Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 381

Figure 8.33 schematically illustrates the dendritic pore growth along the <100> directions
for both (100)- and (111)-oriented substrates. 763 The nondendritic branched pores formed
on (100) substrate are not strictly perpendicular to the primary pores but deviate to various
extents from the <100> direction toward the source of holes. 35,60,1083 Table 8.3 shows the
orientation of the pores observed under different conditions.
Ronnebeck et al . 1083 found that the macropores formed on misoriented n (100)
substrates from 15° to 35°, as well as on n (111) are ori-
ented along the <100> direction when they are formed in the dark at high anodic poten-
tials. However, when the samples are illuminated from the back, the pores formed on
the (111) sample and the side pores formed on the 35° misoriented (100) sample are
oriented along <113>. Figure 8.34 shows that the orientation of the main pores formed
on the substrate 35° from the (100) surface is <100>, while that of the side pores is
<113>. According to Ronnebeck et al., crystal defects aligned in the <113> direction
are a possible cause of the unusual pore orientation. In another study by the same group
of researchers, the pores formed in dimethylforamide (DMF) are principally aligned in
the (100) direction independent of the substrate orientation. 1138 Also, while macro pores
may grow along the <113> direction on non-(l00) substrates, the growth of the highly
branched dendritic pores appears to be along the <100> direction independent of sub-
strate orientation. 60,1126
The dependence on substrate orientation for the macropores formed on p

-Si is

. 1085,1138 The macropores formed on p- Si in anhydrous HF organic
solutions are straight and well oriented along the <100> or <113> direction on sub-
strates of different orientations, while those formed in solutions containing water tend
to be perpendicular to the surface (less anisotropic effect) and are poorly aligned.
According to Christophersen et al ., 1085,1138 the supply of oxygen and hydrogen in the
solution are important to the anisotropic growth of pores based on their current burst
theory (see next section); oxygen is necessary for smoothing the pore tips, whereas

similar to that on n -Si