Chemistry Reference
In-Depth Information
which reflects the orientation-dependent dissolution nature of the reactions. The shape
of the pores formed on (100) substrate is a square bounded by {011} planes with corners
pointing to the <100> directions.
46,60
The shape of individual pores formed on
tends
to change from circular to square to starlike and to dendritelike with increasing poten-
tial.
763
Low formation voltage tends to favor circular shape and high voltage, starlike
shape. Near-perfect square shape of pores can be obtained for the PS formed on
n-
Si
at a certain current density, HF concentration, anodic bias, and illumination intensity.
For macropores formed on lowly doped
p
-Si the shape of pores tends to be round at
low current densities but tends to be square at high current densities on a (100) sub-
strate and triangular on a (111) substrate.
177
n
-Si
The bottom of pores is always curved, varying from a shallowly curved semi-
circle to an elongated cone depending on the formation conditions as pictured in Fig.
8,10,12,763
Table 8.4 shows that the shape of pore bottoms, in terms of the ratio of
the length to the diameter, can vary from as small as 0.3 for very shallow bottoms to
as large as 5 for highly elongated bottoms. As will be discussed in Section 8.6, the cur-
vature of the pore bottom is a principal factor in the reaction kinetics required for for-
mation of PS and its morphology.
8.22.
8.3.4. Pore Branching
As illustrated in Fig. 8.19(5), individual pores, depending on the formation conditions,
may propagate straight in the preferred direction with very little branching or with for-
mation of side pores. In general, the conditions that favor the formation of small pores
