Chemistry Reference
In-Depth Information
The current under a given potential is generally constant as a function of anodiza-
40,41
tion time or of PS thickness. Figure 8.17 shows that the PS growth on heavily doped
-Si is a linear function of time for thickness up to about Such constant current
and growth rate mean that the PS formed is uniform in thickness, a finding that agrees
with the results from morphological investigations. At large thicknesses the growth may
deviate from linearity. This occurs at different thicknesses under different conditions;
it starts on a smaller PS thickness at high current densities in concentrated solutions.
41
The deviation from linearity, which indicates that either the effective area at the disso-
lution front is increased or the effective dissolution valence is increased, is due to the
effect of diffusion in the electrolyte within the pores.
12,1133
PS growth rate is higher on
(100) samples than on (111) samples.
1132
n
8.2.5. Mass Transport
The exponential current-potential relationship in the PS formation region indicates that
mass transport has little influence on PS formation. This is further confirmed by the
fact the current during PS formation is essentially independent of the thickness of
2,36,40
Also, the concentration of HF inside pores is found to be essentially the same
as outside.
40
However, as the PS layer gets thicker, diffusion may start to limit the rate
under high current densities. For example, Fig. 8.17 shows that the thickness of PS
increases linearly with time under various current densities and potentials below
At a certain thickness depending on HF concentration and current density, the growth
slows down. It has been found that for a very thick PS layer
PS.
there is a dif-
12
ference of about 20% in HF concentration at the tips of pores and the bulk solution.
The morphology of the PS formed under a diffusion-controlled situation will change
with increasing thickness. Due to the depletion of HF at pore tips, the distribution of
