Charge transport at the interface of $n$-GaAs (100) with an aqueous HCl solution: Electrochemical impedance spectroscopy study

Charge transport processes at the interface of $n$-GaAs (100) with an aqueous HCl solution are studied by electrochemical impedance spectroscopy. It is found that when open-circuit potential and anodic potentials are applied to the semiconductor the impedance spectra contain two capacitive semicircles corresponding to the capacitances of the space charge layer and surface states. In the case of open-circuit potential, semiconductor band bending at the interface with the solution is about 0.7 eV and the density of occupied surface states in the dark and under daylight conditions is 1.6 and 2.8 $\times$ 10$^{12}$ cm$^2$ eV$^{-1}$, respectively. When cathode potentials are applied to GaAs, hydrogen evolution begins at the semiconductor/electrolyte interface and an additional inductive loop appears in the impedance spectra. At the same time, the density of occupied surface states increases considerably both due to a straightening of the semiconductor bands and to the appearance of As-H bonds. Thus, charge transport through the $n$-GaAs (100)/aqueous HCl solution interface is always mediated by semiconductor surface states.