Anodic Oxide - Electrochemistry of Silicon and Its Oxide - page 119

ing and a trace amount of fluorine, the anodic oxide is incorporated with fluo-
rine, and the fluorine content increases with the oxide thickness and the fluorine in the
solution. 835 Fluoride content on the order of 10 15 is reached for a fluoride

concentration of in the solution. Most of the fluorine is located in a region close
to the interface and the fluorine is linked to the silicon atoms as is oxygen in
the oxide. The SiOH groups in the anodic oxides show hydrogen bonding. 1039 The
hydrogen bond may form between a SiOH and the nonbridging oxygen atoms associ-
ated with defects in the oxide. 357 Large concentrations of hydroxyl ions in film
enable penetration of protons from the electrolyte into a hydrogen-bonded network at
low cathodic field strengths. 427
The tendency for the solution-bond species to be incorporated into the structure
of anodic oxide films can be used for oxide doping. For example, phosphorus-doped
oxides of different concentrations can be produced by anodization in diethylphosphate
(DEP)-nitrite-tetrahydrofurfuryl alcohol. 459 Figure 3.24 shows that the phosphorus
concentration increases with increasing concentration of DEP in the solution. Low tem-
perature and current density give higher doping levels, indicating that DEP adsorption
decreases with increasing temperature.
According to Nannoni and Musselin, 793 the properties of anodic oxides tend to
change with time after being placed in humid air or water. The action of water causes

incorporation of hydrogen in the interstitial positions in the oxide. The concentration
is an increasing function of exposure time,
and a decreasing function of the pH of the water and the anodic voltage applied across
the oxide. The diffusivity of the species associated with water has been determined to
be at 25 °C for the oxide formed in water and in
ethylene glycol. 836 Surface roughness of anodic films grown in eth-
ylene glycol containing 0.04 M is found to increase with increasing water content
in the electrolyte. 228 The silicon substrate after the removal of the oxide films shows
similar roughness dependence on water content.

of hydrogen, on the order of