Oxidation process of Mg films by using high-concentration ozone for magnetic tunnel junctions

Ozone oxidization process of metal Mg film for the barrier formation in magnetic tunnel junctions (MTJs) is investigated. Ozone exposure method is expected to oxidize ultra-thin metal films more mildly than with the plasma oxidization method, since the energy level of atomic oxygen is ∼2 eV lower in the ozone method than in the plasma method. The main results were as follows: (1) In the case of ozone oxidation, the diffusion coefficient of oxygen in the insulator is much smaller than that in plasma oxidation. (2) Mg-O film thickness, which is formed by reaction immediately on the metal Mg surface, is thicker as compared with the Al case. (3) In the ozone oxidation method of metal films with the thickness of more than the film thickness formed by reaction, the oxidation is spontaneously stopped at the interface to the bottom Co-Fe. As a result, we succeeded in inducing a TMR ratio of 25% at room temperature in MTJs with Mg(1.3 nm)-O barrier with wider exposure range than in the plasma case.