Nonfluoride Solutions

In Nonfluoride Solutions. In water or nonfluoride aqueous solutions the disso- 

FIGURE 4.29. The elemental steps involving breaking Si-0 bond, (a) Adsorption (b) activated complex (c) hydrolysis. (Reprinted from Dove and Rimstidt. 1994 by permission of the Mineralogical Society of America.) 

The nature of the silica-water interface is determined by adsorption/desorption of the species in the water. When a silicon oxide, e.g., quartz, is fractured, the initial surface is composed of dangling silicon and oxygen bonds (Fig. 4.30a) which are not stable and hydroxylate easily with available waterThe hydroxylated surface is dominated by SiOH groups (Fig. 4.30b). The initial adsorbed water adjacent to the surface is oriented and has properties different from the bulk water. As this adsorbed water layer increases to more than three monolayers, its properties become more like bulk water. The surface potential changes as a result of the adsorption of the ionic species in the water. ° 

The interaction of the hydroxyl groups with the protons in the water establishes an interfacial layer with a pFl-dependent surface charge and potential. According to 

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The photocurrent in nonfluoride solutions is affected by the amount of preanodic current passed through the sample as shown in Fig. 5.10. It is also seen that the photocurrent onset potential is shifted to more anodic values with formation of an oxide film and the amount of shift is related to the thickness of the film. This shift is due to the potential drop across a growing oxide layer and is one of the reasons for the difference between the photocurrent onset potential and the flatband potential. ... 

In nonalkaline and nonfluoride aqueous solutions, silicon substrates behave as essentially inert electrodes due to the presence of a thin oxide film. Even in alkaline solutions, silicon is passivated by an oxide film at anodic potentials beyond the passivation peak. Very small current can pass through the passivated silicon surface of n- or p-type materials in the dark or under illumination. Depending on the pH of the electrolyte, oxidized surface sites Si—OH are more or less ionized into anionic species Si—0 owing to the acido-basic properties of such radicals so that the passivation current can vary in a wide range from a few... 

Anodic behavior of sihcon can best be characterized by i-V curves. Neglecting the details associated with a silicon substrate such as doping, the current-potential relationship of silicon in aqueous solutions can be considered to be principally determined by the pH and HE concentration as illustrated in Pig. 5.1. In nonalkaline and nonfluoride aqueous solutions, silicon as an electrode is essentially inert, showing a very small current at anodic potential due to the presence of a thin oxide film. In alkaline solutions, silicon is also passivated by an oxide film at anodic potentials but is active below the passivation potential, Vp. In fluoride solutions, the silicon electrode is active in the whole anodic region as shown by the large anodic current. 

FIGURE 5.1. Typical i V curve in HF and nonfluoride, nonalkaline solutions. 

In nonfluoride acids such as H2SO4 solutions, the anodic current is due to the formation and thickening of the oxide fihn. Addition of fluoride species to these... 

The silicon surface in nonfluoride and nonalkaline solutions is spontaneously passivated due to the formation of a thin native oxide film at a rate depending on many factors as discussed in Chapter 2. For n-Si samples in aqueous solutions under illumination the occurrence of passivation causes a decrease of the photocurrent as shown in Fig. 5.11. " In the absence of HF, photocurrent rapidly reduces to near zero due to the formation of an oxide film. The stationary photocurrent increases with increasing HF concentration. For a given light intensity, there is a HF concentration above which the photocurrent does not decrease from the initial value. The surface is free of oxide film at this HF concentration. 

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