Anodic Dissolution and Electroluminescence of p-Si at High Potentials in Fluoride Media

Abstract

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...

Macromorphologies in electrochemically formed porous silica

“…All these observations, together with the characteristic potential range 15-30 V where most of the morphological transitions take place, can be understood when one realizes that oxygen evolution starts to take place around 15-20 V and becomes the dominant Faradaic mechanism above 30 V [10]. As it has been explained earlier, the mechanism of porous silica formation in neutral fluoride medium is associated with the two-step mechanism of silicon dissolution:…”

“…Among aspects that have aroused special interest are the elaboration of porous silicon [3], or the intriguing oscillations observed in the electropolishing regime [4][5][6][7][8][9]. More recently, we found that, at higher potentials, the oxide film normally present under electropolishing conditions becomes mesoporous [10,11], and it can be grown to micron-thickness by using a neutral or weakly alkaline fluoride electrolyte of low buffer strength [12,13]. This process appears related to that leading to porous oxide formation on various metals, especially porous alumina on aluminium [14,15] or TiO 2 nanotubes on titanium [16,17].…”

Self-Organized Macrostructures in Anodically Formed Mesoporous Silica

“…9 Several models have been invoked to explain these oscillations. 10-14 They more or less survive for potentials up to 20 V. 15 Oscillating behavior under high potential conditions was also reported by Parkhutik et al 16 for galvanostatic conditions. Above 30 V, water electrolysis leading to oxygen evolution strongly dominates the electrode reactions.…”

“…Above 30 V, water electrolysis leading to oxygen evolution strongly dominates the electrode reactions. 15 The oxide thicknesses produced in nonfluoride media can reach up to several hundred of nanometers by applying voltages of up to 500 V. 7,8 Beyond that limit, the physical breakdown of the oxide layer prohibits further thickening. In dilute fluoride media, oxide layers of comparable thickness have been observed.…”

(Invited) Anodic Dissolution of Si: Electrochemical Oscillations and Porous Silica Formation

“…At still higher potentials and in similar electrolytes, the current is found to exhibit two successive increases (13). The last one, at around 20 V, is clearly associated with the onset of oxygen evolution.…”