Investigation of AlGaZnO pH Sensors Fabricated by Using Cosputtering System

In this study, the extended gate field-effect transistor (EGFET) pH sensors with the aluminum–gallium–zinc-oxide (AGZO) sensing film were fabricated by the radio frequency (RF) cosputtering method at various aluminum–gallium-oxide (AGO):zinc-oxide (ZnO) power ratios. The transfer characteristic curve was measured by the constant voltage mode and the constant current mode at various pH values of 2–12. The pristine AGO sensing film exhibited 23.00 mV/pH and $12.00~\mu \text{A}$ /pH of the pH voltage sensitivity and pH current sensitivity, respectively. The pH sensitivity increased with the increased AGO:ZnO power ratio and achieved the highest value at AGO:ZnO of 80 W:50 W. At this condition, the pH voltage sensitivity and the pH current sensitivity were improved to 30.01 mV/pH and $31.06~\mu \text{A}$ /pH, respectively, and both with the great linearity around 0.99. This was attributed to both the carrier transport improved by the ZnO and the increased surface roughness. From the surface morphology analysis, it was observed that the AGO:ZnO power ratio of 80 W:50 W demonstrated the highest surface roughness of 1.08 nm, related to more surface contact area and more binding sites than other conditions, resulting in the highest pH sensitivity. Therefore, cosputtering AGO and ZnO to form the AGZO sensing film can improve the carrier transport, increase the surface roughness, lead to higher surface reactivity, and enhance the pH voltage sensitivity and the pH current sensitivity.