Breakdown of ultrathin anodic valve metal oxide films in metal-insulator-metal-contacts compared with metal-insulator-electrolyte contacts

The anodic breakdown of thin valve metal oxide films on aluminium, hafnium, niobium, titanium, tantalum and zirconium in the system valve-metal/valve-metal-oxide/silver was investigated. For all systems valve metal wires covered by an anodically formed oxide with an evaporated silver film were used. For comparison three different types of oxide were used in the case of aluminium: anodic oxide, gas phase oxide and physical vapour deposited oxide. Due to an improved technique for the preparation a high reproducibility and reliability could be achieved. In the case of anodic oxide it is shown, that the formation field strength or reciprocal film formation factor and the breakdown field strength are equal. The initial step of anodic breakdown is clearly an ionic one. This was concluded from the strong correlation between film thickness and breakdown potential. An equation for the absolute quantitative calculation of tunnel currents is derived that takes the deformation of the barrier due to the image potential into account. The simulations are compared with the experimental results and the breakdown process is discussed in terms of ions which move into the tunnel barrier and deform the tunnel barrier.