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Abstract: We report measurements on single electron memory devices where the memory island, a floating gate, is charged through aluminum oxide tunnel barriers, fabricated through plasma oxidation of aluminum and atomic layer deposition (ALD) of aluminum oxide. These devices are characterized at 300mK and show a definite threshold for tunneling through the oxide barriers indicating a potential for nonvolatile memory. [Copyright &y& Elsevier]

Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit's structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. In this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at grain boundary, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • All Nb films deposited on Si exhibit columnar growth with strong [1 1 0] textures. • The amorphous surface NbO x layer is dominated by Nb 2 O 5 with suboxide(s) • The amorphous NbO x decomposed into nanocrystalline FCC Nb in the amorphous NbO matrix. • The observed larger lattice parameter of FCC Nb is due to O at its interstitial sites. • The original metal-oxide interface remains almost unchanged after the heat treatment. Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. Structural defects at the Nb/Si and air/Nb interface may be a major source of two-level systems (TLS), which are detrimental to the device's coherence time. Thus, identifying and understanding the microscopic origin of possible TLS in Nb-based devices and their relationship to processing is key to superconducting qubit performance improvement. This work studied the structure and thermal stability of surface oxide on physical vapor deposited Nb films on Si wafers, using aberration-corrected (scanning) transmission electron microscopy and spectroscopy. All Nb films exhibit columnar growth with strong [1 1 0] textures. After in-situ heating of the heterostructure at 360 °C inside the microscope, the initial amorphous niobium surface oxides decompose into face-centered cubic Nb nanograins in the amorphous Nb-O matrix, which may reduce microwave dissipation. Despite changes in the microstructure and chemistry of the niobium oxide surface layer due to heat treatment, the interface between the Nb and the surface oxide layer remains almost unchanged. Our comprehensive study of the Nb surface oxide decomposition mechanism may guide future superconducting qubit device optimization through interfacial scattering center and TLS minimization. [ABSTRACT FROM AUTHOR]