Porous InP layers can be formed electrochemically on (100) oriented n-
InP substrates in aqueous KOH. A nanoporous layer is obtained
underneath a dense near-surface layer and the pores appear to propagate
from holes through the near-surface layer. In the early stages of the
anodization transmission electron microscopy (TEM) clearly shows
individual porous domains which appear to have a square-based pyramidal
shape. Each domain appears to develop from an individual surface pit
which forms a channel through this near-surface layer. We suggest that the
pyramidal structure arises as a result of preferential pore propagation
along the directions. AFM measurements show that the density of
surface pits increases with time. Each of these pits acts as a source for a
pyramidal porous domain. When the domains grow, the current density
increases correspondingly. Eventually, the domains meet forming a
continuous porous layer, the interface between the porous and bulk InP
becomes relatively flat and its total effective surface area decreases
resulting in a decrease in the current density. Numerical models of this
process have been developed. Current-time curves at constant potential
exhibit a peak and porous layers are observed to form beneath the
electrode surface. The density of pits formed on the surface increases with
time and approaches a plateau value