Structural and Sensing Characteristics of NiO_xSensing Films for Extended-Gate Field-Effect Transistor pH Sensors

In this article, the impact of post-deposition annealing (PDA) on the structural features and sensing properties of the NiOx sensing films deposited on a n+-type Si substrate was studied for an extended-gate field-effect transistor (EGFET) pH sensor. X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscope, and transmission electron microscopy were applied to explore the crystal structure, elemental composition, film morphology, and film microstructure of the NiOx sensing films after PDA at five different temperatures, respectively. The NiOx sensing film after PDA at 500 °C showed a higher pH sensitivity of 60.65 mV/pH, a smaller hysteresis width of 1.8 mV and a lower drift coefficient of 0.28 mV/h than those at different PDA temperatures. This result may be attributable to the promoted proton-exchange process and increased the number of surface bind-sites due to the NiOx film featuring the column-like polycrystalline structure, possessing a high Ni2+ content and forming of a thinner silicide layer at the NiOx/Si interface. In addition, this PDA temperature can minimize the oxygen vacancies and passivate the trap sites, thus reducing the formation of a hydrated layer on the NiOx film surface.