# Investigation of High-Sensitivity pH Sensor Based on Au-Gated AlGaN/GaN Heterostructure
## 2 Materials and Methods  
### Epitaxy Growth and Device Fabrication  
The device was fabricated using a commercial epitaxial GaN wafer based on a silicon substrate. Figure 1 shows a schematic cross-section diagram of the Au-gated AlGaN/GaN pH sensor. The epitaxial structure was grown by metal-organic chemical vapor deposition (MOCVD) on the silicon substrate. Starting from the substrate, one can find the structure consisted of a 1 \(\upmu\)m thick GaN buffer layer, a 2 \(\upmu\)m thick GaN channel layer, a 1 nm thick AlN interlayer, a 25 nm thick Al\({}_{0.26}\)Ga\({}_{0.74}\)N barrier layer, and a 3 nm thick GaN cap layer. The sheet electron density and electron mobility of the 2 DEG at 25 \({}^{\circ}\)C were 1 \(\times\) 10\({}^{13}\) cm\({}^{-2}\) and 1500 cm\({}^{2}\)/(V-s), respectively.  
The fabrication process and characterize method are similar to our previous report [20]. The fabrication process flow started with a mesa etching using a Cl\({}_{2}\)/BCl\({}_{3}\) inductively coupled plasma (ICP) to define the sensor geometry. After that, Ti/Al/Ti/Au (20/110/40/50 nm) metal contacts were deposited via electron-beam evaporation (EBE) for the drain (D) and source (S) electrodes followed by annealing at 850 \({}^{\circ}\)C for 45 s in N\({}_{2}\) ambient. Then, a 200 nm thick SiO\({}_{2}\) was deposited via plasma-enhanced chemical vapor deposition (PECVD). The SiO\({}_{2}\) layer was etched in a buffered oxide etch (BOE) solution to open the contact pads and gate windows. A 10 nm thick nanolayer of Au or SiO\({}_{2}\) was deposited as the sensing membrane by EBE or PECVD, respectively. All the conducting areas, except the sensing membrane, were covered with a 5 \(\upmu\)m thick photoresist to prevent them from contacting the solution.  
After dicing, the discrete pH sensor samples were mounted on a self-designed PCB substrate, as shown in Figure 2. The Au quasi-reference electrode was used to apply a voltage bias to the electrolyte solution as well as control the charge density of the 2 DEG beneath the sensing membrane. It can provide a stable electrochemical interface, especially when used with a suitable electrolyte solution that forms a stable layer on the Au surface, contributing to a stable potential and reducing unnecessary electrolysis.  
Figure 3 shows the scanning electron microscope (SEM) images of the Au-gated device, with a detailed zoom-in view of the sensing membrane given in Figure 3b. Figure 4 shows the energy dispersive spectroscopy (EDS) spectra of the Au-gated sensing membrane. The inset of Figure 4 shows an SEM image of the sensing area and the elemental mapping of Ga, N, Au, Al, and O in the Au-gated sensing membrane. The measured elemental atomic percentages were Ga (45.83%), N (45.29%), Au (2.23%), Al (2.71%), and O (3.94%), respectively. Correspondingly, the elemental weight percentages were Ga (72.55%), N (14.40%), Au (9.96%), Al (1.66%), and O (1.43%), respectively. Thus confirming the deposition of Au on the sensing membrane.  
Figure 1: Schematic cross-section diagram of the Au-gated AlGaN/GaN heterostructure-based pH sensor.  
Figure 2: Photograph of the Au-gated pH sensor: (**a**) photograph of the PCB; (**b**) microscope image of the sensor active area.