Numerical simulation of the anodic formation of nanoporous indium phosphide

Anodic etching of n-type InP in KOH electrolytes under suitable conditions leads to the formation of a nanoporous region beneath a ∼40 nm dense near-surface layer. The early stages of the process involve the formation of square-based pyramidal porous domains and a mechanism is proposed based on directional selectivity of pore growth along the <100> directions. A numerical model of this mechanism is described in this paper. In the algorithm used the growth is limited to the <100> directions and the probability of growth at any pore tip is controlled by the potential and the concentration of electrolyte at the pore tip as well as the suitability of the pore tip to support further growth. The simulated porous structures and their corresponding current versus time curves are in good agreement with experimental data. The results of the simulation also suggest that, after an initial increase in current caused by the spreading out of the porous domains from their origins, growth is limited by the diffusion rate of electrolyte along the pores with the final fall-off in current being caused by irreversible processes such as the formation of a passivating film at the tips or some other modification of the state of the pore tip.