Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 385

conditions, perfectly straight pores with smooth walls can be formed on n -Si in the

dark, as, for example, shown in Fig. 8.35. 1153 Also, on

smooth and straight pores
without branching can be obtained under back illumination using a surface-patterned
substrate. On and samples, the pores grow perpendicular to the silicon surface
with many secondary pores on the sides of these pores which sometimes results in
branching. 35,572 The macropores formed on lowly doped

n -Si

p-Si generally have no side

pores longer than the pore diameter. 136,177,1027

The branches of the main pores are due principally to the tendency for anisotropic
dissolution along the <100> direction. Like the main pores the branched pores are highly
directional, propagating preferentially in the {100} planes and the long <100> direc-
46,60,763 Such directional branching can produce regularly spaced three-dimensional
structures as shown in Fig. 8.36. 1153 The tendency to branch is stronger on (110) and (111)

tions.