Superconducting energy gap features of MgB$_{2}$ thin films on different substrates and orientations

We report a detailed study of tunneling spectra of MgB$_{2}$/I/Pb planar junctions with MgB$_{2}$ films on various substrates and of various doping levels. Planar trilayer junctions were fabricated using MgB$_{2}$ films with native oxide barrier grown by the Hybrid Physical-Chemical Vapor Deposition technique. Both $\pi $ and $\sigma $ bands contribute to the tunneling spectra of tilted-axis films on MgO (211) substrate and mainly $\pi $ band was observed on c-axis MgB$_{2}$ films on SiC (0001), MgO (111) and c-sapphire substrates. We observed $\sigma $ gap value of $\sim $7.9 meV in MgB$_{2}$ films on SiC substrates which display higher T$_{c}$ due to the lattice strain. This is larger than 7.4 meV on unstrained substrates. However, the $\pi $ gap value of all samples is $\sim $2.3 meV. We concluded that the strain in MgB$_{2}$ films on SiC substrates mainly affects the $\sigma $ band of MgB$_{2}$. In addition, small amount of nitrogen gas was added during film growth to introduce more scattering in MgB$_{2}$ films. We systematically studied the change of two gaps with nitrogen doping from the tunneling spectra of MgB2/I/Pb junctions.