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Register a new userGe thin-films with tantalum diffusion-barriers for use in Nb-based superconductor technology
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Germanium thin films are an excellent candidate for use as a low-loss dielectric in superconducting microwave resonators, a low-loss inter-layer metal wiring dielectric, and passivation layers in microwave and Josephson junction devices. In Ge/Nb structures deposited at 400 {deg}C, we observe intermixing over as much as 20 nm. The addition of a 10 nm Ta diffusion barrier layer reduces the superconductor/dielectric intermixing to less than 5 nm and enhances the structural properties of deposited a-Ge layers based on Raman spectroscopy. Additionally, superconducting microwave resonators fabricated at room-temperature on crystalline Ge substrates with a Ta barrier layer show marked improvement in total and power-dependent two-level system microwave losses.
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We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high $I_cR_n$-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes lead and niobium, respectively. By tuning the barrier thickness we were able to change the junctions behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe$_2$As$_2$ thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5,nm we observe clear Josephson effects with $I_cR_n$,$approx$,90,$mu$V at 4.2,K. These junctions behave SNS-like and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0,nm and higher no Josephson but SIS- or SINS-like behavior with a tunnel-like conductance spectrum was observed.
Nb-doped SrTiO$_{3}$ epitaxial thin films have been prepared on (001) SrTiO$_{3}$ substrates using pulsed laser deposition. A high substrate temperature ($>1000^{circ}{C}$) was found to be necessary to achieve 2-dimensional growth. Atomic force microscopy reveals atomically flat surfaces with 3.9 AA $ $ steps. The films show a metallic behavior, residual resistivity ratios between 10 and 100, and low residual resistivity of the order of 10$^{-4}$$Omega$cm. At 0.3 K, a sharp superconducting transition, reaching zero resistance, is observed.
We report on the layer-by-layer growth of single-crystal Al2O3 thin-films on Nb (110). Single-crystal Nb films are first prepared on A-plane sapphire, followed by the evaporation of Al in an O2 background. The first stages of Al2O3 growth are layer-by-layer with hexagonal symmetry. Electron and x-ray diffraction measurements indicate the Al2O3 initially grows clamped to the Nb lattice with a tensile strain near 10%. This strain relaxes with further deposition, and beyond about 5 nm we observe the onset of island growth. Despite the asymmetric misfit between the Al2O3 film and the Nb under-layer, the observed strain is surprisingly isotropic.
We study the two-dimensional superconductor-insulator transition (SIT) in thin films of tantalum nitride. At zero magnetic field, films can be disorder-tuned across the SIT by adjusting thickness and film stoichiometry; insulating films exhibit classical hopping transport. Superconducting films exhibit a magnetic field-tuned SIT, whose insulating ground state at high field appears to be a quantum-corrected metal. Scaling behavior at the field-tuned SIT shows classical percolation critical exponents $z u approx$ 1.3, with a corresponding critical field $H_c ll H_{c2}$. The Hall effect shows a crossing point near $H_c$, but with a non-universal critical value $rho_{xy}^c$ comparable to the normal state Hall resistivity. We propose that high-carrier density metals will always exhibit this pattern of behavior at the boundary between superconducting and (trivially) insulating ground states.
We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing metalorganic precursor bis(cyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the films increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to the clean films. The self-field Jc in the carbon-doped films is lower than that in the clean films, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic field applications.
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