Ta2O5-gates of ph-sensitive devices : comparative spectroscopic and electrical studies

Abstract

Thin films of tantalum pentoxide (Ta2O5) were prepared on Si/SiO2 substrates by thermal oxidation of tantalum. In systematic oxidation studies we followed the growth of the Ta2O5/SiO2 interface. The oxide layers and their interfaces were characterized by SIMS, SAM, XPS, by comparative C-V measurements and by pH-(ISFET) sensitivities. Depending on the oxidation procedure, we find non-ideal stoichiometries of the Ta2O5/SiO2 interface, whose widths vary as a function of the oxidation time of the previously evaporated metallic tantalum. Specific annealing procedures lead to unexpectedly high leakage currents, which correlate with the formation of voids in the oxide layers. Even in the absence of voids, non-ideal interfaces provide high concentrations of electrically-active states in gate oxides of ISFETS, which in turn determine the results of C-V measurements and ISFET characteristics. For ideal stoichiometric and atomically abrupt interfaces, we observe long-term stability and ideal Nernstian behaviour in the pH-(ISFET) sensitivities.