Abstract
The growth of very thin aluminum oxide and aluminum nitride tunnel barriers on top of an about 7 nm aluminum layer is deduced by measurements of the reflectivity change of a laser beam due to the decrease of the aluminum laser thickness. Within the first seconds of the process thermal aluminum oxide grows much faster than aluminum nitride in a nitrogen plasma. For both barrier types the reflectivity change can be correlated with the Josephson current density of the finished junctions. In a semi-logarithmic scale the current density versus reflectivity change can be approximated by a straight line up to 20 kA/cm 2. High current densities with AIN x seem to be more easily controllable than with AlO x .
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