Comparison between the properties of amorphous and crystalline Ta 2O 5 thin films deposited on Si

Abstract

In this study, the structural and electrical properties of amorphous and crystalline Ta 2O 5 thin films deposited on p-type Si by low pressure metalorganic chemical vapour deposition from a Ta(OC 2H 5) 5 source have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta 2O 5 (hexagonal phase) was obtained after post-deposition O 2-annealing at 800°C. Physico-chemical analysis of our layers shows that the O 2-treatment leads to the growth of a thin (∼1 nm) interfacial SiO 2 layer between Ta 2O 5 and Si but, contrary to other studies, was not sufficient to reduce the level of carbon and hydrogen contaminants. Crystalline Ta 2O 5 shows better leakage current properties than amorphous Ta 2O 5. The conduction mechanism in amorphous Ta 2O 5 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 Ta 2O 5, the situation remains unclear since no simple law can be invoked due to the presence of the SiO 2 interlayer: a double conduction process based on a tunnelling effect in SiO 2 followed by a trap-modulated mechanism in Ta 2O 5 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.