  We demonstrate that good superconductor/insulator/normal-metal tunnel
junctions can be fabricated on as-grown superconducting MgB2 thin films. The
as-grown films were prepared by coevaporation at low growth temperatures of
around 280deg. The insulating barrier was formed by an Mg overlayer, which is
subsequently oxidized in air. The tunneling spectra for Au/MgOx/MgB2 junctions
showed a reproducible and well-defined superconducting gap (delta= ~ 2.5 meV).
The resultant 2delta/ kBTc was significantly smaller than the predicted BCS
value of 3.52.
