Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB₂

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB₂ by measuring planar tunnel junctions made from epitaxial MgB₂ thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB₂ thin films on SiC substrates which had T_c values over 40 K due to epitaxial tensile strain. As the MgB₂ film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB₂ film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB₂ film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.