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Atomic-scale characterization of quantum information material systems using correlative scanning transmission electron microscopy and atom probe tomography
Supple, Edwin J.
Supple, Edwin J.
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2024
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2025-11-26
Abstract
Material properties and defects are one key area reducing qubit performance. These material defects and features are predominantly nanoscale and their characterization requires techniques like scanning transmission electron microscopy (STEM) and atom probe tomography (APT).
Amorphous material has been identified as a host to detrimental two-level systems that reduce superconducting qubit performance by causing decoherence of the stored quantum state. Epitaxial growth of nitrides provides a path to amorphous oxide-free fabrication of Josephson junctions. In this study, the crystal structure and morphology of a NbTiN/AlN/NbTiN trilayer grown on sapphire by plasma-assisted molecular-beam epitaxy is studied by APT and STEM. The lower NbTiN layer and AlN layer grow epitaxially, but with variable thickness and pinhole shorts in the barrier. The NbTiN overlayer is polycrystalline, randomly oriented, and highly strained. Deviation from intended cation:cation and cation:anion ratios are observed in all layers.
Highly doped monolayers of B in Si have been found to be superconducting, and if atomically-precise lithography of the B $\delta$-layer is realized, could be used to fabricate dense arrays of qubits, and are additionally of interest in working towards the Kane qubit. In this study APT and STEM are used to determine B concentration and distribution in a $\delta$-layer grown in Si using adsorbed BCl$_3$ as a precursor dopant. The $\delta$-layer thickness is found to remain less than 2 nm, with areal concentrations near $10^{14}\ \text{atoms}/\text{cm}^2$ with nanoscale lateral variation. Limitations of APT characterization of B in Si due to B preferential retention are also explored.
A novel specimen preparation technique for APT, useful for regions of interest within a few-hundred nm of the sample surface as found in many quantum information systems is demonstrated. Permanent marker is used to apply a protective carbon coating before focused ion beam milling. After a specimen has been sharpened the remaining carbon is removed using a plasma cleaner, leaving the sample surface directly accessible at the end of the specimen. The technique is demonstrated on the B $\delta$-layer described above as well as Ge quantum wells in SiGe and CdSeTe solar cell devices.
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