Anodization behavior of Al, and physical and electrical characterization of its oxide films

We have investigated the physical and electrical properties of anodic oxide films treated by a variety of aqueous electrolyte solutions. All anodic oxide diodes revealed consecutive current regimes varying with bias voltage as V, V2, and Vn (n≳2) in the steady-state current-voltage characteristics. Here, the transition voltage from the V2 to Vn current regime and the voltage exponent n varied depending on the oxide forming electrolyte and the bias direction of the diodes. These electrical properties were interpreted as a space-charge-limited current modified by a distributed gap state density in space and in energy. The oxide had a double-layer structure, a layer containing an anion characteristic of each electrolyte and an anion-free layer. The anion-containing layer had a varying gap state distribution. This was due to the anion characteristics, which reduce a deep-lying gap state density and modify a tail state density. We propose two practical anodizing techniques to control the depth profile of the anion. This allows us to form an oxide with a small leakage current equivalent to an anion-free control oxide. We also show that the dielectric loss for the anion-free oxide was reduced through the formation in an organic electrolyte and was increased in an inorganic electrolyte.