Synchrotron-Based X-ray Reflectivity of Al/AlO_x/Al Josephson Junctions

Aluminum oxide Josephson junctions are critical to most modern superconducting qubits and yet little is known about how their atomic-scale structure impacts qubit performance. While TEM is a common recourse for insight into the structure of the ~ 2 nm oxide barrier, characterization methods which sample large areas afford better prospects for correlating junction microstructure with qubit batch performance. We will present synchrotron-based X-ray reflectivity measurements that reveal the thickness, density profile, and roughness of Al/AlO_x/Al junctions, with sub-nanometer thickness resolution, sampled over centimeter-scale areas. Analysis of reflectivity data suggests asymmetry in the electron density profile of the junction layer. Departures of the electron density profile from that of either bulk Al or bulk Al₂O₃ can be attributed to incommensurate nucleation and growth of Al on top of the oxidized bottom Al film. Beyond this model Al/AlOx/Al system I will discuss our results examining the Ta-Al interface and other structures relevant to contemporary qubit design. Our results demonstrate the utility of XRR in assessing the structural composition and heterogeneity of JJs.