Chemistry Reference
In-Depth Information
oxides for different properties. For none of the anodic oxides reported in the literature
do all of the properties overlap those of thermal oxides.
3.5.1. Physical and Chemical Properties
The oxide films formed by anodization generally have a loose structure and contain
a significant amount of water, hydroxyl ions, and other species, which are present in the
electrolyte. Many of the physical, chemical, and electrical properties of anodic silicon
oxides are determined by this loose structure and the incorporation of foreign species.
This defective nature is required for the growth of an anodic oxide because a large number
of charged carriers are moving within the oxide during its formation.
As-formed anodic oxides may contain Si-O bonds, SiOH and SiH groups,
absorbed water, oxidation products of the solvent, and ionic impurities.
776
For example,
there are hydroxyl and silicon hydride SiH groups in the
anodic oxide films formed in tetrahydrofurfuryl.
424
By comparison,
the thermal film produced in wet oxygen at 1050°C contains OH groups and
that produced in dry oxygen contains no OH groups. The anodic oxide grown in 0.1 M
HCl contains about 4 wt % OH in the form of water and 4 wt % OH as isolated silanol.
98
Due to the incorporation of OH and water, anodic oxides are generally nonstoi-
chiometric with silicon-deficient structures. For example, the oxide grown in 1.35 M
has a stoichiometry
of
with total (Si + O) density of
449
and contains approximately 1 atom % N.
The oxides formed in
may contain 1-5% water and have O : Si ratios varying from 1.8 to 3.4
depending on current density, oxide thickness, and water content in the solution as
shown in Fig. 3.23.
919
They contain a large amount of carbon compounds in the form
of C—H, S—O—C, and C—C groups.
1039
The anodic oxide grown in a phtha-
late aqueous solution is incorporated with a substantial amount of water and/or hydroxyl
groups with thinner films exhibiting a higher content.
604
In diethylene glycol contain-
