Electrode Effects on Aluminum Oxide Tunnel Junctions

The properties of evaporated Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-$M_x$ tunnel junctions have been investigated by systematic variation of the upper electrode metal ($M_x$) using comparison techniques on air-grown oxide layers of known uniformity. It was observed with the metals $M_x=\mathrm{N}\mathrm{i},\mathrm{C}\mathrm{u},\mathrm{A}\mathrm{l},\mathrm{A}\mathrm{g},\mathrm{A}\mathrm{u},\mathrm{S}\mathrm{n},\mathrm{S}\mathrm{b},\mathrm{P}\mathrm{b},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{B}\mathrm{i}$ that relative tunnel resistance increases with increasing atom size up to atomic radius approximately 1.6 Å, leveling off thereafter. The effect of large differences in evaporation temperature for the different metals was found to be small, and the influence of atom size was shown to be correlated with the spinel-like structure of the oxide. Contrary to the assumptions of other workers, the upper metal-oxide transition zone was found to exert a powerful influence on the properties of the junction. The influence of electrode work function was investigated using barium, and an average decrease in resistivity of the order predicted by the Holm-Kirschstein equations for an ambient barrier height of 3 to 4 ev was observed.