Superconducting Qubit Performance Enabled by Controlling Oxygen Segregation with 0.3 Precision

Oxygen impurities within superconducting materials represent a significant obstacle to building more robust quantum computers. Jaeyel Lee, Dieter Isheim, and Zuhawn Sung, alongside colleagues from the Superconducting Quantum Materials and Systems Center and Northwestern University, have now investigated oxygen distribution at the atomic scale within niobium thin films, a key material for superconducting qubits. Using a combination of atom-probe tomography and transmission electron microscopy, the team examined niobium films both with and without a tantalum encapsulation layer. Their research reveals that oxygen preferentially segregates at grain boundaries, and critically, that higher oxygen levels within the niobium itself exacerbate this segregation. This detailed understanding of oxygen behaviour offers a potential pathway to mitigate decoherence and improve the performance of superconducting quantum devices.

Oxygen Control in Nb and Ta Films

Oxygen distribution and segregation at grain boundaries in Nb and Ta-encapsulated Nb thin films for superconducting qubits is critically limited by oxygen impurities, which induce energy dissipation and decoherence. This work investigates oxygen distribution and segregation behaviour in Nb thin films encapsulated with either Nb or Ta, employing a combination of magnetron sputtering and atomic layer deposition. Characterisation using transmission electron microscopy and secondary ion mass spectrometry reveals that Ta encapsulation provides an effective barrier against oxygen diffusion, leading to improved material quality for superconducting qubit applications. Detailed analysis of the grain boundary structure indicates that oxygen segregation is strongly correlated with the presence of extended defects and amorphous regions, and a direct link between oxygen concentration at grain boundaries and the critical current density of the Nb films was established, demonstrating a reduction in critical current density with increasing oxygen content.

Oxygen Mapping in Nb Films via APT and

The study employed a multi-faceted approach, combining atom-probe tomography (APT) and transmission electron microscopy (TEM) to investigate oxygen distribution within Nb and Ta-encapsulated Nb thin films. Researchers fabricated 200nm thick Nb films with columnar grain diameters ranging from 20-50nm, subsequently analysing these samples using APT to reconstruct three-dimensional representations of the material at the atomic scale. This technique enabled precise mapping of oxygen segregation at grain boundaries relative to the oxygen concentration within the Nb grains. To quantify oxygen distribution, the team developed a proximity histogram methodology to obtain atomic concentration profiles across the surface oxide/Nb heterophase interface. APT reconstructions were performed on Nb thin film samples, with additional analyses conducted at depths of 25, 50, 75, and 100nm from the surface, allowing for depth-resolved observation of oxygen segregation.

Further investigation involved the creation of Nb thin films capped with a 10nm thick Ta layer, allowing scientists to assess the impact of this capping layer on oxygen behaviour. High-resolution ADF-STEM imaging revealed surface grooves at grain boundaries within the Ta capping layer, coinciding with underlying undulations in the Nb thin film. Subsequent APT analyses confirmed oxygen segregation not only at grain boundaries within the Ta layer, but also a continuation of this segregation along the grain boundaries of the underlying Nb film, with oxygen concentrations at grain boundaries reaching 6.0 ±0.6 at.%, compared to 2.1 ±0.2 at.% in the Nb grain interiors. The resulting enrichment factor for oxygen segregation in Nb films was determined to be 2.7 (±0.3), slightly reduced to 2.3 (±0.3) in Ta-capped films, while the Ta capping layer itself exhibited a higher enrichment factor of 3.0 (±0.3).

Oxygen Segregation at Grain Boundaries in Nb Films

Scientists achieved atomic-scale analysis of oxygen distribution within niobium (Nb) and tantalum-encapsulated niobium (Ta/Nb) thin films, employing both atom-probe tomography (APT) and transmission electron microscopy (TEM). Experiments revealed oxygen segregation at grain boundaries in both Nb and Ta-capped Nb films, with higher oxygen concentrations within the grains correlating to increased segregation at these boundaries. This finding underscores the importance of controlling oxygen impurities during Nb film deposition and fabrication to minimise oxygen accumulation. Detailed analysis of Nb films demonstrated an oxygen enrichment factor, the ratio of oxygen concentration at the grain boundary to that within the grains, of 2.7, with a margin of error of 0.3. Ta-capped Nb films exhibited slightly reduced enrichment factors of 2.3 (±0.3), while the Ta capping layer itself displayed higher enrichment factors reaching 3.0 (±0.3).

Researchers hypothesize that the Ta capping layer traps oxygen, influencing its diffusion and subsequent segregation at the grain boundaries within the underlying Nb films. Further investigations into a 200nm thick polycrystalline Nb film revealed a surface oxygen concentration of approximately 10 at.%, decreasing to 2.0 ±0.2 at.% within the bulk of the Nb. Oxygen concentration profiles across two grain boundaries, measured 60nm from the surface, showed local increases to 6.0 ±0.6 at.% and 5.5 ±0.6 at.%, compared to 2.0-2.2 ±0.2 at.% in the adjacent grain interiors. Depth-resolved analyses confirmed consistent enrichment factors around 2.6 ±0.3 along the grain boundary from the surface to a depth of 100nm, demonstrating sustained oxygen accumulation.

Notably, similar enrichment factors of 2.7 ±0.3 were measured for a second Nb film, despite a significantly lower oxygen concentration within the Nb grains of 0.6 at.%. This suggests that the segregation mechanism is robust, even with varying overall oxygen levels. The study also established a correlation between increased oxygen concentrations in both Nb grains and grain boundaries and a suppression of the critical temperature, indicating a potential mechanism contributing to decoherence within the material.

Oxygen Segregation Drives Boundary Enrichment in Niobium

Atomic-scale investigation using atom-probe tomography and transmission electron microscopy has revealed a clear relationship between oxygen concentration within niobium grains and oxygen segregation at grain boundaries in both bare and tantalum-capped niobium thin films. Researchers found that higher oxygen levels within the niobium grains directly correlate with increased oxygen segregation at the grain boundaries, with enrichment factors of 2.7 for bare niobium films. While tantalum capping slightly reduced this enrichment in the niobium, the tantalum layer itself exhibited even higher oxygen concentrations at its own grain boundaries. The significance of these findings lies in understanding the behaviour of oxygen impurities within superconducting materials. This study demonstrates that controlling oxygen content during the deposition and fabrication of niobium films is crucial for minimising oxygen segregation, a phenomenon linked to the suppression of critical temperature and potential decoherence in superconducting qubits and radio frequency cavities. The authors acknowledge limitations inherent in nanoscale analysis, but suggest future work could focus on exploring methods to further reduce oxygen impurities and mitigate their effects on superconducting performance.