Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 362

of inside the pores, because if hydrogen bubbles were formed inside pores they would
block the current path and stop the growth of the pore.
The formation of hydrogen does not occur in anhydrous organic solvents. 136,248,1085
Because of lack of hydrogen evolution the dissolution reactions are entirely electro-
chemical in nature with a dissolution valence of 4 at all current densities. Addition of
water to the organic solvents reduces the dissolution valence as shown in Fig. 8.10. 1085

8.2.4. Growth Rate of Porous Silicon

The growth rate of a PS layer can vary over a wide range depending on formation con-
ditions as shown in Table 8.1. It can be as low as a few angstroms per second and as
high as 4000 Å/s. 36,41 Such rates are well within the limits observed for planar etch rates
of solid silicon and its oxides. The etch rates of silicon in solutions of certain
mixtures can be as high as (see Table 7.1), and those of anodic oxides, accord-
ing to Fig. 4.2, can be on the order of 1000 Å in concentrated HF solutions.
For p -Si the growth rate of PS at a given current density appears to increase lin-
early with HF concentration as shown in Fig. 8.12. 36,1128 For n- Si the growth rate also
increases with HF concentration but the relation is not linear (Fig. 8.13). 12,1153 At a given
HF concentration it increases linearly with increasing current density as shown in Fig.
8.14. 36 It also increases linearly with logarithmic dopant concentration (Fig. 8.15). The
growth rate of PS on n -Si is more complicated. Figures 8.14 and 8.15 show that the
growth rate on n- Si does not follow a linear function with current density nor with
dopant concentration and under identical conditions the PS formed on n- Si type gen-
erally has a higher growth rate than that on p -Si. Also, the growth rate of PS on n -Si
becomes relatively constant at current densities that are a significant fraction of the

as shown in Fig. 8.16. 12,763 The temperature of HF solutions generally current peak

33,170

has little influence on the growth rate of PS. The formation of PS on n -type silicon

in the dark does not occur below certain anodic potentials in the dark but can occur
after roughening or formation of a thin PS layer under illumination. 36,40