Comparison between the properties of amorphous and crystalline Ta_2O_5 thin films deposited on Si

摘要

In this study, the strucutural and electrical properties of amorphous and crystalline Ta_2O_5 thin films deposited on p-type Si by low pressure metalorganic chemicla vapour deposition from a Ta(OC_2h_5)_5 souce 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 800degC. Physico-chemicla analysis of our layers shows that the O_2-treatment leads to the growth of a thin (～1nm) 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 hydrogne contraminants. Crystalline Ta_2O_5 is clearly attributed to the Poole-Frenkel effect with a barrier height separating the traps form 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 proccess 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.