Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 356

roughening or passing through an anodic current prior to PS formation will greatly
increase the current density at lower potentials, at which the current density is very
small without the surface roughening. The current rise occurs at a potential several volts
lower on a rough surface than that on a smooth surface.
The i-V curve of n -Si under a high illumination intensity, when the reaction is
no longer limited by the availability of photogenerated carriers, is identical to that for
p -Si except for a shift along the potential axis. As for p -Si, formation of PS on n -Si
occurs only below the critical current, The i-V relationship at a current density much
lower than the saturation photocurrent density is similar to that observed on p -Si. When
the saturation photocurrent density is lower than the critical current density, the current
is limited by the saturation photocurrent.
The current at a given potential increases with HF concentration. The current
density in the exponential region is linearly proportional to HF concentration, giving a
reaction order of about 1, while in the electropolishing region the HF reaction order is
about 1.35. 2 Also, the potential at peak current shifts to high values with increasing HF
concentration. The i-V curves measured in concentrated HF solutions are very noisy
due to the violent hydrogen evolution at high current densities in concentrated solu-
tions. Unlike in diluted HF solutions, the current in concentrated HF solutions at high
potential values depends on scanning rate and decreases on successive curves measured
on the same sample, indicating the role of mass transport in the growing PS layer in
these solutions. 1153
8.2.2. Conditions for PS Formation and Electrochemical Polishing
Examination of the sample surfaces that are anodized at different potentials indicates
that the potential corresponding to the maximum slope of the i - V curve is the upper
limit for formation of a uniform PS layer. At potentials between the maximum slope
and current peak, the porous layer may still form but the surface coverage is not
uniform. Thus, the anodic behavior is characterized by three regions: PS formation,
transition, and electropolishing. 2