In situ scanning tunneling microscopy observations of polycrystalline MgO(001) tunneling barriers grown on amorphous CoFeB electrode

Topological surface analysis using in situ scanning tunneling microscopy was performed for highly oriented polycrystalline (textured) MgO(001) tunneling barrier layers grown on amorphous CoFeB electrode layers. The microscopy revealed a MgO surface structure in which nanosized grains were dispersed on clusters that originated from the CoFeB underlayer. In situ annealing reduced this surface roughness. Local tunneling spectroscopy measurements revealed the formation of a nearly perfect and uniform tunneling barrier in spite of grain boundaries in the textured MgO(001) layer, which is consistent with the fact that textured CoFeB∕MgO∕CoFeB and fully epitaxial MgO-based magnetic tunnel junctions exhibit comparable spin-dependent tunneling properties.