Pairs of Nb (110) and Re (0001) RHEED pictures showing the alignment of the two lattices. With the RHEED beam parallel to the Nb axis (a) the Re diffraction image was an asymmetric pattern, indicating a slight misalignment. Contrast this with RHEED along the Nb  direction (b) where the Re diffraction image was a symmetric pattern. Similarly, it was found that the Re axis was aligned with Nb (c). Note that all of the Re RHEED patterns had elongated streaks, and the image also showed a faint pattern. Both observations indicate domain growth with multiple in-plane orientations.
XRD scans from Nb/Re bilayers grown on A-plane sapphire. The scan exhibits single orientations for both metal layers: (110) for bcc Nb and (0001) for hcp Re. The scan of off-axis Re peaks reveals six different in-plane orientations of the Re layer, each of which are depicted here with respect to the underlying Nb (110) surface lattice. (Numbers on the diagrams correspond to the numbered peaks in the scan.) 98% of the Re film is comprised of KS- (1,2) and NW-oriented (3) domains.
AFM scans covering the full range of Re film thicknesses investigated. Bare Nb (110) was found to be atomically smooth, while Re (0001) overlayers exhibited evolving surface morphologies characterized by increasing roughness and average grain size. Scans shown here are all with the height scale shown at right.
The evolution of Re (0001) surface roughness as a function of film thickness as measured by AFM. Thin films less than about 150 Å thick retained the atomically smooth surface of the underlying Nb, while thicker films showed very rough surface topologies with large grains. The scan size used for these measurements was .
In-plane strain relaxation in Re (0001) films grown on Nb (110) as measured in situ with RHEED. The wafer’s azimuthal angle is fixed during Re growth, and assuming an initial surface of bulk-like Nb the lattice constant of the Re layer can be determined. The measured Re lattice spacing at large thickness agrees well with bulk values, with a fully relaxed lattice by about 200 Å.
A bright-field TEM image of the Nb/Re interface looking down the Nb axis. Atomic planes are clearly visible in both layers but the interface is not atomically sharp. Instead, there appears to be some mixing over a range of about five monolayers, which may responsible for the imperfect registration of the Re (0001) layer with the underlying Nb.
Schematic of a SIS “trilayer” employing buffer layers between both superconducting electrodes and the insulating tunnel barrier. While the structure shown here consists of five distinct layers, the structure is designed to behave electrically as a trilayer. The Nb/Re bilayers described in this letter may be utilized for the bottom electrode.
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