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Modeling of differential admittance behaviour of thin amorphous semiconducting film

  • Authors: La Mantia, F; Stojadinovińá, J; Santamaria, M; Di Quarto, F
  • Publication year: 2012
  • Type: Proceedings (TIPOLOGIA NON ATTIVA)
  • Key words: electronic properties; thin oxide film; passive films; M-S theory; a-Nb2O5
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The understanding of the electronic properties of thin oxide film is an important step toward the understanding of the mechanisms of film dissolution and breakdown as well as for their application in the field of electrolytic capacitors and solar energy conversion. From this point of view the correct location of the characteristic energy levels (flat band potential, Ufb, and conduction (valence) band edge EC (EV)), of a passive film/electrolyte junction is the preliminary task for a deeper understanding of the mechanism of charge transfer at oxide/electrolyte interface. At this aim the most frequently employed method to locate such characteristic energy levels of semiconductor oxide/electrolyte junction is by means of differential capacitance measurements and their representation in the classical Mott-Schottky plots. Yet, as frequently reported in literature, passive films are usually very thin and with a variable degree of lattice disorder (from amorphous to polycrystalline phase) and both these characteristics should invalidate the use of the simple M-S theory (1,2,3). In order to highlight the role of film thickness on the differential admittance behaviour of thin anodic oxide/electrolyte interface we will present, in this work, the results of an extensive study on the electronic properties of amorphous niobia (a-Nb2O5) film anodically grown up to different thickness by anodizing in H2SO4 1 N solution at different formation potentials, namely 5, 10, and 20 V. The results of an electrochemical impedance spectroscopy (EIS) and differential admittance (DA) study in a large range of electrode potentials and ac frequency will be presented and discussed. The experimental results strongly evidence the limits of the traditional approach based on the M-S theory. Moreover a new model is proposed, to take into consideration both the amorphous nature of the film as well the effect of the metal back contact, when the space charge region in the oxide is approaching the metal/oxide interface. The experimental results evidenced that to cope with the simulated theoretical behaviour it is necessary to introduce a field-dependent dielectric constant for the a-Nb2O5 film as previously reported by Ord et al. and attributed to the electrostriction effect (2).