Effect of Titanium Content on the Sensing and Impedance Characteristics of High-κ TbTixOy Electrolyte-Insulator-Semiconductor pH Sensors

We report the effect of titanium content on the sensing and impedance characteristics of TbTixOy sensing membranes deposited on Si(100) substrates through reactive cosputtering for electrolyte-insulator-semiconductor (EIS) pH sensors. X-ray diffraction (XRD) and Auger electron spectroscopy (AES) were used to investigate the structural and chemical features of these films prepared under different growth conditions (Ti plasma power from 80 to 120 W). The EIS sensor featuring the TbTixOy), sensing membrane prepared at the 100 W condition exhibited the best sensing characteristics (pH sensitivity, hysteresis voltage, and drift rate). We attribute this behavior to the optimal titanium content in this sensing film forming a well-crystallized Tb2Ti2O7 structure. In addition, the effect of titanium content on the impedance characteristics of TbTixOy EIS sensors was studied using the capacitance-voltage (C-V) method (frequency-dependent C-V curves). The membrane resistance and capacitance of TbTixOy sensing films were determined in different frequency ranges under accumulation, depletion, and inversion regimes. From the impedance spectroscopy analysis, we find that the diameter of a semicircle of a TbTixOy EIS sensor becomes bigger due to a gradual increase in the bulk resistance of the device with degree of pH value.