Structural and Sensing Characteristics of NiOx Sensing 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 NiO
_x
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 NiO
_x
sensing films after PDA at five different temperatures, respectively. The NiO
_x
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 NiO
_x
film featuring the column-like polycrystalline structure, possessing a high Ni
²⁺
content and forming of a thinner silicide layer at the NiO
_x
/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 NiO
_x
film surface.