Using Atomic Layer Deposition for Josephson Junction Quantum Bits

Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic point defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators that contribute to the decoherence of the qubits. Atomic Layer Deposition (ALD) of aluminum oxide (Al$_{2}$O$_{3})$ is a promising alternative to resolve the issue of oxygen vacancies in the Al$_{2}$O$_{3\, }$tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. ALD has been implemented in a high-vacuum magnetron sputtering system for \textit{in situ} deposition of ALD-Al$_{2}$O$_{3}$ tunneling barriers in superconductor-insulator-superconductor (SIS) JJs. The modifications made to the Al surface during ALD were explored with ellipsometry and atomic force microscopy, and ALD-Al$_{2}$O$_{3}$ barriers were grown on Nb to form Nb/Al2O3/Nb JJs. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al$_{2}$O$_{3}$ barrier layer.