MgB$_{2}$ Tunnel Junctions with Native or Thermal Oxide Barriers

MgB$_{2 }$tunnel junctions (MgB$_{2}$/barrier/MgB$_{2})$ were fabricated using oxides of Mg and stoichiometric MgB$_{2}$, as the tunnel barrier. The sum of the superconducting gaps ($\pi $-gap) observed in conductance-voltage (G-V) measurements was as high as 4.3mV at 4.2 K and a finite value was found for temperatures above 30K. The G-V data exhibit smeared BCS densities of states, indicative of a degraded layer at the electrode/barrier interface. The presence of such an interface might also explain the lack of supercurrents above 20K in junctions exhibiting gap structures above 30K and even in shorted junctions. A subgap current was also observed and was not found to strongly depend on the oxide stoichiometry.~