Abstract
Over the past years, to achieve better sensing performance, hafnium dioxide (HfO2) has been studied as an ion-sensitive layer. In this work, thin layers of hafnium dioxide (HfO2) were used as pH-sensitive membranes and were deposited by atomic layer deposition (ALD) process onto an electrolytic-insulating-semiconductor structure Al/Si/SiO2/HfO2 for the realization of a pH sensor. The thicknesses of the layer of the HfO2 studied in this work was 15, 19.5 and 39.9 nm. HfO2 thickness was controlled by ALD during the fabrication process. The sensitivity toward H+ was clearly higher when compared to other interfering ions such as potassium K+, lithium Li+, and sodium Na+ 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−1], 10.63 p[H−1], 12.72 p[H−1] for 15, 19.5 and 30 nm thickness of HfO2 ions sensitive layer, respectively. The developed pH sensor was highly sensitive and selective for H+ ions for the three thicknesses, 15, 19.5 and 39.9 nm, of HfO2-sensitive layer when compared to the other previously mentioned interferences. However, the pH sensor performances were better with 15 nm HfO2 thickness for the Mott–Schottky technique, whilst for EIS analyses, the pH sensors were more sensitive at 39.9 nm HfO2 thickness.
Highlights
The detection and control of pH are challenging for many environmental, biological and chemical processes that impact human lives [1]
The hafnium dioxide (HfO2 ) substrate was fabricated by the atomic layer deposition (ALD) technique
The hafnium dioxide pH sensor was fabricated from a p-type silicon wafer with 100 mm diameter, orientation and 4–40 W·cm−1 resistivity
Summary
The detection and control of pH are challenging for many environmental, biological and chemical processes that impact human lives [1]. One of the methods for controlling water and food quality is through the change in the pH value. If the measured pH is not in the normal pH range, the quality of used water and food is questionable and should be discarded from normal use. In the case of water, for instance, leaching and nitrifying are indicated by low pH values as seen in the case of the presence of the proliferation of microorganisms in water [2]. The conventional analytical process for water quality monitoring consists of multiple steps: water sampling, sample transportation to laboratories and laboratory analysis. This approach is time-consuming, expensive and laboratory-dependent. The results are affected by anthropogenic interference as well as long-term
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