Steady and transient conduction processes in anodic tantalum oxide

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 −1·6 m 2 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 0·44 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.