Rupert Lewis

Probing the Nucleation of

Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators and contribute to the decoherence of the qubits. Development of alternative approaches for improved tunneling barriers becomes urgent and imperative. Atomic layer deposition (ALD) of aluminum oxide is a promising alternative to resolve the issue of oxygen vacancies in the tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. A critical issue in ALD of on metals is the lack of hydroxyl groups on metal surface, which prevents nucleation of the trimethylaluminum. In this work, we explore modifications of the aluminum surface with water pulse exposures followed by trimethylaluminum pulse exposures to assess the feasibility of ALD as a viable technique for JJ qubits. ALD films from 40 to 100 were grown on 1.4 to 500 of Al, and were characterized with ellipsometry and atomic force microscopy. A growth rate of 1.2 was measured, and an interfacial layer was observed. Because the interfacial layer thickness depends on the availability of Al and saturated at 2 nm, choosing ultrathin Al wetting layers may lead to ultrathin ALD tunneling barriers.

\hbox{Al}_{2}\hbox{O}_{3}

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

in Atomic Layer Deposition on Aluminum for Ultrathin Tunneling Barriers in Josephson Junctions

Niobium and niobium nitride thin films are transitioning from fundamental research toward wafer scale manufacturing with technology drivers that include superconducting circuits and electronics, optical single photon detectors, logic, and memory. Successful microfabrication requires precise control over the properties of sputtered superconducting films, including oxidation. Previous work has demonstrated the mechanism in oxidation of Nb and how film structure could have deleterious effects upon the superconducting properties. This study provides an examination of atmospheric oxidation of NbN films. By examination of the room temperature sheet resistance of NbN bulk oxidation was identified and confirmed by secondary ion mass spectrometry. Meissner magnetic measurements confirmed the bulk oxidation not observed with simple cryogenic resistivity measurements.

Fabrication of

Properties of NbN and Ta_xN thin films grown at ambient temperatures on SiO₂/Si substrates by reactive-pulsed laser deposition and reactive magnetron sputtering (MS) as a function of N₂ gas flow were investigated. Both techniques produced films with smooth surfaces, where the surface roughness did not depend on the N₂ gas flow during growth. High crystalline quality, (111) oriented NbN films with T_c up to 11 K were produced by both techniques for N contents near 50%. The low temperature transport properties of the Ta_xN films depended upon both the N₂ partial pressure used during growth and the film thickness. The root mean square surface roughness of Ta_xN films grown by MS increased as the film thickness decreased down to 10 nm.

\hbox{Nb/Al}_{2}\hbox{O}_{3}/\hbox{Nb}

Presents corrections to the paper, “Degradation of superconducting Nb/NbN films by atmospheric oxidation,” (Henry, M.D., et al), IEEE Trans. Appl. Supercond., vol. 27, no. 4, Jun. 2017, Art. no. 1100505.

Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

This work demonstrates the use of focused ion beam (FIB) implanted Ga as a lithographic mask for plasma etching of Nb films. Using a highly collimated Ga beam of a FIB, Nb is implanted 12 nm deep with a 14 nm thick Ga layer providing etch selectivity better than 15:1 with fluorine based etch chemistry. Implanted square test patterns, both 10 μm by 10 μm and 100 μm by 100 μm, demonstrate that doses above than 7.5 × 1015 cm−2 at 30 kV provide adequate mask protection for a 205 nm thick, sputtered Nb film. The resolution of this dry lithographic technique is demonstrated by fabrication of nanowires 75 nm wide by 10 μm long connected to 50 μm wide contact pads. The residual resistance ratio of patterned Nb films was 3. The superconducting transition temperature (Tc) = 7.7 K was measured using a magnetic properties measurement system. This nanoscale, dry lithographic technique was extended to sputtered TiN and Ta here and could be used on other fluorine etched superconductors such as NbN, NbSi, and NbTi.