Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 359

density, On the other hand, at a given current density, n increases with HF concen-
tration. Also, the dissolution valence decreases with increasing PS layer thickness as
shown in Fig. 8.7; 41 also, a dissolution valence as low as 0.5 is measured for thick PS
films formed at low current densities. The low n values are due to the chemical disso-
lution of PS; the thicker the PS layer, the longer the sample is present in the electrolyte
and the larger the amount of chemical dissolution. For p- Si, on which the PS has
an extremely fine pore structure, the chemical dissolution during the growth of PS is
significant and is responsible for the low n values below 2.
For p- Si in the PS formation region, n tends to increase with doping concentra-
tion particularly at high doping concentrations as shown in Fig. 8.8; 35,651 for n -Si the
effect of doping concentration on dissolution valence is seen to depend on current
density. It decreases with increasing illumination intensity as shown in Fig. 8.9.
Substrate orientation has little effect on n as shown in Fig. 8.10. 1085
Evolution of hydrogen gas occurs during the formation of PS. It is the result of
the chemical reaction responsible for the effective dissolution valence of less than 4.
Figure 8.11 shows that the amount of hydrogen gas is proportional to the time of
anodization. 34 When the anodization is stopped, the hydrogen evolution still continues
at a lower rate. In situ FTIR studies indicate that the H-termination of the silicon surface
is preserved during PS formation on n -Si. 477,775,1123 The silicon atoms on the surface are