Comparison between the properties of amorphous and crystalline Ta2O5 thin films deposited on Si

In this study, the structural and electrical properties of amorphous and crystalline Ta2O5 thin films deposited on p-type Si by low pressure metalorganic chemical vapour deposition from a Ta(OC2H5)5 source have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta2O5 (hexagonal phase) was obtained after post-deposition O2-annealing at 800°C. Physico-chemical analysis of our layers shows that the O2-treatment leads to the growth of a thin (~l nm) interfacial SiO2 layer between Ta2O5 and Si but, contrary to other studies, was not sufficient to reduce the level of carbon and hydrogen contaminants. Crystalline Ta2O5 shows better leakage current properties than amorphous Ta2O5. The conduction mechanism in amorphous Ta2O5 is clearly attributed to the Poole-Frenkel effect with a barrier height separating the traps from the conduction band of ~0.8 eV. For crystalline Ta2O5 the situation remains unclear since no simple law can be invoked due to the presence of the SiO2 interlayer: a double conduction process based on a tunnelling effect in SiO2 followed by a trap-modulated mechanism in Ta2O5 may be invoked. From capacitance-voltage measurements, the permittivity was found to be ~25 for amorphous samples, but values ranging from 56 to 59 were found for crystalline layers, suggesting a high anisotropic character.