The electrochemical behaviour of p-type silicon in dilute fluoride electrolyte has been investigated in a potential range extending up to +100 V vs SCE. Beyond the known range where the system is prone to exhibiting an oscillatory behaviour (3 -8 V), two marked current rises are observed, at around 10 V and 20 V. Evidence for electronic transport through the oxide is given by the appearance of two distinct electroluminescence bands, a red band above 10 V, and a "blue" band above 20 V. The observation of strong gas evolution above 20 V confirms that in this potential range transport through the oxide is mainly electronic. However, in the range 10 -20 V, no gas evolution is seen and electrochemical impedance indicates that the oscillatory behaviour somewhat survives. Furthermore, the electrical thickness of the oxide film appears one order of magnitude lower than its thickness determined by infrared spectroscopy, which proves that the oxide film is largely porous. The appearance of this porosity is associated with the sharp increase in current around 10 V. Direct scanning-electron-microscopy observation of the oxide film indicates a close similarity with porous alumina formed in similar conditions, which suggests that ordered structures may be realisable in this system.

The electrochemical dissolution of silicon in fluoride media under very high potentials ͑5-100 V͒ was investigated. The interface was characterized by voltammetry and in situ infrared spectroscopy. The infrared results indicate that the interfacial oxide becomes porous above a critical potential of ϳ20 V. Light emission from the interface was observed for potentials above ϳ10 V. The electroluminescence signal was found to be composed of a red component and a blue component. Both components exhibit an increase in intensity as a function of applied potential, but their distinct dependence suggests that they arise from two distinct types of oxide defects and excitation mechanisms.In the last ten years many studies have been devoted to the anodic dissolution of silicon in fluoride media. Most of these studies have been directed to the understanding of porous silicon formation, 1 or to the complex behavior occurring in the electropolishing region, up to potentials of 5-10 V. 2-9 An especially intriguing feature was the observation of an electrochemically oscillating ͑or more exactly resonant͒ regime in the 3-8 V region, whose exact origin is not as yet perfectly understood. [10][11][12][13][14][15][16][17][18][19][20] Meanwhile, potentials above 10 V have been scarcely explored. 21 However, such studies are of interest for several reasons. Light emission has been observed from thin silica films in the presence of a large applied voltage. [22][23][24] This offers a way for characterizing defects in the SiO 2 film, including in the presence of the electrolyte. Also, it is in a similar high-potential regime that anodization of aluminum leads to the formation of porous alumina, a material of high interest for the elaboration of nanostructured materials. 25 Electrodissolution conditions offer more reproducible conditions for analyzing these phenomena in a steady state, which should bring useful pieces of information on the transport mechanisms in the oxide film.The present paper represents an attempt to explore p-Si dissolution in dilute fluoride electrolytes, in a potential range extending up to ϩ100 V. The electrochemical behavior was investigated using voltammetry on a rotating disk electrode, and the oxide film was characterized by in situ infrared spectroscopy. Finally, the electroluminescence spectrum was studied as a function of electrolyte and applied potential. After describing the experimental procedures in the following section, we then present the results on these three aspects and discuss the information that they bring. ExperimentalVoltammetric and electroluminescence measurements.-The samples used for the voltammetric and electroluminescence measurements were cut from p-type boron-doped silicon ingots with a resistivity of 0.31-0.5 ⍀ cm. Samples are disks of 4 mm diam and 3 mm thickness. Ohmic contacts were made on the back side using p ϩ doping ͑Au:1% Al evaporation while cooling the sample from 600°C͒ and indium or silver-epoxy soldering. The electrode was sealed with wax in a polytrifluorochloroethylene b...

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