Structural and sensing characteristics of Gd₂Ti ₂O₇, Er₂TiO₅ and Lu ₂Ti₂O₇ sensing membrane electrolyte-insulator- semiconductor for bio-sensing applications

This paper describes the structural and sensing characteristics of Gd ₂Ti₂O₇, Er₂TiO₅, and Lu₂Ti₂O₇ sensing membranes deposited on Si substrates through reactive co-sputtering for electrolyte-insulator- semiconductor (EIS) pH sensors. In this work, the structural properties of Gd₂Ti₂O₇, Er₂TiO₅, and Lu₂Ti₂O₇ membranes were investigated by X-ray diffraction, atomic force microscopy and X-ray photoelectron spectroscopy. The observed structural properties were then correlated with the resulting pH sensing performances. The EIS device incorporating a Lu₂Ti ₂O₇ sensing film exhibited a higher sensitivity (59.32 mV pH^-1), a larger drift rate (0.55 mV h^-1), a higher hysteresis voltage (5 mV), and a larger hysteresis gap (∼70 mV) compared to those of the other sensing films. This result is attributed to the higher surface roughness and the formation of a thicker interfacial layer at the oxide-Si interface. Furthermore, the impedance effect of EIS sensors has been investigated using capacitance-voltage (C-V) method (frequency-dependent C-V curves). From the impedance spectroscopy analysis, we find that the diameter of a semicircle of an EIS sensor becomes smaller due to a gradual decrease in the bulk resistance of the device with degree of pH value.