Selective oxidation of an individual layer in a magnetic tunnel junction through the use of thermodynamic control

Oxidation of an ultrathin metal layer (less than 1 nm) to form a tunnel barrier oxide, without oxidizing adjacent layers, is of critical importance in making nanoscale devices such as magnetic tunnel junctions. It is extremely difficult, if not impossible, to achieve this objective using conventional methods that rely on kinetic control of the oxidation process. We present an alternative approach using a gas mixture with a fixed chemical potential of oxygen as the oxidizing medium. This mixture, chosen with thermodynamic calculations, tends to uniformly oxidize the tunnel barrier to the thermodynamically favored stoichiometry without oxidizing the adjacent layers. Experiments on a model system show that a thin-film layer such as Al can be oxidized without oxidizing common ferromagnetic alloys, such as Co-Fe, using a mixture of CO2 CO or H2 H2 O. The chemical states of the Al and Co-Fe based example were characterized using x-ray photoelectron and synchrotron-source Fourier transform infrared spectroscopy.