Structural Properties and Sensing Performance of CeY_xO_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)O₂ 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 (Ce⁴⁺ → Ce³⁺) and resulting in less than one electron transferred per proton in the redox reaction.