Aluminum oxide tunnel junctions: Influence of preparation technique, sample geometry and oxide thickness

Tunnel junctions of the type Al/Al oxide/Ag can emit hot electrons of about 2 eV to an adjacent electrolyte and cause redox reactions. We developed a production technique of such tunnel systems, which are stable at room temperature in atmosphere and in electrolyte. Two types of Al base electrodes (wires and evaporated films on glass), three types of oxide films (gas phase oxidation, anodic oxidation and physical vapour deposition) were combined with an Ag top electrode film of 15 nm thickness. PVD oxide films are porous with a large thickness distribution. This causes a rough silver top electrode and therefore a weak corrosion resistance. Stable oxide films were only obtained for the pairs Al wire/anodic oxide and Al film/gas phase oxide. The oxides are almost equal in their properties and form dense homogenous films independent of the sample geometry. Anodic films can be formed with various thicknesses, but ionic currents may exceed the tunnel currents. A successful layer sequence was Al film/gas phase oxide (2.5 nm)/Ag (15 nm) with an emission current of hot electrons of about 1 mA/cm2.