(Invited) Characterization of Thin Passive Film-Electrolyte Junctions. Amorphous Semiconductor (a-SC) Schottky Barrier Approach
Authors: Di Quarto Francesco , Di Franco Francesco , Miraghaei Shahab , Santamaria Monica , La Mantia Fabio 
Published: February 2020

The knowledge of the solid-state properties of passive film is a preliminary task for a full understanding of the electron and ion transfer processes at the metal/oxide and oxide/electrolyte interface. Both processes are of paramount importance in determining the mechanism of film growth and dissolution as well as in determining the nature of the breakdown during the growth of anodic oxide films or the onset of generalized or localized corrosion process (1-2). With very few exceptions, it is a common belief, that most of anodic oxide films, grown on metals and alloys in aqueous solutions, display a semiconducting or insulating behaviour. It is also very well known that in many cases the initial air-formed or electrochemically grown films have a highly defective or amorphous structure. In any case the location of the characteristic energy levels of the metal/passive-film/electrolyte junction is a necessary task to reach a deeper understanding of the mechanism of charge transfer at the metal/oxide/electrolyte interface so that it is not surprising that in-situ electrochemical techniques like electrochemical impedance spectroscopy (EIS) and differential admittance (DA) have been routinely employed to get information both on the electrical properties as well as on the mechanism of growth of thin and thick oxide films. In order to account for the amorphous or scarcely crystalline nature of anodic oxide films we have suggested, in several papers (3-5), an approach to the description of physicochemical properties of semiconducting passive films based on the theory of amorphous semiconductors. We have stressed in previous papers that the characterization of semiconducting passive film/electrolyte junction based on the use of simple M-S theory to get information on the solid state properties of anodic films presents some drawbacks in terms of assumptions as well as for a correct location of the characteristics energy levels of the passive film/electrolyte junction: flat band potential, Ufb, and the nearest characteristic level of the semiconductor EC (n-type SC) or EV (p-type SC). In an attempt to overcome the intrinsic limits of this traditional approach we advocated the use of the theory of a-SC Schottky barrier as developed in (6-10). In more recent papers (4-5) we try to extend the results of the a-SC Schottky barrier theory to the case of thin film amorphous semiconductors, i.e. when the space charge region of the semiconductor reaches an extension near to the 70% of the film thickness. In the last paper (5b) we introduced also a new parameter directly related to the series resistance of the space charge region which allows to extract information on the homogeneity of the DOS distribution within the space charge region as well as on the relative weight of the volumetric charge presents in the semiconductor with respect to the surface charge located on the metal. Although some very preliminary results strongly supported the novel proposed approach to the use of a-SC theory for thin semiconducting film, a more detailed experimental study was undertaken in order to exploit the robustness of our approach to the case of thin semiconducting anodic films. In this work the results of an extensive characterization, by means of EIS and DA technique, of thin semiconducting passive films grown on Ti metal will be presented in order to highlight the validity of the proposed approach. Further information on the amorphous nature and on the possible dependence of the dielectric constant on the electric field will be taken into account in an attempt to rationalize the complex behaviour of barrier-like passive film on Ti.

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