Structure of tunnel barrier oxide for Pb-alloy Josephson junctions

The oxide formed on Pb-In and Pb-In-Au alloy films by processes similar to those used to fabricate oxide tunnel barriers for experimental Josephson junction devices has been investigated with transmission electron microscopy and diffraction (TEM/TED), Auger electron and x-ray photoelectron spectroscopies (AES and XPS), and ellipsometry. Thermal oxidation of Pb-In(13 at%) alloys at room temperature results in a noncrystalline oxide, whereas oxides formed at ≥60°C in low pressures of O2, result in a continuous stable epitaxial layer of cubic In2O3 ≅2.5 nm thick. The oxide formed by sputtering such a thermal oxide in an rf-excited O2, glow discharge (rf oxidation) results in a layered structure ≅6.5 nm thick, the bulk of which consists of an upper layer of epitaxial In2O3 and a lower layer of crystalline orthorhombic and tetragonal PbO. The thickness of the PbO layer depends on the availability of In at the metal-oxide interface, and thus, on the alloy composition and the temperature and rate of oxidation. For In concentrations above ≅18 at%, the bulk of the oxide was found to be entirely epitaxial In2O3. An additional ≅0.3-nm-thick surface layer of PbO is observed, which arises from material sputtered from the Pb-coated rf electrode and subsequently backscattered onto the surface of the oxide. Altering this backscattered material from lead oxide to indium oxide increases the current densities of completed junctions by more than a factor of 40. In contrast, variations in the composition of the lower portions of the oxide have little effect on the junction characteristics. Factors affecting the composition and reproducibility of the oxide are discussed.