Highly charged ion interactions with thin insulating films

The electrical conductance of magnetic tunnel junction (MTJ) devices whose ultra-thin aluminum oxide tunnel barriers were irradiated by highly charged ions (HCIs) increases linearly with the fluence of HCIs, while retaining a current-voltage relationship indicative of a tunnel junction. The slope of the MTJ conductance σc as a function of fluence varies with different tunnel barrier thicknesses d, levels of oxidation (stoichiometry) and charge state q. Since the MTJ conductance after HCI irradiation is due to tunneling, the increased conductance can result from thinning the barrier, reducing the effective tunnel barrier height φ, or both. Measurements of the current-voltage profile provide sufficient degrees of freedom to substantially constrain d and φ provided the reduction of the barrier remains within the assumptions of the commonly used WKB (Wentzel-Kramers-Brillouin) tunneling formalism. For the Xe32+ ions discussed here the perturbation of the tunnel barrier is much weaker than in our previously reported measurements of Xe44+ and application of WKB is still reasonable. This analysis reveals a trend of decreasing d while φ changes little.