Jacques Gautron

Photoelectrochemical determination of minority carrier diffusion length in II–VI compounds

Abstract The minority carrier diffusion length L of different II–VI compounds (n-ZnSe, n-CdTe, p-CdTe, p-ZnTe and n-CdSe) has been measured by a photoelectrochemical technique based on the Ga¨rtner model. The conditions used in this technique are defined accurately to avoid erroneous interpretations. Five experimental parameters which may influence drastically the measurement of L are pointed out. A linear relationship between L and free carrier density in CdTe and ZnSe has been found.

Diffusion length of minority carrier in n‐type semiconductors: A photoelectrochemical determination in aqueous solvents

Photoelectrochemical measurements with aqueous electrolyte‐semiconductor junctions under monochromatic illumination for five n‐type semiconductor compounds are presented: ZnSe, CdSe, CdTe (II‐VI) and GaP, InP (III‐V). The Schottky behavior of these junctions is demonstrated in the dark and under illumination. Only if conditions such as surface preparation, proper band bending, and doping level as well as energy of incident photons are achieved, the photocurrent versus potential characteristics are described using Gartner’s equation. Values of the diffusion length L of holes are determined by an extrapolation and a fitting method. They depend on the particular properties of the samples, particularly the concentration of free electrons. Results obtained are: ZnSe (7×1017 cm−3) L=0.025 μm; CdSe (2.5×1018 cm−3) L=0.124 μm; CdTe (8.5×1017 cm−3) L=0.14 μm; GaP (4×1016 cm−3) L=0.24 μm; InP (3×1017 cm−3) L=0.15 μm.

Analysis of photocurrents at the semiconductor—eletrolyte junction

Abstract The photocurrent vs potential characateristics for three different electrolyte—semiconductor junctions, representative of those generally found in semiconductor photoelectrochemistry, are analyzed in detail using parameters which define both the semiconductor and the electrolyte. It is shown that, in general, the behaviour of junctions including semiconductors with sufficiently wide energy pags and large free carriers densities, may be accurately described using the Gartner model in the potential region which does not include the onset of the photocurrent. In this case, it is the characteristics of the semiconductor which control the photoresponse and the electrolyte does not induce limiting steps in the charge transfer across the interface. If certain restrictive conditions are fulfilled concerning the relative orders of magnitude of the semiconductor space charge region, diffusion length of minority carriers and penetration depth of light into the semiconductor, the value of the flat band potential may be easily determined by ploting i 2 ph vs V and extrapolating to i 2 ph = 0. In the other cases, the relation established by Gartner between i ph and V has been verified provided that the free carriers density be sufficiently large.

The semiconductor-electrolyte interface : Optical transitions in CdMnTe alloys

Abstract The junction p-Cd1−xMnxTe-electrolyte is studied under monochromatic illumination for the alloy composition range 0 ≤ x ≤ 0.7. Optical transitions in the semiconductor are evidenced by photocurrent measurements. In the composition range 0 ≤ x ≤ 0.45, the nature of the fundamental transition appears to be the same as in CdTe (valence band Γ → conduction band Γ ). For x > 0.45, the observed photocurrent p ak, pinned at a consta t energy value ca. 2.1 eV, may presumably be related to a transition Γ → d(Mn) level whereas the photocurrent edge associated to the forme transition increases linearly with x.

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.

Electroluminescence of the electrolyte/n-ZnSe junction under anodic polarization

Abstract The electroluminescent properties of the electrolyte/ZnSe junction give important information on electron transfer across the interface. Adsorption of OH− groups on it, that enables electron injection into the semiconductor, accounts for a luminescence at 2.0 eV, produced by an impact-ionization mechanism already observed in solid state diodes. Addition of Cu2+ ions in the electrolyte gives an additional luminescene at an energy of 2.2 eV. This phenomenon is connected to the adsorption of Cu0 and Cu2+ onto the semiconductor surface in relation with a corresponding energy level CuXZn in the semiconductor bulk.

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.

Flat band potential determination of an electrolyte/semiconductor junction by an electro‐optical method

When excitons have a lifetime sufficiently large at room temperature and when the semiconductor presents a uniform dopant distribution in the near surface region, if a convenient doping level is used it is possible to demonstrate an excitonic interference phenomenon. The latter may be modulated by varying the dc polarization bias V of the semiconductor/electrolyte junction and then oscillations of the ΔR/R electroreflectance response versus V appear. In this particular case of junctions, it is possible to correlate simply the period of the observed oscillations with (V−Vfb )1/2, where Vfb is the flat band potential value. A comparison of values obtained for Vfb in this way with those obtained by classical methods for three different semiconductors (CdS, GaAs, and ZnSe) shows the interest of this electro‐optical method.

Photoelectrochemical study of (Ti,V)O2 and (Ti,Nb)O2 alloys

Abstract The ceramic alloys (Ti,Nb)O2 and (Ti,V)O2 are studied photoelectrochemically in an aqueous electrolyte. A n-type electrical conductivity is noted for the whole composition range. The energy gaps E g for the (Ti,V)O2 alloys are 0.8 eV (VO 2 ) g eV (TiO 2 ) whereas for the (Ti,Nb)O2 alloys E g remains nearly constant and ≈ 3 eV . With an energy gap of ≈ 2 eV , the alloy Ti0.4V0.6O2, which is electrochemically stable, seems especially attractive for solar energy conversion. However, its characteristics are counterbalanced by a too much anodic flat-band potential and by a low quantum efficiency. A correlation between the photocurrents and the associated optical transitions leads to precise band diagrams of both alloys. The study of their photoelectrochemical stability enables one to determine the nature of the optical transitions, giving rise to the photocurrent.