Tunnel barriers on Pb-In-Au alloy films

Abstract

Tunnel barrier oxides consisting primarily of In2O3 formed on Pb-In-Au alloys by various thermal and rf-plasma oxidation techniques were characterized in situ using ellipsometry and Auger Electron Spectroscopy (AES). The results were compared to measurements of the Josephson critical current density j1 and the specific capacitance of tunnel junctions fabricated on the same wafers. It was found for junctions with oxides consisting entirely of In2O3 that j1 exceeded 3 KA/cm2 and did not depend strongly on oxide thickness from about 3 nm (for thermally-grown oxides) to 4.5-7 nm (for rf-grown oxides). The addition of a thin layer of PbOx at the top of the barrier, either by backscattering during rf oxidation or by deposition onto a thermal oxide, caused by a substantial decrease in j1, while the presence of PbO in the bulk of the oxide had little effect. The specific capacitance of the lower current density junctions was essentially the same for either 3- or 4.5-6-nm thick oxides and was consistent with a barrier thickness of order 2 nm. These results are interpreted as evidence that tunneling occurs through a Schottky barrier in the indium oxide at the interface with the counterelectrode. Such a barrier, which is consistent with the known behavior of In2O 3 as a degenerately-doped semiconductor, would account for effective tunneling and capacitive thicknesses less than the physical oxide thickness.