Structural Properties and Sensing Performance of CeY(x)O(y) Sensing Films for Electrolyte–Insulator–Semiconductor pH Sensors

In this study we developed CeY(x)O(y) sensing membranes displaying super-Nernstian pH-sensitivity for use in electrolyte–insulator–semiconductor (EIS) pH sensors. We examined the effect of thermal annealing on the structural properties and sensing characteristics of the CeY(x)O(y) sensing membranes deposited through reactive co-sputtering onto Si substrates. X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy revealed the structural, morphological, and chemical features, respectively, of the CeY(x)O(y) films after their annealing at 600–900 °C. Among the tested systems, the CeY(x)O(y) EIS device prepared with annealing at 800 °C exhibited the highest sensitivity (78.15 mV/pH), the lowest hysteresis voltage (1.4 mV), and the lowest drift rate (0.85 mV/h). Presumably, these annealing conditions optimized the stoichiometry of (CeY)O(2) in the film and its surface roughness while suppressing silicate formation at the CeY(x)O(y)–Si interface. We attribute the super-Nernstian pH-sensitivity to the incorporation of Y ions in the Ce framework, thereby decreasing the oxidation state Ce (Ce(4+) → Ce(3+)) and resulting in less than one electron transferred per proton in the redox reaction.