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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Luminescence measurements in aqueous solutions were performed upon n-GaN layers grown on sapphire substrates and on Si substrates. Photoluminescence ͑PL͒ measurements at n-GaN/sapphire and n-GaN/Si electrodes show an identical emission band centered at 2.20 eV ͑the well-known yellow luminescence band͒, showing that the same deep acceptor level is present in both materials. Additional reddish luminescence is observed when the holes are injected from the solution ͓electroluminescence ͑EL͔͒, which may be ascribed to the occurrence of radiative ͑near͒ surface recombination. From an analysis of the potential dependence of both the PL and EL intensity of the 2.20-eV band, it may be concluded that this band possesses a significant contribution from the ͑near͒ surface.

In this paper, an electrochemical impedance study of the Ge/electrolyte interface has been carried out. At n-type Ge͑100͒ and Ge͑111͒ electrodes with different donor densities, linear Mott-Schottky plots were observed over a wide potential region, showing that the depletion region is conserved up to a band bending exceeding the bandgap of Ge. In most cases, the Mott-Schottky plots show some frequency dependence. A more elaborate investigation of the total interfacial impedance using electrochemical impedance spectroscopy shows that this frequency dependence may be caused by an additional impedance in parallel with the depletion layer capacity in the equivalent circuit describing the Ge/electrolyte interface. Under very weak depletion conditions, i.e., close to the flatband potential, an additional impedance is also observed, which may be related to electron-hole recombination at the semiconductor surface. For p-type Ge electrodes, linear Mott-Schottky plots are much more difficult to obtain, probably because electrochemical reactions and/or surface recombination strongly complicate the total impedance of the Ge/electrolyte interface.Due to the continuous downscaling of transistor sizes in accordance with Moore's law, Si-based devices are expected to run into performance limits. As a possible solution to this problem, germanium is put forward as a material for future high-speed transistors, because of its much higher charge carrier mobility as compared to Si. However, in order for the development of performant Ge transistors to be successful, some important drawbacks need to be resolved first, such as the problems with high n-type dopant activation and the stronger deterioration of channel mobility caused by interface states for a negative metal oxide semiconductor than for a positive metal oxide semiconductor. 1 Also, whereas very performant wet cleaning procedures have been developed for Si, these procedures do not seem to yield equally good results with Ge because of the apparent much higher reactivity of Ge toward etching agents. 2 Therefore, a more fundamental understanding of the Ge surface appears useful. As it is known that impedance measurements may yield more insight into some fundamental properties of the semiconductor surface and processes occurring at the semiconductor/ electrolyte interface, a detailed electrochemical impedance study of the Ge/electrolyte interface is described in this paper. The impedance of the Ge/electrolyte interface has been studied since the very early days of semiconductor electrochemistry. [3][4][5] In these studies, it was found that the capacitance of the Ge/electrolyte interface shows a minimum as a function of the potential, which, at least for intrinsic Ge, occurs at the flatband potential. Our results are compared to these measurements. ExperimentalMeasurements were performed on wafers cut from Czochralskigrown n-and p-type Ge crystals ͑Umicore, Olen, Belgium͒ with either ͑100͒ or ͑111͒ orientation. The p-type Ge͑100͒ was Ga-doped ͑resistivity between 0.020 and 0.026 ⍀ ...

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