Self-oxidation-formed boron oxide as a tunnel barrier in SmB6 junctions

Topological surface states in the Kondo insulator YbB12 revealed via planar tunneling spectroscopy

“…3b compares the sharpness of the Pb superconducting features observed in different junctions prepared with different parameters for the ion-beam etching and plasma oxidation. Similar sensitiveness of the junction quality to such processing parameters was also reported for SmB6 [67], wherein it was speculated that the layer of disrupted boron octahedra resulting from the ion beam etching process might be too thin if the ion beam energy is low or it becomes inhomogeneous if the ion beam energy is high. The asymmetry of the coherence peaks is highly reproducible across junctions irrespective of the details of the barrier formation, which originates from the intrinsic DOS of the YbB12, as seen below.…”

“…On the other hand, changing the processing parameters slightly result in a poor tunnel barrier (S2) with very different fitting parameters (d = 24.85 Å , H = 2.02 eV, and Γ = 3.13 eV) in which the barrier height is less than the barrier asymmetry. This further attests the high sensitivity of the barrier properties to the processing parameters [67].…”

“…Atomic force microscopy of the polished surfaces reveals a peak-to-dip roughness less than 10 Å (see Supplemental Material [66]), showing that smooth and clean crystal surfaces are achieved after polishing. These polished crystals are etched with an argon ion beam in a high vacuum chamber to remove any contaminants on the surface and to obtain a boron-rich top layer, which is then plasma-oxidized in the same chamber to form boron oxide to serve as the tunnel barrier [67]. Since the characteristics of the tunnel barrier are extremely sensitive to both ion-beam etching and plasma oxidation processes, the etching/oxidation power and durations were optimized through multiple runs.…”