Depth-dependent electrical impedance distribution in Al2O3 films on Al(111)-detection of an inner barrier layer

A method to probe the depth dependence of the electrical impedance of an Al 2O3 film on Al metal has been developed. The deposition of high electron af finity species (O-3, NO2, Cl-2) at 90 K on the outer surface of an oxide fi lm (d = 20-25 Angstrom) on Al(111) produces negatively charged adsorbate mo lecules as a result of the trap-mediated electron tunneling from the metal. The capacitor produced in this way exhibits an electrostatic field gradien t across the Al2O3 film which has been depth profiled by measuring the shif ts in the Al3+(2p) and O2-(1s) X-ray photoelectron spectroscopy (XPS) featu res originating from the film. Analysis of the XPS peak shifts and shape va riation shows that most of the potential gradient exists in the inner thin layer of Al2O3 (d = 7 Angstrom) adjacent to the metal surface. We postulate that the inner oxide layer detected here is the crucial region for corrosi on protection of Al.