Chemistry Reference
In-Depth Information
the current efficiency increases for water content up to 20%, but drops to much lower
values for water content higher than 30%.
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In
solutions, ionic current efficiency increases in the presence of
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chloride or fluoride ions.
For the anodization in ethylene glycol containing
the efficiency increases rapidly with increasing KF concentration above which
is associated with disturbances in the oxide layer and an excessive incorporation of
fluorine.
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Relatively high current efficiency can be obtained in solutions.
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Figure 3.9 shows the efficiency of oxide growth in 1.35M as a function of
potential and time. The efficiency is higher at low potentials and short anodization
times, indicating that the growth efficiency decreases with oxide thickness. The low
ionic current efficiency during anodization is due to the side reactions. In aqueous
solutions, the side reaction is that of water oxidation leading to oxygen evolution
which occurs at potentials greater than
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In organic solvents, oxidation of the
solvent molecules may also be side reactions.
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The field required for oxide growth also strongly depends on electrolyte com-
position. Table 3.1 shows that the field during oxide growth is in the range from 2.4 ×
to The field is in general larger in organic solvents than in
aqueous solutions. Higher fields are generally required with larger current densities. It
has been reported that the anodization of silicon in dry electrolytes requires a field
strength of about
oxide growth rate falls to essentially zero if the field
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strength is lowered by 10% from the above value.
Anodization in different solvents
requires different fields for growth as shown in Fig. 3.6.
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Addition of water in organic
solvent generally reduces the growth field.
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3.3.3. Effect of Silicon Substrate
There are few data on the effect of substrate on the formation of anodic oxide.
In 2M KOH, the formation characteristics of anodic oxides on (100) and (111) surfaces
in the potential range from 6 to 15V are the same.
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Ion implantation of substrate
silicon has been found to greatly affect the anodization behavior of silicon.
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An
increase in the anodization rate occurs after implantation of
