Oxygen distribution and segregation at grain boundaries in Nb and Ta-capped Nb superconducting thin films

We investigated oxygen distribution and segregation at grain boundaries in Nb and Ta-capped Nb thin films for superconducting qubits employing atom probe tomography (APT) and transmission electron microscopy (TEM). We find a strong correlation between oxygen concentration in Nb grain interior and the amount of oxygen segregation at GBs in Nb. Higher oxygen concentration in Nb grain interiors correlates with higher oxygen segregation at GBs in Nb. This finding emphasizes that controlling oxygen impurity in Nb during film deposition and fabrication process is important to reduce the amount of oxygen segregation at GBs in Nb. Enrichment factors (C_GB/C_Grain) of oxygen segregation at GBs in Nb is 2.4-2.9 for Nb films, and Ta-capped Nb thin films tend to show slightly lower enrichment factors ranging from 1.6-2.4 within the Nb, indicating that Ta capping layer acts as a barrier for oxygen diffusion into underlying Nb films along GBs. Superconducting transport measurements indicate that increased oxygen concentration in the interior of Nb grains and at GBs results in reduced critical temperature (T_c) of Nb. Finally, we discuss the impact of oxygen impurities and segregation at GBs on the performance of Nb and Ta-capped Nb thin film superconducting qubits.