Surface Metal Cleaning of GaN Surface Based on Redox Potential of Cleaning Solution
Authors: Nagao Kenji , Nakamura Kenichi , Teramoto Akinobu , Shirai Yasuyuki , Imaizumi Fuminobu , Suwa Tomoyuki , Sugawa Shigetoshi , Ohmi Tadahiro 
Published: February 2020

I. INTRODUCSION
Gallium nitride (GaN) -based materials are used for HEMTs, MESFETs, and MOSFETs. GaN-based transistors operate under high power, high frequency, and high temperature. These devices have surface or interface related problems, such as drain current collapse and excess gate leakage. Previous studies have reported that the current collapse is caused by the electron traps at the surface of the GaN-based materials and interface between the insulator and GaN [1]. In this work, we studied the cleaning of GaN surface and the removal of the metal impurities causing the ionization of metal.
II. EXPERIMENTAL

Solutions were prepared with different pH values and redox potentials as shown in table I. Sulfuric acid, hydrogen peroxide, nitric acid and ozonated water were used as oxidizer and hydrogenated water was used as the reducing agent. The pH level and redox potential of solutions are measured by a pH and redox potential meter (D-53, HORIBA STEC Corporation, Japan). Redox potential measurements were performed with a Pt electrode with an Ag/AgCl2 reference corrected to the normal hydrogen electrode (NHE). Sapphire substrates with epitaxially grown GaN were employed. In order to prepare a GaN wafer sample deposited with Cu, a pre-cleaned convex n-type GaN wafer, formed under vacuum was filled with a solution of CuCl2 (using UPW) and then dried by the heat (50 °C) of a halogen lamp. Initial Cu concentration was 1×1013 atoms/cm2. The concentration on GaN surface was measured by total-reflection X-ray fluorescence analyzer (TXRF). The GaN samples were cleaned with the following steps: (1) immersing the wafers in a cleaning solution for 10 minutes, (2) rinsing the wafers in a continuous flow of UPW for 5 minute, and (3) drying the wafers with N2blow.
III. RESULTS AND DISCUSSION

Fig. 1 shows the residual Cu on the GaN surface as a function of redox potential after immersion in the prepared solutions. The residual Cu deposition is significantly suppressed when the redox potential exceeds approximately 0.8 V vs NHE. Fig. 2 shows the effect of pH and redox potential of the solutions on the states of Cu, such as Cu2+, Cu(OH)2, etc. Cu deposition is found to be suppressed when the pH is lower than 7.6 and the redox potential is higher than 0.8 V. This is considered as follows, when the redox potential of the solution exceeds 0.8 V, the oxidizing agent possesses a higher thermodynamic driving force than Cu2+ and removes electrons from the GaN surface [2].
IV. CONCLUSION

Cu contamination on GaN surfaces can be reduced by low pH and high redox potential. Cu is usually oxidized to Cu2+ at a redox potential of 0.16 V. However, the redox potential increased to 0.8 V to oxidise Cu on GaN surface. Because Cu can receive electrons from GaN surface, And then Cu can not be oxidized to Cu2+ by solutions.The Cu contamination on GaN can be reduced by the solutions with low pH of 7.6 and high redox potential of 0.8V are required. This method is very useful for metal cleaning and very effective for chemical consumption.
V. REFERENCES

1. Watanabe et al., IEEE Trans. on Electron Devices, 60, 6, p.1916, 2013
2. Morinaga et al., IEICE Trans. E79-C, p.349, 1996

Figure 1