J.-N. Chazalviel

The p-Si/fluoride interface in the anodic region: Damped and/or sustained oscillations

The anodic dissolution of silicon in fluoride media has given rise to much excitement in recent years [l-lo], particularly concerning the intriguing current oscillations sometimes observed in the anodic region (2.5 V < Es,, < 7 V). Several authors [1,5,10-121 have found that the current exhibits spontaneous oscillations above a critical potential = 3 Vs,,. On the contrary, other authors [8,9] have found damped oscillations, which appear only upon a perturbation of the electrode potential. This has allowed them to perform a detailed investigation of the electrochemical impedance in that regime, and to propose a model accounting for their experimental results [9,13]. The origin of the discrepancy between the various authors was somewhat of a mystery. However, the compositions of the electrolytes were different in the various groups, and, interestingly, the electrolytes where sustained oscillations had been observed are in a fluoride concentration range higher than those where the damped ones had been reported. We will show here that, irrespective of the electrolyte composition in a wide fluoride concentration and pH range, the oscillations can be made intentionally damped or sustained, depending upon the value of the uncompensated resistance in series with the interface. All our experiments have been performed on low-doped p-Si electrodes (resistivity l-4 R cm). We performed two kinds of experiments. The first experiments demonstrate that, on a given interface, with damped oscillations (“steady” interface), a sustained oscillation can be created by adding an external resistor in series with the working electrode. We have taken the

In Situ Characterization of the p ‐ Si / NH 4 F Interface during Dissolution in the Current Oscillations Regime

Several physicochemical properties of the p-Si/NH 4 F interface have been monitored by in situ techniques in the regime of current oscillations. Comparison of evolution of infrared absorption, microwave reflectivity, electrode admittance, hydrogen evolution, and electron injection rate shows interesting correlations. An integrated description of the processes involved is attempted on the basis of the current models for the Si/acidic fluoride interface.

Kinetic and diffusional current contributions in the anodic dissolution of p-Si immersed in fluoride electrolytes

Leffet de la vitesse de rotation de lelectrode sur les caracteristiques courant-tension est etudie lors de la dissolution anodique dune electrode de silicium de type p

In situ luminescence and IR study of porous silicon during and after anodic oxidation

Abstract When porous silicon is transferred into a non-fluoride electrolyte and anodically oxidized, the onset of red electroluminescence during anodic oxidation appears correlated with a decrease in the OH IR absorption bands, indicating significant electrolyte removal from the pores. The electron states whose population is affected by carrier injection or light excitation have been investigated using in situ electromodulated or photomodulated IR spectroscopy. The modulated IR absorption of red-luminescent electro-oxidized porous silicon exhibits an extra absorption of localized carriers in the 1000–2500 cm −1 region, suggesting that the red luminescence occurs through carriers trapped in localized states. The localization process may be efficiently affected by the dielectric constant of the medium surrounding the silicon nanocrystallites.

In situ infrared spectroscopy of the semiconductor ∣ electrolyte interface

Abstract Infrared spectroscopy has been developed as a powerful tool for the in situ study of the electrochemical interface. The possibility of using a multiple-internal-reflection geometry makes it especially suitable for the study of the semiconductorxa0∣xa0electrolyte interface. For high sensitivity to surface species, it is implemented as a differential or a modulation technique. Absorption due to vibrational transitions provides straightforward characterisation of the changes in the chemical state of the surface (interfacial films, adsorbed species, structure of the double layer); it can also be used to probe the composition of the electrolyte in the diffusion layer (redox species, reaction intermediates, pH) or even of the bulk of the electrode, in the case of intercalation reactions. Electronic absorption provides direct information on the semiconductor space-charge layer and on electronic states at the interface. Inherent possibilities of the infrared technique include the use of the polarisation of the infrared light, which gives information on the orientation of interface species, and the analysis of the time response of the infrared absorption, which gives direct information on the interfacial kinetics. Careful analysis of the changes in electrolyte absorption and spectrum baseline may provide valuable indirect information, for instance about infrared non-active species, surface roughening, or the formation of a porous layer at the interface. In situ infrared spectroscopy clearly offers a wealth of information about the semiconductorxa0∣xa0electrolyte interface, and it is sufficiently convenient and versatile that it could be used to a much wider extent than has been the case so far.

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.

In situ Spectroelectrochemical Study of the Anodic Dissolution of Silicon by Potential-Difference and Electromodulated FT-IR Spectroscopy:

The anodic dissolution of p-Si has been investigated by in situ infrared spectroscopy. The combination of potential-difference and electromodulated spectroscopies allows for the acquisition of a rather complete picture of the various regimes of the dissolution. After a review of general principles for studying electrochemical interfaces, a study of the interfacial oxide layer formed in the electropolishing regime is presented. Quantitative analysis shows that the thickness and quality of the oxide (density and defect content) depend upon electrode potential. Free-carrier absorption detected in electromodulated spectra shows that the blocking character of the oxide is correlated with the buildup of a stoichiometric oxide of low defectivity at sufficiently positive potentials. Furthermore, the dynamic response to the modulation reveals that oxides formed at weak positive potentials interact with electrolyte species through electro-induced adsorptions/desorptions on charged SiOH sites. At more positive potentials, charge is transported across the oxide by charged defects which could be associated with tricoordinated, positively charged SiO species. Finally, results obtained during porous silicon formation at weak positive potentials are presented. Potential-difference spectroscopy indicates that the electrode exhibits a very large specific surface area, and that the surface is covered by SiH bonds. Electromodulated infrared spectroscopy reveals that the SiH species are generated upon anodic current flowing and that the breaking of these bonds is the rate-limiting step of the anodic reaction. These unexpected results have given rise to the elaboration of new microscopic models for the direct anodic dissolution of silicon in fluoride electrolytes.

In-situ photoluminescence studies of porous silicon in liquids

Abstract Photoluminescence of porous silicon in HF-based liquids has been revisited. When the porous layer is kept wet after fabrication, the luminescence is initially red and weak. Upon exposure to the photoluminescence-excitation light, a brighter luminescence gradually builds up and shifts to higher energies. If the sample is removed from the liquid, the blue shift is decreased, but the intensity of the luminescence then appears much larger than previously. In the liquid, the evolution rate is faster and the spectral shift is slightly larger for more energetic excitation light. Under intense light, one observes a saturation of the ageing rate and of the photoluminescence intensity, which can be ascribed to Auger recombination. The evolution of the luminescence under illumination is explained by photoelectrochemical etching of porous silicon by HF. The etching creates small-size structures responsible for the high-energy luminescence. These structures are destroyed upon drying, due to capillary forces. A quantitative interpretation of the evolution rate is attempted on the basis of optical absorption of porous silicon. The screening capability of the liquid is suggested as a possible origin for quenching of the luminescence in the electrolyte.

Modulated infrared spectroscopy at the electrochemical interface

Performing infrared spectroscopy of chemical species at the electrochemical interface represents a difficult challenge in terms of sensitivity (1 monolayer ∼1015 species/cm2) and selectivity (presence of the electrolyte). These problems are efficiently addressed by using modulation coupled with lock-in detection of the optical signal. The electrode potential, which governs the interface behavior, is the most straightforward physical quantity that can be modulated. Such a modulation technique may be combined with Fourier transform spectroscopy by using an interferometer with a very slow scanning speed of the movable mirror (∼1–10 μm/s). This approach allows one to reach high sensitivity (typical minimum detectable signal ΔI/I ∼ 10−6 in a single-reflection arrangement). In some special cases, other modulations may be of interest, for example, modulation of the light at a semiconducting photoelectrode. A common benefit of these modulation techniques is that the resulting response can be analyzed as a function of the modulation frequency or by consideration of the phase of the signal at a given frequency. As can be shown for several examples, this analysis allows one to distinguish between the various physical and electrochemical processes taking place at the interface: change of free-carrier concentration beneath the electrode surface or of ion populations in the ionic double layer, adsorptiondesorption effects, and Faradaic processes, for which useful information on the reaction mechanisms may be obtained.

In-situ infrared investigations of the electrochemistry of heteropolyacids at n-Ge electrodes

Abstract The electrochemical modification of n-Ge electrodes has been carried out using 10 −3 M heteropolyacid solutions (HPA, here α-SiW 12 O 40 4− and α-PW 12 O 40 3− ) in 1 M acidic electrolyte. First, cyclic voltammograms containing the various reduction waves of HPA ions were obtained. The infrared vibrational spectra of the various reduced species were obtained by using Fourier-transform electromodulated infrared spectroscopy (FTEMIRS), with potential modulation in the ranges corresponding to the reversible reduction waves. After subsequent negative polarization, the irreversibly modified surfaces exhibit a considerable shift of the hydrogen evolution potential in the positive direction. The in-situ infrared vibrational spectra show that a heavily hydrated compound, retaining locally the overall structure of the initial ions, is formed on the Ge surface during the cathodic reduction of the HPA. The electromodulated infrared spectra provide direct evidence for the catalytic activity of the silicotungstate deposit in improving the H 2 -evolution kinetics. The hydrogen evolution reaction is shown to be associated with a process involving an increased interaction between water and H 3 O + ions and the Wue5f8O bonds; a corresponding mechanism is suggested.