Steady and transient conduction processes in anodic tantalum oxide

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Abstract
Electrical conduction in anodic tantalum oxide films of thicknesses from 160 to 510 nm with counter electrodes of aluminium have been studied under steady and step-function transient voltage conditions. The system is strongly rectifying; and in forward bias with tantalum negative, the current is space-charge-limited with clear indications of progressive filling of deep traps finally giving way to a shallow-trap conduction process for which the effective carrier (electron) mobility is only 10−16 m2 V−1 s−1. The peaked form of the transient current with this polarity confirms that space-charge limitation is occurring and gives a direct measure of carrier mobility strongly influenced by traps and in agreement with the above. In reverse bias, Schottky-type, field-enhanced emission from the aluminium cathode is believed to occur with a workfunction of 044 eV, but the current is limited to low values by an aluminium-oxide interfacial barrier. The transient current decreases monotonically with this polarity and is taken as evidence for trap emptying. An energy band model for the oxide is deduced with a band of deep trapping levels separated from an effective hopping conduction band.