Surface Dependence of pH Sensor on AlGaN/GaN Heterostructure

Glass electrodes are commonly used for pH measurements. The miniaturization, stability and sensitivity of the pH sensor have been improved by introducing Si ISFETs (ion-sensitive field-effect transistors) [1]. However, Si ISFETs are still limited in practical applications especially in high-temperature operation due to the material limitations of silicon and the sensor structure itself. Gallium nitride (GaN) is a good candidate for developing high-speed, high-sensitive and high-temperature pH sensor because of its wide band-gap, chemical stability, low toxicity, and superior conductivity owing to the high saturation velocity and high sheet carrier concentration of the two-dimensional electron gas (2DEG) layer in a AlGaN/GaN heterostructure [2-4]. In this work, GaN-based pH sensors were developed and the surface dependence was evaluated by using different surface layers on the AlGaN/GaN heterostructure.

The AlGaN/GaN heterostructure used in this experiment was grown on a (0001) sapphire substrate, which consists of a buffer layer, followed by an undoped GaN layer and an undoped AlGaN barrier layer with thickness of around 12-24 nm. To evaluate the surface dependence, the AlGaN barrier layers were designed to have different aluminum mole fraction of 22%, 24%, 25%, and 35%, respectively. Furthermore, some samples are with thin cap layers of i-GaN, n-GaN and p-GaN, respectively. To fabricate the pH sensor, first, isolation region with depth of 600 nm was formed by inductively coupled plasma (ICP) dry etching system. Ohmic contact for drain and source was then formed using Ti/Al/Ti/Au (50/200/40/40 nm) multilayer followed by annealing process with temperature of 850°C and period of 1 minute in N₂ ambient. After dicing step, the chips were mounted on a Teflon substrate by covering all the conducting area with silicone resin to avoid contact with the measurement solutions. The sensing area is an open-gate structure with length of about 600 µm and width of 200-800 µm.

A semiconductor parameter analyzer was used to characterize the current-voltage characteristics of the sensor by applying a drain voltage. A gate bias was applied to the solution via a platinum electrode or a standard Ag/AgCl reference electrode. A reference electrode was also used to monitor the voltage in the liquid. Three kinds of buffer solutions were used for measurement, which were phthalate, phosphate and tetraborate pH standard solution.

The sensors showed good pinch-off characteristics in the three solutions for all the samples with different surface. The drain current decreased and the threshold voltage had a positive shift with the pH value increasing. The estimated average pH sensitivities (ΔV/pH) were 47.2, 49.5, and 51.8 mV/pH for the samples with cap layer of p-GaN, i-GaN and n-GaN, respectively. While for the samples with aluminum mole fraction of 22%, 24%, 25%, and 35%, the estimated average pH sensitivities were 52.2, 53.5, 54.1, and 55.2 mV/pH, respectively. It demonstrates that the sensitivity of the AlGaN/GaN pH sensor depends on the aluminum mole fraction and increases with the aluminum mole fraction increasing.

[1] P. Bergveld, Sensors & Actuators B 88 (2003).

[2] G. Steinhoff, Appl. Phys. Lett. 83, 177 (2003).

[3] T. Kokawa et al., J. Vac. Sci. Technol. B 24,1972 (2006).

[4] B. S. Kang, et al., Appl. Phys. Lett., 91, 012110 (2007).