Impact of junction oxidation parameters on the two-level system densities in superconducting qubits

Superconducting qubits are a promising platform to build large-scale quantum computers. To achieve such scalability, we require the ability to fabricate qubits with low decoherence and perform gate operations with high fidelities. However, superconducting qubits often suffer from the presence of two-level systems (TLSs), which originate from material imperfections and defects induced by various nanofabrication processes. Strongly-coupled TLSs are predicted to arise from defects in the amorphous oxide region of the commonly used Al/AlO_x/Al Josephson junctions. However, a comprehensive understanding of the relation between oxidation conditions, quality of the AlO_x layer and the density of observable defects is currently missing.

Here, we investigate the effects of junction oxidation parameters on the TLS densities and losses observed in transmon qubits containing Al/AlO_x/Al junctions. In this work, we explore how the temperature, pressure, and presence of plasma during oxidation affect the crystalline nature, chemical composition, and defects observed in these junctions. For this we utilize material analysis techniques such as X-ray diffraction, atomic force microscopy and transmission electron microscopy. Finally, we correlate it to the microwave performance of these qubits at cryogenic temperatures.