The Distribution of A‐C Resistance in Oxide Films on Aluminum

Abstract

Films of produced by anodizing superpurity aluminum at 20v in neutral ammonium tartrate are thinned uniformly when placed in sodium chromate solution of pH range 7–9. During film thinning the metal remains passive and the inherent specific resistance of the film substance is not modified by the dissolution process. Uniform thinning can continue from an initial film thickness of around 240Aå down to the passive film thickness 20–25Aå. Measurement of capacitance and dielectric loss at frequencies of 1000 to 100,000 cps during thinning has permitted determination of the variation of electronic and ionic resistivity throughout the majority of the thickness of the alumina films. Corrections for a‐c resistance of the solution have been found to be unnecessary at 1000 cps but mandatory at 100,000 cps in order to obtain accurate results. The films possess a region of low electronic resistivity up to 60–80Aå from the oxide‐metal interface. At greater thickness, electronic resistivity is constant. Ionic resistivity is only constant across a central region of the film around 100Aå thick. Nearer to the oxide solution interface, the ionic resistivity falls gradually and then rises sharply in the outer 20Aå of the film. A defect model is advanced to account for this resistivity distribution.