High-k Tm₂O₃ sensing membrane-based electrolyte-insulator-semiconductor for pH detection

For high-sensitive pH sensing, an electrolyte-insulator-semiconductor (EIS) device with a Tm₂O₃ sensing membrane fabricated by reactive sputtering and the succeeding postdeposition annealing treatment was proposed. In this study, the influence of thermal annealing (700, 800, and 900 °C) on the structural characteristics of the Tm₂O₃ sensing membrane was 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 performance. Results revealed that the EIS device with a Tm₂O₃ sensing film annealed at 800 °C exhibited a higher sensitivity of 58.02 mV/pH (within the pH range of pH 2-12), a lower hysteresis voltage of 2 mV (measurement cycle, pH = 7→4→7→10→7), and a lower drift rate of 1.04 mV/h (at the condition of pH 7) compared with those at other annealing temperatures. These results are attributed to the formation of a well-crystallized Tm ₂O₃ structure, a thinner low-k interfacial layer at the oxide/Si interface, and the higher surface roughness at such thermal annealing conditions. As a whole, this study provided some fundamental data regarding the application of Tm₂O₃ sensing membrane-based EIS devices for pH detection.