The formation of amorphous alloy oxides as barriers used in magnetic tunnel junctions

The quality of a tunnel barrier is of critical importance for the success of a magnetic tunnel junction. An amorphous state of the precursor metal films is beneficial for the formation of an ultrasmooth and likely defect-free oxide tunnel barrier layer, thus leading to an improved performance of a magnetic tunnel junction. However, prior to oxidation, most sputter-deposited thin films of pure metals are polycrystalline instead of amorphous. In contrast, a sputter-deposited alloy thin film could have a better amorphous-forming ability than a pure metal. Using an Al-Zr binary as an example, the amorphous-forming ability of alloy thin films was investigated thermodynamically and experimentally. A thermodynamic model was formulated to predict the compositions of a binary alloy exhibiting favorable amorphous-forming abilities when sputter-deposited alloy thin films are made. The thermodynamically calculated compositions of Al-Zr alloys with great tendencies to form amorphous metallic thin films were confirmed experimentally. The sputter-deposited alloy thin films were characterized using transmission electron microscopy and x-ray diffraction. The thermodynamic argument, applicable to any binary systems, can serve as a guide to preselect alloy compositions of any selected metallic binary with high amorphous-forming ability as precursor metals to form an oxide tunnel barrier used in a magnetic tunnel junction.