Superconducting transmon qubits with niobium-buffered α-tantalum on sapphire substrates

Tantalum (Ta) is a promising material for superconducting qubits due to its stable chemical properties, formation of a single oxide, and resistance to reaction with other substances. However, achieving the desired \alpha-Ta typically requires high-temperature substrate heating, leading to a time-consuming process and stringent requirements for the film deposition system. Here we demonstrate the deposition of high-quality \alpha-Ta at room temperature using a niobium (Nb) film as a buffer layer. The buffered films are characterized using x-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Transmons with capacitors, transmission lines, and readout resonators fabricated with such Nb-buffered Ta exhibit excellent performance. We observed an "inflection point" in the dependence of the Q_\rm i of the resonators on the input microwave photons, which can be fitted well with an expression for the superposition of the buffered resonators. These experimental results show that the buffered films deposited at room temperature have promising applications in superconducting devices.