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

Samarium hexaboride (SmB6) belongs to a novel class of quantum matter known as topological Kondo insulators. By utilizing planar tunneling spectroscopy, signatures of spin excitons that interact with the topological surface states have been observed. To uncover additional details about this interaction, the effect on the characteristic features by plasma cleaning and ion beam etching processes, employed to clean the SmB6 crystal surface, are examined. Additionally, the effect of the plasma oxidation process used to form the tunnel barrier is analysed. Comparing the conductance spectra to simulations based on the Blonder-Tinkham-Klapwijk model revealed that the junctions with the highest resistances also had the weakest barrier strengths. Such seemingly contradictory observations may be explained by considering a barrier containing layers of B6O or B3O in addition to B2O3. This suboxide formation appears to be most dominant at higher ion beam energies as well as lower plasma oxidation powers.