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Register a new userTunneling spectroscopy of superconducting MoN and NbTiN grown by atomic layer deposition
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A tunneling spectroscopy study is presented of superconducting MoN and Nb$_{0.8}$Ti$_{0.2}$N thin films grown by atomic layer deposition (ALD). The films exhibited a superconducting gap of 2meV and 2.4meV respectively with a corresponding critical temperature of 11.5K and 13.4K, among the highest reported $T_c$ values achieved by the ALD technique. Tunnel junctions were obtained using a mechanical contact method with a Au tip. While the native oxides of these films provided poor tunnel barriers, high quality tunnel junctions with low zero bias conductance (below $sim$10%) were obtained using an artificial tunnel barrier of Al$_2$O$_3$ on the films surface grown $textit{ex situ}$ by ALD. We find a large critical current density on the order of $4times 10^6$A/cm$^2$ at $T=0.8T_c$ for a 60nm MoN film and demonstrate conformal coating capabilities of ALD onto high aspect ratio geometries. These results suggest the ALD technique offers significant promise for thin film superconducting device applications.
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Despite many efforts the origin of a ferromagnetic (FM) response in ZnMnO and ZnCoO is still not clear. Magnetic investigations of our samples, not discussed here, show that the room temperature FM response is observed only in alloys with a non-uniform Mn or Co distribution. Thus, the control of their distribution is crucial for explanation of contradicted magnetic properties of ZnCoO and ZnMnO reported till now. In the present review we discuss advantages of the Atomic Layer Deposition (ALD) growth method, which enables us to control uniformity of ZnMnO and ZnCoO alloys. Properties of ZnO, ZnMnO and ZnCoO films grown by the ALD are discussed.
A method to treat the surface of Nb is described which potentially can improve the performance of superconducting RF cavities. We present tunneling and x-ray photoemission spectroscopy (XPS) measurements at the surface of cavity-grade niobium samples coated with a 3 nm alumina overlayer deposited by Atomic Layer Deposition (ALD). The coated samples baked in ultra high vacuum (UHV) at low temperature reveal at first degraded superconducting surface. However, at temperatures above 450C, the tunneling conductance curves show significant improvements of the superconducting density of states (DOS) compared with untreated surfaces.
Atomic layer deposition was used to synthesize niobium silicide (NbSi) films with a 1:1 stoichiometry, using NbF5 and Si2H6 as precursors. The growth mechanism at 200oC was examined by in-situ quartz crystal microbalance (QCM) and quadrupole mass spectrometer (QMS). This study revealed a self-limiting reaction with a growth rate of 4.5 {AA}/cycle. NbSi was found to grow only on oxide-free films prepared using halogenated precursors. The electronic properties, growth rate, chemical composition, and structure of the films were studied over the deposition temperature range 150-400oC. For all temperatures, the films are found to be stoichiometric NbSi (1:1) with no detectable fluorine impurities, amorphous with a density of 6.65g/cm3, and metallic with a resistivity {rho}=150 {mu}{Omega}.cm at 300K for films thicker than 35 nm. The growth rate was nearly constant for deposition temperatures between 150-275oC, but increases above 300oC suggesting the onset of non-self limiting growth. The electronic properties of the films were measured down to 1.2K and revealed a superconducting transition at Tc=3.1K. To our knowledge, a superconducting niobium silicide film with a 1:1 stoichiometry has never been grown before by any technique.
Rapid proliferation of hyperspectral imaging in scanning probe microscopies creates unique opportunities to systematically capture and categorize higher dimensional datasets, toward new insights into electronic, mechanical and chemical properties of materials with nano- and atomic-scale resolution. Here we demonstrate similarity learning for tunneling spectroscopy acquired on superconducting material (FeSe) with sparse density of imperfections (Fe vacancies). Popular methods for unsupervised learning and discrete representation of the data in terms of clusters of characteristic behaviors were found to produce inconsistencies with respect to capturing the location and tunneling characteristics of the vacancy sites. To this end, we applied a more general, non-linear similarity learning. This approach was found to outperform several widely used methods for dimensionality reduction and produce a clear differentiation of the type of tunneling spectra. In particular, significant spectral weight transfer likely associated with the electronic reconstruction by the vacancy sites, is systematically captured, as is the spatial extent of the vacancy region. Given that a great variety of electronic materials will exhibit similarly smooth variation of the spectral responses due to random or engineered inhomogeneities in their structure, we believe our approach will be useful for systematic analysis of hyperspectral imaging with minimal prior knowledge, as well as prospective comparison of experimental measurements to theoretical calculations with explicit consideration of disorder.
Atomic Layer Deposition (ALD) is a promising technique for producing Josephson junctions (JJs) with lower defect densities for qubit applications. A key problem with using ALD for JJs is the interfacial layer (IL) that develops underneath the tunnel barrier. An IL up to 2 nm forms between ALD Al2O3 and Al. However, the IL thickness is unknown for ALD films less 1 nm. In this work, Nb-Al-ALD-Al2O3-Nb trilayers with tunnel barriers from 0.6 - 1.6 nm were grown in situ. Nb-Al-AlOx-Nb JJs with thermally oxidized tunnel barrier were produced for reference. RN was obtained using a four-point method at 300 K. JC, and its dependence on barrier thickness, was calculated from the Ambegaokar-Baratoff formula. The Al surface was modeled using ab initio molecular dynamics to study the nucleation of Al2O3 on Al. Current voltage characteristics were taken at 4 K to corroborate the room temperature measurements. Together, these results suggest that ALD may be used to grow an ultrathin, uniform tunnel barrier with controllable tunnel resistance and JC, but a thin IL develops during the nucleation stage of ALD growth that may disqualify Al as a suitable wetting layer for ALD JJ based qubits.
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