Capacitance Electrochemical pH Sensor Based on Different Hafnium Dioxide (HfO2) Thicknesses

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.

Miniaturized pH‐Sensitive Field‐Effect Capacitors with Ultrathin Ta2O5Films Prepared by Atomic Layer Deposition

“…However, while providing a stable Si-SiO 2 interface with a low density of states, it suffers from a low pH sensitivity, a restricted linear pH range as well as a relatively large drift and hysteresis. [9,26,27] Therefore, plenty of thin-film high-κ materials (e.g., Si 3 N 4 , [28][29][30] Al 2 O 3 , [30][31][32] Ta 2 O 5 , [1,[33][34][35] ZrO 2 , [36,37] HfO 2 , [38][39][40] CeO 2 , [41] TiO 2 , [42] SnO 2 , [43] Gd 2 O 3 , [44] and barium strontium titanate, [45,46] just to name a few) have been studied as pH-sensitive gate insulators in electrolyte-gated field-effect devices, in particular, in EISCAPs. These films were deposited either directly on the Si substrate to replace the SiO 2 or on a SiO 2 layer as stacked gate insulators.…”

Investigation of the Atomic Layer Deposition of the Titanium Dioxide (TiO2) Film as pH Sensor Using a Switched Capacitor Amplifier

“…ISFETs are common metal-oxidesemiconductor (MOS) transistors where the gate of the transistor is floating and is controlled by a reference electrode in an electrolyte solution through the capacitive coupling between the passivation layer and the gate oxide. So far, different types of materials have been used as the passivation layer in the ISFET such as titanium dioxide (TiO 2 ) [2], silicon nitride (Si 3 N 4 ) [3], aluminum oxide (Al 2 O 3 ) [4], Si nanowire/SiO 2 /Al 2 O 3 [5], tantalum pentoxide (Ta 2 O 5 ) [6], tin oxide (SnO 2 ) [7], and hafnium dioxide (HfO 2 ) [8]. TiO 2 is known as a good candidate for the pH sensitive layer due to the high dielectric constant, high stability, and high sensitivity.…”

Effect of Plasma-Enhanced Atomic Layer Deposition on Oxygen Overabundance and Its Influence on the Morphological, Optical, Structural, and Mechanical Properties of Al-Doped TiO2 Coating

“…Among them, ALD stands out for its unique capabilities, which include the complex shapes coverage embedded in high conformal 3D areas [ 23 ], the growth of stacked monolayers of different nanomaterials [ 24 ], and the growth of thin films precisely defined by self-limited surface reactions [ 25 , 26 , 27 , 28 , 29 ]. The ALD’s versatility allows its application in a broad range of fields, such as micro and nanoelectronics [ 30 , 31 ], biomedical engineering [ 32 ], on food packaging against corrosion [ 33 , 34 ], fuel cells [ 35 ], solar cells [ 36 ], anti-tarnish coatings on jewels surfaces [ 37 ], smart textiles [ 38 ], membranes, and optoelectronics [ 39 , 40 , 41 ]. Despite the wide range of applications of the ALD technique, fundamental studies are needed to understand essential aspects of the chemical, morphological, mechanical, and optical properties of the thin films that influence devices’ properties and their applications.…”