A critical assessment of the Mott-Schottky analysis for the characterisation of passive film-electrolyte junctions

The widespread use of the Mott-Schottky plots to characterize the energetics of passive film/electrolyte junction is critically reviewed in order to point out the limitation of such approach in describing the electronic properties of passive film as well in deriving the correct location of the characteristic energy levels of the junction. The frequency dependency of M-S plots frequently observed in the experimental data gathered in a sufficiently large range of frequency is extensively discussed for a relatively thick (160 nm) thermally aged amorphous niobia (alpha-Nb2O5) film immersed in electrolytic solution. The relatively simple equivalent electrical circuit describing an ideally blocking behaviour of the junction allows a direct comparison of the experimental data analysis based on the use of Mott-Schottky or amorphous semiconductor Schottky barrier interpretative models. Moreover the theoretical simulations of the M-S plots based on the theory of crystalline semiconductor suggest an electronic structure of the investigated passive film containing a distribution of localized electronic states deep lying in energy in agreement with the model of amorphous semiconductor Schottky barrier.