Structural properties and sensing performance of high-k Sm2O3 membrane-based electrolyte–insulator–semiconductor for pH and urea detection

For high sensitive pH sensing, an electrolyte–insulator–semiconductor (EIS) device with a samarium oxide (Sm2O3) sensing membrane fabricated by reactive sputtering and the subsequent post-deposition annealing (PDA) treatment was proposed. In this work, the effect of thermal annealing (700, 800, and 900 °C) on the structural characteristics of Sm2O3 membranes was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The observed structural properties were then correlated with the resulting pH sensing performances. For enzymatic field-effect-transistors-based urea biosensing, a hybrid configuration of the proposed Sm2O3 membrane with urease-immobilized alginate film attached was established. The EIS device with the Sm2O3 membrane annealed at 700 °C exhibited a higher detection sensitivity of 56.2 mV/pH, a lower hysteresis voltage of 6.2 mV, and a lower drift rate of 1.29 mV/h compared to those at other annealing temperatures. These results are attributed to the formation of a thinner low-k interfacial layer at the oxide/Si interface and the higher surface roughness. Furthermore, the presented urea biosensor was proved to be able to detect urea with good linearity (R2 = 0.99) and reasonable sensitivity of 2.457 mV/mM in the urea concentration range of 5–40 mM.