Electrochemical characteristics of n-type GaN in oxalic acid solution under the pre-breakdown condition

•Electrochemical impedance spectroscopy is used to evaluate the etching of n-GaN.•The oxide accumulation on the GaN surface can lead to an increase of the resistance.•In the etching process, charging current depends on the active area of GaN films.

Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy are used to evaluate the etching of n-type GaN in oxalic acid solution under the pre-breakdown condition which leads to the pit formation on the GaN surface. The chronoamperograms are analyzed using a multivariate curve resolution alternative least square model, indicating that faradaic current increases with the applied bias rising, and that charging current depends on the active area of GaN thin films. To study the electrochemical characteristics during the pit formation, we correlated the changes of parameters (e.g. capacitance and resistance) at the GaN working electrode/electrolyte interface with a parametric mapping of electrolysis variables. Under the same etching condition, three notable trends were observed (i) the capacitance decrease and the resistance increase with the applied bias rising, (ii) the decrease of capacitance and resistance as the oxalic acid concentration increases, and (iii) the capacitance increase and the resistance decrease with the rise of doping concentration. The increase of resistance could be ascribed to the oxide accumulation on the GaN surface under different acid concentrations and applied biases, respectively. However, the conclusion cannot be applied in different doping concentrations.