Atomic Scale Characterization of Engineered Interfaces in Superconducting Qubits.

다국어 초록 (Multilingual Abstract)

Superconducting microwave circuits are a promising physical implementation of quantum computing. A significant challenge with the development of superconducting qubits is the materials source of loss leading to qubit decoherence. Much of this loss is dielectric loss or quasiparticle dissipation at superconductor surfaces and interfaces. In order to understand the loss mechanisms microscopically, it is necessary to have an accurate picture of the atomic structure and microstructure. In this work, we explore the structure of three interfaces. First, we present nondestructive X-ray characterization of the NbH surface precipitation in Nb thin films. Unwanted hydride precipitation in niobium-based superconducting circuits is a side effect of hydrofluoric acid etching of the Nb surface oxide. The precipitate microstructure is challenging to probe because of the high mobility of hydrogen in niobium. Using X-rays diffraction, we show evidence supporting phase-field simulations that the nucleation of NbH occurs at free surfaces. Using darkfield X-ray nanoprobe microscopy, we identify a complex microstructure suggesting a martensitic nucleation that transitions to a dendritic growth. Next, we present X-ray standing wave excited X-ray photoelectron spectroscopy of the annealed, Nb(110) ordered oxide surface layer. We discover the existence of an oxygen interstitial rich subsurface layer and identify the origin of two distinct oxygen chemical states at the surface: one coming from this subsurface layer, and the other from the surface NbO termination. Last, we present the heteroepitaxy of single crystal Al2O3 with a TiN. We identify TiN as an ideal substrate for the epitaxial growth of Al2O3 in a capacitor geometry based on the high degree of crystallinity and sharp interfaces with minimal diffusion and we measure the two-level-state loss of the Al2O3 junction dielectric layer. We present this interface as an alternative to the commonly used amorphous alumina in Josephson Junctions.