Impedance study of GaN and InGaN semiconductor anion selective electrodes

The response of potentiometric anion selective electrodes employing undoped GaN or In0.2Ga0.8N films as sensing element to detect various anions was investigated in solutions of KF, KNO3, KCl, HOC6H4COONa, KSCN, CH3 COOK, KClO4 and KBr salts. The calibration plots for the GaN and In0.2Ga0.8N semiconductor electrodes contained linear regions extending over four decades of activity change in most solutions. The structure of the GaN and In0.2Ga0.8N semiconductor electrode/ electrolyte interface was studied through electrochemical impedance spectroscopy. Analogous equivalent circuits modeling the GaN or In0.2Ga0.8N electrode/electrolyte interface were proposed and their parameters were calculated. The space charge layer of the GaN and In0.2Ga0.8N semiconductors dominated the impedance of the electrochemical system at high frequencies (> 10 kHz), whereas at low frequencies (< 10 kHz), the impedance was controlled by the diffusion of electroactive species across the layer of adsorbed ions at the surface of the electrode. Results imply a strong dependence of the electrodes performance on the adsorption capacity of tested anions.