Capacitance Electrochemical pH Sensor Based on Different Hafnium Dioxide (HfO<sub>2</sub>) Thicknesses
2021
Zina Fredj | Abdoullatif Baraket | Mounir Ben Ali | Nadia Zine | Miguel Zabala | Joan Bausells | Abdelhamid Elaissari | Nsikak U. Benson | Nicole Jaffrezic-Renault | Abdelhamid Errachid
Over the past years, to achieve better sensing performance, hafnium dioxide (HfO<sub>2</sub>) has been studied as an ion-sensitive layer. In this work, thin layers of hafnium dioxide (HfO<sub>2</sub>) were used as pH-sensitive membranes and were deposited by atomic layer deposition (ALD) process onto an electrolytic-insulating-semiconductor structure Al/Si/SiO<sub>2</sub>/HfO<sub>2</sub> for the realization of a pH sensor. The thicknesses of the layer of the HfO<sub>2</sub> studied in this work was 15, 19.5 and 39.9 nm. HfO<sub>2</sub> thickness was controlled by ALD during the fabrication process. The sensitivity toward H<sup>+</sup> was clearly higher when compared to other interfering ions such as potassium K<sup>+</sup>, lithium Li<sup>+</sup>, and sodium Na<sup>+</sup> ions. Mott−Schottky and electrochemical impedance spectroscopy (EIS) analyses were used to characterise and to investigate the pH sensitivity. This was recorded by Mott–Schottky at 54.5, 51.1 and 49.2 mV/pH and by EIS at 5.86 p[H<sup>−1</sup>], 10.63 p[H<sup>−1</sup>], 12.72 p[H<sup>−1</sup>] for 15, 19.5 and 30 nm thickness of HfO<sub>2</sub> ions sensitive layer, respectively. The developed pH sensor was highly sensitive and selective for H<sup>+</sup> ions for the three thicknesses, 15, 19.5 and 39.9 nm, of HfO<sub>2</sub>-sensitive layer when compared to the other previously mentioned interferences. However, the pH sensor performances were better with 15 nm HfO<sub>2</sub> thickness for the Mott–Schottky technique, whilst for EIS analyses, the pH sensors were more sensitive at 39.9 nm HfO<sub>2</sub> thickness.
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