduced tunnel junctions are of extremely high quality with Josephson critical current densities as high as 5000 A/cm2. Auger, electron energy loss, and ellipsometry studies reveal that there is a thinner and more uniform oxide coverage on the acid-oxidized films when compared to air-oxidized films. The absence of a pronounced Auger carbon peak on the tunnel junction samples, together with infrared evidence for acetate ion formation on indium samples exposed to significantly higher partial pressures of acetic-acid vapor, suggest that there is an absorption/desorption process in which a submonolayer amount of ionized acetic acid serves as a structural relaxation catalyst at oxide sites which are at higher energy than would occur in a uniform lattice.