Jean-Lou Sculfort

Kinetic and diffusional limitations to the anodic dissolution of p-Si in fluoride media

Abstract The anodic dissolution of single-crystal p-Si in fluoride media in a large range of pH and nominal fluoride concentration c F has been studied using a rotating-disc electrode. The process is found to be under mixed control, and the measured current is split into kinetic and diffusional contributions. A zero diagram for both kinetic and diffusion currents as a function of c F , pH and electrode potential is constructed. A critical concentration c * F is introduced to define the c F value at which kinetic and diffusion contributions are the same: while values higher than c * F lead to a diffusion-controlled process, kinetic control becomes dominant at values lower than c * F . The results also indicate that, unlike the kinetic current, the diffusion current is pH independent and constant in the potential region beyond the domain of porous Si generation. In these conditions, quantitative analysis of the diffusion current via Levichs relation is consistent with a tetravalent dissolution scheme leading to a SiF 2− 6 reaction product.

A study of the mechanisms of O2 reduction at n- and p-InP in acid aqueous electrolyte

Abstract The oxygen reduction reaction has been studied at semiconductor InP (n- and p- type) electrodes in an aqueous acid medium. The existence of a hole injection step has been demonstrated. The injection power, weaker than that observed at other III–V compounds, has been explained. The oxygen reduction mechanism has been indentified by means of electroluminescence (E.L.) with the n- type, coupled with photocurrent measurements with the p-type semiconductor.

Charge-transfer kinetics at an indium phosphide semiconductor electrode: Hole injection process in the presence of the Ce4+/Ce3+ couple

Abstract Electrochemical and electroluminescence measurements using rotating disk (n- and p-type InP) electrodes have been used to prove that the mechanism of the reduction of Ce 4+ ions occurs by a pure hole injection process in the valence band. The holes injected into the valence band induce the corrosion of InP samples in the dark. During the dissolution electrons are injected into the conduction band from decomposition intermediates.

Taking advantage of liquid ammonia to control the surface modification of silicon electrodes

Abstract This work shows the advantages of using a non-aqueous solvent such as ammonia, which can be used at cryogenic temperatures and can be prepared anhydrously, to control the state of a silicon electrode surface in contact with the electrolyte. Depending on the in-situ experimental conditions, it was possible to change the oxidized material surface (native oxide) to an oxide-free surface and conversely. An original technique for pretreatment of the electrodes in a solvated electron solution in ammonia was used to corroborate the results obtained with the usual treatment, HF under an inert atmosphere. The influence of a superficial oxide layer and the growth of this layer under various conditions, was studied carefully by following the variations of two experimental parameters: the flatband potential and the photocurrent onset potential. By changing the pH, in liquid ammonia, the growth of the superficial oxide layer could be completely controlled; moreover, the oxide could be obtained either by a chemical reaction with traces of water or by a photoelectrochemical route.

Electron excitation during anodic decomposition of III–V compounds induced by hole injecting species (Ce4+)

Abstract Evidence is presented for electron excitation steps during the anodic decomposition of III–V compounds induced by hole injecting species such as Ce4+. Their number depends on the nature of the compound. Thus, the behaviour of GaAs on the one hand and phosphorus containing compounds on the other are quite different. The nature of the decomposition intermediates connected with the existence of the electron excitation steps is related to the phosphorus induced gap states.

Effect of alkali-metal and some quaternary-ammonium cations on the anodic dissolution of p-Si in fluoride media

Abstract A systematic study of the anodic dissolution rate upon changing the cations of the supporting electrolyte has been performed for a wide range of electrolyte compositions. The usable range of fluoride concentrations is limited by the solubility of the fluoride salt in the case of Li+, and by the solubility of the hexafluorosilicate salt in the case of K+, Rb+ and Cs+. When limitations due to solubility are avoided, the anodic dissolution rate has been found to depend markedly upon the concentration of the supporting electrolyte and the nature of its cation (alkali-metal or quaternary-ammonium). Changing the cation may result in variations in anodic currents of more than one order of magnitude. This variation appears entirely due to a variation of the kinetic contribution to the anodic current, and is associated with a catalytic effect in the dissolution rate of the interfacial oxide by the fluoride species.

Properties of semiconductor—molten salt electrolyte interface: review of the semiconductor—ammoniate of sodium iodide junctions

Abstract The influence of the supporting electrolyte on the charge transfer mechanism at the semi-conductor/electrolyte interface is often primordial in the utilization of semiconducting electrodes in solar energy conversion. This work specifies the behaviour of a range of S.C. (SiSe, (III, V) and (II, VI) compounds) in a “non-aqueous” protonic solvent: the liquid ammoniate of sodium iodide (LASI). In each case, the flat band potential was unambiguously determined and the influence of solvated protons, NH + 4 and water was studied.

Photoelectrochemical behavior of the junction: Cu2O/liquid ammoniate of sodium iodide (NaI·3.3NH3)

Abstract At room temperature, the characteristics of the junction Cu 2 O/liquid ammoniate of sodium iodide (NaI·3.3 NH 3 ) are: • - flat-band potential E fb = +2.28 V vs. Ag/Ag + (5 × 10 −3 M) in situt reference, • - E gap = 2.00 eV. The role of water in the photoreduction of the semiconductor has been carefully investigated.

Electrochemical behavior of n-and p-type GaxIn1−xP electrodes in sodium iodide liquid ammoniate at room temperature

Abstract The electrochemical behavior of p- and n-type Ga x In 1- x P electrodes in liquid ammoniate (NaI+3.3 NH 3 ) in the dark and under illumination has been investigated at room temperature. The flatband potentials of the semiconductors (SC) have been measured either by impedance determinations or by the photoperturbation method. Some specific physical parameters of the polycrystalline material have been deduced, such as E gap , the electronic transition modes and the carrier densities. The SC behavior in water and in liquid ammoniate has been compared.

nInP flat band potential: A pH probe in nonaqueous and mixed solvents?

Abstract The interface structure of the semiconductor/electrolyte junctions in nonaqueous solvents is an open question, and as in water, the study of the influence of pH on the flat band potential can lead to an important part of its knowledge and to applications such as pH measurements. In water, the flat band potential shifts of many semiconducting materials can be connected with pH shifts. In this paper we show it is the case for nInP electrodes in acetonitrile and methanol (96% in weight in water) at room temperature, and in liquid ammonia at low temperature, −55°C. Therefore this material can be used as a probe of pH. Using this property we were able to find again, for example, the values of the self-ionization constants of the studied solvents and the acidity constants of water in liquid ammonia and of acetic acid in methanol.