Elemental Semiconductor Encapsulation of Niobium Thin Films for Superconducting Qubits

Niobium is a widely used material for the fabrication of superconducting qubits, however it readily forms an amorphous native oxide. This amorphous native oxide, primarily composed of Nb₂O₅ but including other sub-oxides, has been shown to be a source of two-level systems (TLSs) and loss that result in decoherence.¹ In this study we explore encapsulation of Nb with elemental semiconductor materials. Based on materials loss measurements, amorphous Si is lower loss than Nb₂O₅ and this trend is extended even further with amorphous Ge, which is measured to have low loss in comparison to a large range of materials.^2,3 We explore a range of deposition methods for Si and Ge, including thermal and e-beam evaporation and pulsed laser deposition. Furthermore, we explore the effects of encapsulating Nb before ambient exposure vs oxide removal and subsequent encapsulation in later processing. X-ray diffraction, X-ray reflectivity, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and transmission electron microscopy techniques are used to characterize the materials interfaces and validate the effectiveness of Nb₂O₅ exclusion.

[1] Altoe, M., et al. PRX Quantum 3 (2022)

[2] McRae, C. R. H., et al. Rev. Sci. Instrum. 91 (2020)

[3] Kopas, C. J., et al. AIP Advances 11 (2021)