Barrier diffusion and optical properties of the Au-Al2O3-Al thin-film system

The diffusion across the oxide barrier in a Au-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$Al thin-film sandwich at temperatures below 300 °C has been studied in real time with use of an angular-reflectance method. Assuming a uniformly mixed alloy, the Maxwell Garnett equation was used to calculate the concentrations in the Al-diffused–in–Au layer from the reflectance data. The ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ layer between the Au and Al films was formed by oxidation of Al in air. The rate of diffusion through the oxide did not have the usual inverse dependence on barrier thickness, but rather decreased exponentially with the oxide thickness and thus resembled the Al oxidation process. The data are consistent with a model in which Al ions migrate through the oxide under the influence of a strong electric field (about 5×${10}^6$ V/cm) caused by the difference in the Fermi levels of Au and Al. The change of the work function of the Au-Al alloy for increasing Al concentration in Au is also calculated from the data.