Anodization of highly doped (10
) n-InP in 2
5 mol dm
KOH under potentiostatic or potentiodynamic conditions results in
the formation of a nanoporous sub-surface region. Pores originate
from surface pits and an individual, isolated porous domain is
formed beneath each pit in the early stages of anodization.
Each such domain is separated from the surface by a thin non-porous
layer (typically ~40 nm) and is connected to the electrolyte by its
pit. Pores emanate from these points along the <111>A crystallographic directions to form domains with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the surface of the electrode. We propose a three-step model of electrochemical pore formation: (1) hole generation at pore tips, (2) hole diffusion and (3) electrochemical oxidation of the semiconductor to form etch products. Step 1 determines the overall etch rate. However, if the kinetics of Step 3 are slow relative to Step 2, then etching can occur at preferred crystallographic sites leading to pore propagation in preferential directions.