W. P. Gomes

Impedance spectroscopy at semiconductor electrodes: Review and recent developments

Abstract It is demonstrated that impedance spectroscopy can be used for the energetic characterization of the semiconductor/electrolyte interface. Furthermore, by means of several examples, it is shown how the mechanisms of electrochemical reactions are studied by the electrical impedance technique. Also the use of the opto-electronic admittance method for the investigation of photoelectrochemical processes is discussed.

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.

A comparative study of electrochemically formed and vacuum-deposited n-GaAs/Au Schottky barriers using ballistic electron emission microscopy (BEEM)

A comparative study between n-GaAs/Au contacts, formed by electrochemical deposition or by vacuum evaporation, is presented. The main parameter, the barrier height ?B, was determined using three methods, i.e.?classical current-voltage and capacitance-voltage measurements as well as STM-based ballistic electron emission microscopy (BEEM). The latter method allowed us to determine the distribution of ?B over the contact area on a nanometre scale and showed that the electrochemically made contacts are inhomogeneous. The main result, confirmed by the three methods, was that ?B was higher for the electrochemically deposited contacts than for the evaporated ones. This higher value is attributed to O- groups, present at the interface during the electrochemical metallization, and forming interfacial dipoles with the Au, leading to an increase of the barrier.

On the frequency-dependence of the impedance of n- and p-type gallium arsenide electrodes

The impedance of n- and p-type GaAs electrodes has been studied as a function of applied voltage and of frequency. Simple frequency laws were found to exist for the series capacitance and resistance. To explain these laws, a distribution of time constants has been assumed, associated with dielectric relaxation phenomena in the double-layer at the semiconductor/electrolyte interface. This distribution was found to be independent of the distance from the crystal surface for n-type samples but to depend upon it for p-type specimens. An investigation on the role of the applied voltage in these frequency laws yielded additional evidence for the mathematical model which was originally introduced in a previous paper in order to explain similar laws observed at CdSe, CdS and TiO2 electrodes. The frequency-dispersion of the p-type samples was found to be strongly influenced by an appropriate pretreatment of the surface, in contrast with the behaviour of the n-type specimens. In both cases, the experimental results indicate that the source of the frequency dispersion has to be sought in structural irregularities of the depletion region of the electrode. The possibility of determining the flat-band potential from frequency-dependent impedance data is discussed.

On the application of the Kramers-Kronig relations to problems concerning the frequency dependence of electrode impedance

Abstract It is emphasized that, under certain conditions, the frequency dependences of the real and imaginary parts of an electrical impedance are correlated by the Kramers-Kronig relations. The importance of these relations in electrochemistry is illustrated by the examples of certain electrochemical impedance types. Some misapprehensions appearing in the electrochemical literature are pointed out.

Impedance Measurements at the N‐ and P‐Type GaP Single Crystal Electrode

Impedance measurements were made at the dark n- and p-type GaP single crystal electrodes in aqueous indifferent electrolyte solutions as a function of applied voltage and of frequency. The results allow the determination of the flatband potential and hence of the position of the Fermi level and the conduction and valence band edges at the electrode surface. The observed frequency dependence of the capacitance can be attributed to dipole relaxation phenomena in the space-charge layer of the semiconductor electrode. The influence of preliminary etching on the frequency behavior is discussed.

Study of Stabilization and Surface Recombination on n ‐ GaP Photoelectrodes: Mechanisms and Interrelation

The competition has been studied between three processes occurring with photogenerated holes at the n-GaP/aqueous electrolyte interface, i.e., reaction with Fe/sup 2+/ ions, reaction with GaP itself, and surface recombination. The experimental method used was the rotating ring-disk technique. The influence of the reducing agent concentration, of the light intensity, and of the electrode potential was investigated. The results can be interpreted on the basis of models, in which the stabilization process, and under certain circumstances also the recombination process, takes place through surface intermediates of the photoanodic decomposition reaction. 10 refs.

Bromine‐Methanol as an Etchant for Semiconductors: A Fundamental Study on GaP I . Etching Behavior of n‐ and p‐Type

Electrochemical and etching experiments were performed at n- and p-type GaP single crystals in the commonly used etchant bromine-methanol to investigate the fundamental aspects of the etching reaction. The etching properties of these methanolic bromine solutions were similar to those of bromine solutions in which water is used as the solvent; thus, e.g., as in water, the etching kinetics and morphologies at the (111) and (111) faces are markedly different. In many cases of practical etching, methanol may be substituted by water as the solvent for bromine. The results allow us to propose an overall reaction equation for the etch process as well as a detailed mechanism involving radical decomposition intermediates of the semiconductor

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 behaviour of (1 0 0) GaAs in copper(II)-containing solutions

Abstract The electrodeposition of copper from acidic chloride and sulfate solutions onto the surface of n - and p -GaAs electrodes and the dissolution of copper from the GaAs surface have been studied by means of cyclic voltammetry, rotating-ring-disk voltammetry and impedance measurements. The results show, that the reduction of Cu 2+ is due to the injection of holes. In chloride solutions, reduction to Cu + precedes the reduction to Cu 0 , whereas in sulfate solutions no Cu + intermediates are detected. At open-circuit potential, Cu 2+ is reduced to Cu + in chloride solutions, Cu 2+ hence being an electroless etchant for GaAs. At p -GaAs, dissolution of copper by holes is observed both in Cl − and SO 4 2− medium, whereas at n -GaAs in chloride solutions, copper is seen to dissolve chemically. Mechanisms explaining the observed potential-dependence of the hole injection rate are proposed, and the morphology of the copper layers as well as the electrical properties of the resulting GaAs/Cu dry junctions are discussed.