Isotropic plasma-thermal atomic layer etching of superconducting TiN films using sequential exposures of molecular oxygen and SF6/_6/H2_2 plasma

Microwave loss in superconducting titanium nitride (TiN) films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with Angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications; or the etch rate lacks the desired control. Further, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF$_6$/H$_2$ plasma. For certain ratios of SF$_6$:H$_2$ flow rates, we observe selective etching of TiO$_2$ over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 \r{A}/cycle at 150 $^\circ$C to 3.2 \r{A}/cycle at 350 $^\circ$C using ex-situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits.Comment: 17 pages, 7 figure

Directional atomic layer etching of MgO-doped lithium niobate using sequential exposures of H2_2 and SF6_6 plasma

Lithium niobate (LiNbO$_3$, LN) is a ferroelectric crystal of interest for integrated photonics owing to its large second-order optical nonlinearity and the ability to impart periodic poling via an external electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is presently limited by optical loss arising from corrugations between poled regions and sidewall surface roughness. Atomic layer etching (ALE) could potentially smooth these features and thereby increase photonic performance, but no ALE process has been reported for LN. Here, we report a directional ALE process for $x$-cut MgO-doped LN using sequential exposures of H$_2$ and SF$_6$/Ar plasmas. We observe etch rates up to $1.01 \pm 0.05$ nm/cycle with a synergy of $94$%. We also demonstrate ALE can be achieved with SF$_6$/O$_2$ or Cl$_2$/BCl$_3$ plasma exposures in place of the SF$_6$/Ar plasma step with synergies above $90$%. When combined with a wet post-process to remove redeposited compounds, the process yields a 50% decrease in surface roughness. With additional optimization to reduce the quantity of redeposited compounds, these processes could be used to smoothen surfaces of TFLN waveguides etched by physical Ar$^+$ milling, thereby increasing the performance of TFLN nanophotonic devices or enabling new integrated photonic capabilities

Demonstration of Universal Parametric Entangling Gates on a Multi-Qubit Lattice

We show that parametric coupling techniques can be used to generate selective entangling interactions for multi-qubit processors. By inducing coherent population exchange between adjacent qubits under frequency modulation, we implement a universal gateset for a linear array of four superconducting qubits. An average process fidelity of $\mathcal{F}=93\%$ is estimated for three two-qubit gates via quantum process tomography. We establish the suitability of these techniques for computation by preparing a four-qubit maximally entangled state and comparing the estimated state fidelity against the expected performance of the individual entangling gates. In addition, we prepare an eight-qubit register in all possible bitstring permutations and monitor the fidelity of a two-qubit gate across one pair of these qubits. Across all such permutations, an average fidelity of $\mathcal{F}=91.6\pm2.6\%$ is observed. These results thus offer a path to a scalable architecture with high selectivity and low crosstalk

Configurational Thermodynamics of Alloyed Nanoparticles with Adsorbates

Changes in the chemical configuration of alloyed nanoparticle (NP) catalysts induced by adsorbates under working conditions, such as reversal in core–shell preference, are crucial to understand and design NP functionality. We extend the cluster expansion method to predict the configurational thermodynamics of alloyed NPs with adsorbates based on density functional theory data. Exemplified with PdRh NPs having O-coverage up to a monolayer, we fully detail the core–shell behavior across the entire range of NP composition and O-coverage with quantitative agreement to in situ experimental data. Optimally fitted cluster interactions in the heterogeneous system are the key to enable quantitative Monte Carlo simulations and design

Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation

Carbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior Here we report a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)(3) precursor in the presence of poly(vinylpyrrolidone) stabilizer The measurement of catalytic activity of CO oxidation over two-dimensional Ru NPs arrays under oxidizing reaction conditions (40 Torr CO and 100 Torr O(2)) showed an activity dependence on the Ru NP size The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts The results gained from this study will provide the scientific basis for future design of Ru-based oxidation catalysts.close8

Intrinsic Relation between Catalytic Activity of CO Oxidation on Ru Nanoparticles and Ru Oxides Uncovered with Ambient Pressure XPS

Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle model catalysts were synthesized in the presence of poly­(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir–Blodgett deposition technique. Mild oxidative and reductive characteristics indicate the formation of surface oxide on the Ru nanoparticles, the thickness of which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50–200 °C under reaction conditions, which appears to be correlated with the higher catalytic activity of the bigger nanoparticles. We found that the smaller Ru nanoparticles form bulk RuO₂ on their surfaces, causing the lower catalytic activity. As the size of the nanoparticle increases, the core–shell type RuO₂ becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications

High Catalytic Activity in CO Oxidation over MnOx Nanocrystals

Manganese oxides of various stoichiometry were prepared via Mn-oxalate precipitation followed by thermal decomposition in the presence of oxygen. A nonstoichiometric manganese oxide, MnOx (x = 1.61…1.67) was obtained by annealing at 633 K and demonstrated superior CO oxidation activity, i.e. full CO conversion at room temperature and below. The activity gradually decreased with time-on-stream of the reactants but could be easily recovered by heating at 633 K in the presence of oxygen. CO oxidation over MnOx in the absence of oxygen proved to be possible with reduced rates and demonstrated a Mars—van Krevelen—type mechanism to be in operation. A TEM structural analysis showed the MnOx phase to form microrods with large aspect ratio which broke up into nanocrystalline manganese oxide (MnOx) particles with diameters below 3 nm and a BET specific surface area of 525 m2/g. Annealing at 798 K rather than 633 K produced well crystalline Mn2O3 which showed lower CO oxidation activity, i.e. 100% CO conversion at 335 K. The catalytic performance in CO oxidation of various Mn-oxides either studied in this work or elsewhere was compared on the basis of specific reaction rates.info:eu-repo/semantics/publishe