Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 415

only for the limited range in the continuum of these parameters. For example, it is not

applicable for the pores formed at low current densities relative to the critical current,
of when the growth rate of pores significantly increases with
increasing current. Also, according to Lehmann's data the concentration reduction at
the pore tip due to the diffusion effect is only about 20% at a PS thickness of
which indicates that diffusion has only a minor role in the rate-determining process.
Processes other than diffusion must play a major role particularly for relatively shallow
pores.

i.e., less than 10%

Formation of Two-Layer PS on Illuminated n -Si. Two-layer PS with a micro
PS on top of a macro PS and on the walls of the individual macropores formed on illu-
minated n -Si had been reported in the late 1970s but was little investigated until the
16,247,600,767 The micro PS may have a fractal-like geometry and can vary in the
same layer in structure from amorphous to single crystalline and in diameter from a
few to hundreds of nanometers.
Whereas the formation of a macro PS layer on n -Si under front illumination
follows the same mechanism as the macro PS formed in the dark, the formation of
micro PS is believe to be mainly due to the effect of the photogenerated carriers. Essen-
tially, the dissolution reaction under illumination proceeds with a supply of photo holes
which are generated near the surface and distributed uniformly in the porous structure.
This results in a dissolution process different from that in the dark or back illumina-
tion and the formation of PS of extremely fine and randomly oriented structures. 4
According to Arita, 651 the anodic current under front illumination consists of three parts:

1990s.

where and are the drift current due to carriers generated in the depletion
layer, the diffusion current due to minority carriers outside of the depletion layer, and
the election tunneling current through the barrier into the conduction band. The drift
current has a particularly strong effect on the crystalline structure of the PS:

where is the total flux of incident light, is the light absorption coefficient, and x is
the distance from the surface. The larger the contribution of the drift current, the
stronger is the tendency to form amorphous PS because the amount of amorphous PS
decreases with decreasing illumination intensity and with increasing PS thickness.
Alternatively, Clement et al . 247 proposed that the micro PS found under illumi-
nation could result from shattering of the macro PS into fine filaments due to residual
stress. Dissolution-precipitation was also considered to be a possible mechanism for
the formation of micro PS although it is inconsistent with the single-crystalline nature
of the micro PS in many two-layer PS.
Theories on the Macro PS Formed on Lowly Doped p -Si. The formation of
macro PS and two-layer PS on p -Si was first discovered by Kohl et . 136,248 in the mid-

1990s on lowly doped substrate in anhydrous organic HF solutions. Macropores on p-
Si were later found to also form in aqueous solutions and the walls of the macropores
are not always covered by a micro PS layer. 177,952,1058

al