Capacitance spectroscopy of alumina sol–gel capacitors with Al top contacts

We measured the complex capacitance C(f) of ITO/sol–gel alumina/Al capacitors deposited on glass (and some on stainless steel foil) in the frequency range 15 Hz–10 MHz. The sol–gel films were deposited by dip-coating and following a two-step process. The capacitance C(f) found was much higher than that of a pure Al₂O₃-film due to the remaining porosity of the film and the uptake of H₂O from the environment. The C(f) curves are useful as a sensitive probe for the porosity of the sol–gel film. In particular the evolution of the capacitance curve with time after drying has been measured. The curves can qualitatively be understood by modelling the capacitor as a (nearly) percolating random insulator/conductor network, using the effective medium approximation. However a detailed understanding of the relation between the C(f,t) curves and the structure of the films requires a more elaborate model. Films sintered at 500–550 °C for 1 h initially behave exactly as not sintered films but in contrast with the latter they improve slowly over time. Fast sintering yields better films.