SURFACE EVOLUTION OF N-GAAS IN CONTACT WITH ACETONITRILE SOLUTIONS STUDIED BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Electrochemical impedance spectroscopy (EIS) is used to study oxide film formation onto an n-type GaAs electrode in contact with an acetonitrile solution, in the case where the redox reaction rate is mass-transport limited. In the dark and under anodic polarization, the n-GaAs surface is oxidized leading to dispersed flatband potential determinations. Under white light illumination and anodic polarization, a similar situation occurs. Surface film formation can be studied by electrochemical impedance spectroscopy. The shape of impedance diagrams strongly changes with the applied potential. The observed behaviour is well accounted for by considering that the oxide film is stable at high positive polarization, but it is electrochemically decomposed at more negative potential. In practice, it can be considered that an oxide-free surface is obtained when the GaAs electrode is polarized at a sufficiently negative potential for oxide to be reduced. The stability of such a "reduced" GaAs surface at more positive potential will depend on the redox reaction rate. on the residual water trace content and on the electrode potential.