Anodic Oxide - Electrochemistry of Silicon and Its Oxide - page 107

Si before and after anodization in glycol, the Si ions are the mobile

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ions during the anodization. Croset and Dieumegard found that oxygen during the

growth of the anodic films in ethylene glycol containing 0.4% and 0.4%
using the labeling technique, is the mobile species. Beckmann and Harrick 424 found
that the concentration of hydroxyls in the anodic oxide formed in 0.25 M in
tetrahydrofurfuryl alcohol increases toward the interface as shown in Fig. 3.16.
This indicates that there must be migration of the OH groups to the interface under
the action of the applied field and the oxidation reaction at the interface must be the
rate-limiting process to cause a higher concentration near the interface with respect to
the interior of the film. Bardwell et al ., 449 who used the profiling technique, found
that in the anodic oxide formed in solution, the is concentrated at the
oxide/silicon interface, suggesting that inward migration of oxygen is the main ionic
transport mechanism.
On the other hand, Madou et al ., 404 based on the identification of a small amount
of P and S in the oxides formed in the glycol solutions and none in those formed in

methanol solutions, suggested that Si outward movement may be involved during the
427 , 459 who used as marker,
the growth of anodic films on Si in dry tetrahydrofurfuryl alcohol at high field strengths
occurs almost exclusively by cation drift. However, they also found that at low field
strengths, hydroxyl ions, if present in ample supply, are the mobile species and cause
hydration of the film.
In a study by Schmidt and Ashner, 427 inward motion of hydroxyl ions is also found
in the anodic oxidation of a steam-grown oxide, resulting in a hydrated oxide film. On
contact with the electrolyte there is an induction period during which the steam-grown
oxide surface is hydrated. The electrical field is able to aid the injection of hydroxyl
ions after hydration takes place. According to Schmidt and Ashner, the primary step
involves reaction of the oxide surface with water in the electrolyte to produce
OH groups: The OH groups can then be acted on by
the electrical field to migrate inward into the oxide. The diffusivity of the species

oxidation in methanol. According to Schmidt and Ashner,