Origin of asymmetry of tunneling conductance in CoFeB∕MgO∕CoFeB tunnel junction

Abstract

We investigated the top and bottom interfaces of a CoFeB∕MgO∕CoFeB tunnel junction using transmission electron microscope (TEM) and x-ray photoemission spectroscopy (XPS) in order to understand the origin of the asymmetry of dI∕dV in terms of bias polarity. It was found, from a TEM image, that there is no clear cut at the top interface, while the bottom interface has relatively clean boundary. Furthermore, XPS data show that more hydroxides were formed at the top interface than at the bottom interface. These indicate that the hydroxides would hinder the epitaxial crystallinity at the interface in CoFeB∕MgO∕CoFeB tunnel junctions. Therefore, it is most likely that the asymmetry of dI∕dV is caused by the disappearance of minority Bloch state, which is closely correlated with the existence of hydroxides at the top interface of a CoFeB∕MgO∕CoFeB tunnel junction.