Structural properties and sensing performance of Ni–Ti alloy films for extended-gate field-effect transistor pH sensors

This study presents a facile Ni–Ti alloyed film grown on the n⁺-type Si through a DC sputtering technique for electrochemical pH sensing and its comprehensive structural, morphological, compositional, profiling, microstructural, and elemental characterization. In addition to pH sensing measurements, the properties of the sensing layer were analyzed using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy analysis. The impact of rapid thermal annealing (RTA) treatment (500–700 °C) on the alloyed Ni–Ti sensitive films was extensively studied. The structural feature of the alloyed Ni–Ti sensitive films is more closely related to their relative sensing performances. The performance of the alloyed Ni–Ti sensitive film at the RTA temperature of 600 °C showed a higher pH sensitivity of 60.73 mV/pH in a wide pH range of 2–12 in comparison with other RTA temperatures. Moreover, the stability of this 600 °C-annealed EGFET sensor exhibited a small drift rate of 0.21 mV/h in pH 7 and a low hysteresis voltage of 1 mV in a loop of pH 7 → 4→7 → 10→7. The alloyed Ni–Ti sensitive film is fully compatible with a standard CMOS process and demonstrates better sensing performance than other metal nitride sensitive films (e.g. RuN, TiN), suggesting it may be a strong candidate for future sensor and biosensor applications.