Inge Huygens

Electrochemistry and Photoetching of n‐GaN

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 H 2 SO 4 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, Fe 3+ , Ce 4+ , HIO 3 , in acid medium and Fe(CN) 3- 6 in alkaline medium are electrochemically reduced at a diffusion-limited rate. In I M KOH a high reactivity for O 2 /H 2 O reduction is observed, explaining why n-GaN can be photoetched under open-circuit conditions in this solution.

Photoelectrochemical reactions at the n-GaN electrode in 1 M H2SO4 and in acidic solutions containing Cl− ions

The photoelectrochemical behaviour of n-GaN in contact with 1 M H2SO4 and with acidic solutions containing Cl− 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− ions due to the competing oxidation of Cl− to Cl2. The competition kinetics were interpreted on the basis of a mechanism, in which intrinsic surface states take part in the photoanodic oxidation of Cl−. The participation of such intrinsic surface states may explain the discrepancies found in the literature concerning the photoelectrochemical behaviour of n-GaN.

Electrochemical Impedance Study of the Germanium/Electrolyte Interface

In this paper, an electrochemical impedance study of the Ge/electrolyte interface has been carried out. At n-type Ge(100) and Ge(111) 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.

Subbandgap Photoluminescence and Electroluminescence at n-GaN Electrodes in Aqueous Solutions

Photoluminescence (PL) and electroluminescence (EL) 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 Gartner model, showing that PL originates from the bulk of the semiconductor. Deviations from the simple Gartner 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 blue-shifted 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 (near)surface states.

Anodic and Electroless Etching of Germanium in Alkaline Hexacyanoferrate Solutions

This paper presents results of a study on the electrochemical and etching behavior of Ge in alkaline hexacyanoferrate solutions. A detailed analysis of the current-potential behavior in these solutions allowed important conclusions to be drawn concerning the anodic dissolution mechanism of Ge. It was found that some oxidation steps involve mobile dissolution intermediates instead of, as generally accepted, free holes. Moreover, the electrochemical experiments indicate that hexacyanoferrate etches Ge according to an electroless mechanism, which was confirmed by etching experiments at open-circuit potential. Reproducible etch rates were obtained and only slight surface roughening after etching was observed.

Defect Luminescence at n-GaN Electrodes A Comparative Study Between n-GaN Grown on Sapphire Substrates and on Si Substrates

Luminescence measurements in aqueous solutions were performed upon n-GaN layers grown on sapphire substrates and on Si substrates. Photoluminescence (PL) 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 band), 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 (EL)], which may be ascribed to the occurrence of radiative (near) 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 (near) surface.