Superconducting and structural properties of Nb films covered by plasma enhanced chemical vapour deposited a-Si:H layers for superconducting qubit application

With a view to the fabrication of superconducting qubits with low decoherence time, we have investigated the influence of a-Si:H deposition by the plasma enhanced chemical vapor deposition method at 250◦C on the superconducting and structural properties of a 20 nm thick Nb film treated by two surface protection methods: plasma nitridation and deposition of a thin unhydrogenated Si layer. A suppression of theTc and an increase of the residual resistivity are observed due to hydrogen diffusion and decomposition of the native surface oxide, with subsequent oxygen diffusion caused by sample heating. The unhydrogenated Si layer is found to efficiently protect the Nb films against both diffusion processes.