Difference between chemical structures of the interface at the Al-oxide tunneling barrier prepared by plasma or by radical oxidation

We have studied chemical structures of the interface between the Al-oxide tunneling barrier and the underlying Co₉₀Fe₁₀ layer in magnetic tunnel junctions when a 1-nm thick metallic Al barrier was oxidized by two different methods: plasma oxidation and radical oxidation. Our chemical analyses confirmed that the underlying CoFe layer was unavoidably attacked by oxygen during the oxidation and that this left different oxide states at the AlO_x/CoFe interface, depending on the oxidation method. The radical oxidation required long oxidation time for optimizing tunneling performance and resulted in a large amount of oxygen at the interface, which, in turn, resulted in the formation of mostly α-Fe₂O₃ and Al₂O₃. Conversely, the plasma oxidation required a relatively short oxidation time for optimization and left FeO as a dominant phase at the interface. Our results also show that the thermal treatment helped AlO_x, an oxygen-deficient phase, to be re-oxidized and transformed into Al₂O₃, the thermodynamically stable stoichiometric phase. The oxygen that diffused from the reduced CoFe layer into the barrier is likely responsible for this oxygen enrichment. We show that such differences in the chemical structure of the interface are critical clues to understanding what causes the change in tunneling properties of magnetic tunnel junctions.