Potential barrier modification and interface states formation in metal-oxide-metal tunnel junctions

We show that the barrier profile of in situ grown ${\text{AlO}}_{x}$ tunnel barriers strongly depends on the material choices of the oxide-metal interface. By doing transport measurements on Al and Nb-based metal-oxide-metal tunnel junctions in a wide temperature range and using the phenomenological Simmons' model, we obtain barrier parameters that are qualitatively consistent with the values obtained from the first-principles calculations. The latter suggest that the formation of metal-induced gap states originating from the hybridization between the metallic bands and ${\text{Al}}_{2}{\text{O}}_{3}$ conduction band is responsible for the tunnel barrier modification. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.