The (photo)electrochemical behavior of n‐GaN[0001] in various aqueous solutions was studied using rotating‐disk voltammetry, cyclic voltammetry, and electrical impedance measurements. It was found that the bandedges of the semiconductor shift over 60 mV/pH unit, indicating acid‐base equilibria at the interface. In H2SO4 and KOH solutions, the photocurrent under anodic bias is associated with the oxidation of the semiconductor according to a three‐equivalent reaction, leading to dissolution and roughening of the surface. In 1.2 M HCl solutions, n‐GaN is stabilized for anodic decomposition due to the competing oxidation of Cl− ions. In the presence of oxalic acid and citric acid, anodic photocurrent multiplication was observed. Under cathodic polarization in the dark, Fe3+ , Ce4+ , HIO3 , in acid medium and normalFefalse(CN)63− in alkaline medium are electrochemically reduced at a diffusion‐limited rate. In 1 M KOH a high reactivity for O2/H2O reduction is observed, explaining why n‐GaN can be photoetched under open‐circuit conditions in this solution. © 2000 The Electrochemical Society. All rights reserved.

The photoelectrochemical behaviour of n-GaN in contact with 1 M H 2 SO 4 and with acidic solutions containing Cl À ions was studied using rotating-ring-disk voltammetry, electrochemical impedance spectroscopy and etching experiments. It was found that n-GaN is stabilized against photoanodic decomposition in the presence of Cl À ions due to the competing oxidation of Cl À to Cl 2 . The competition kinetics were interpreted on the basis of a mechanism, in which intrinsic surface states take part in the photoanodic oxidation of Cl À . The participation of such intrinsic surface states may explain the discrepancies found in the literature concerning the photoelectrochemical behaviour of n-GaN.

In this paper, the etching behaviour of germanium in hydrogen peroxide solutions is described. Electrochemical experiments showed that the etching process is most probably purely chemical. The etch rate was found to depend upon the H2O2 concentration, the pH value, the amount of KCl in the solution and , in some cases, upon the rotation rate. Based on all of these findings, possible etching mechanisms for Ge in H2O2 are discussed on a molecular level.

Photoluminescence ͑PL͒ and electroluminescence ͑EL͒ measurements were performed on n-GaN layers in contact with aqueous solutions. In both cases, strong subbandgap emission was observed. The PL spectra consisted of the well-known yellow emission band at 2.2 eV. The potential dependence of the intensity of this band was analyzed according to the Ga ¨rtner model, showing that PL originates from the bulk of the semiconductor. Deviations from the simple Ga ¨rtner model were found, indicating a finite rate of hole consumption at the electrode surface. The EL spectra, recorded at etched electrodes in persulfate solutions, were blueshifted with increasing cathodic current density. It was shown that this shift is correlated to the presence of an extra luminescence band, appearing in the EL spectra at low current densities, which may be ascribed to radiative recombination via ͑near͒surface states.

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