Effect of tantalum content on the structural properties and sensing performance of YbTa_xO_y electrolyte-insulator-semiconductor pH sensors

In this work, we developed YbTa_xO_y sensing membranes displaying super-Nernstian pH-sensitivity for use in electrolyte–insulator–semiconductor (EIS) pH sensors. We examined the effect of tantalum content on the structural properties and sensing characteristics of the YbTa_xO_y sensing membranes deposited through reactive co-sputtering onto Si substrates. X-ray diffraction, atomic force microscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy revealed the structural, morphological, depth, and chemical features, respectively, of these YbTa_xO_y films prepared under various Ta plasma power conditions (from 80 to 160 W). Among the tested systems, the YbTa_xO_y EIS device prepared at the 120 W condition exhibited the super-Nernstian sensitivity (70.24 mV/pH), the lowest hysteresis voltage (1.5 mV), and the lowest drift rate (0.26 mV/h). Presumably, this condition optimized the stoichiometry of YbTaO₄ in the film and its surface roughness while reducing the crystal defect and suppressing silicate formation at the YbTa_xO_y–Si interface. The super-Nernstian pH-sensitivity may be attributed to the incorporation of Ta ions in the Yb₂O₃ forming a YbTaO₄ stoichiometric film, enhancing a change in oxidation state of Yb from trivalent ion to bivalent ion and thus transferring one electron to two protons in the redox reaction.