Numerical Model of Three-Step Mechanism of Pore Formation in n-InP
Authors: Clancy Ian , Quill Nathan , O'Dwyer Colm , Buckley D. Noel , Lynch Robert P. 
Published: February 2020

The anodic formation of porosity in semiconductors has received considerable attention, due to the fundamental insight it offers into semiconductor etching properties and its wide range of possible applications [1]. Although a number of models have been proposed to explain the formation of porosity in semiconductors [2-5], none as yet has been shown to explain the complete range of structures which have been formed in different semiconductor-electrolyte systems.
Recently we proposed a three-step mechanism for electrochemical nanopore formation in n-InP in KOH that explains how crystallographically oriented etching can occur even though non-preferential propagation of etching could be expected since the rate-determining process (hole generation) occurs only at pore tips. [6] The mechanism explains the observed uniform width of pores and its variation with temperature, carrier concentration and electrolyte concentration. [7-9] It also explains pore wall thickness, and deviations of pore propagation from the <111>A directions. [6,9] As a pore etches, propagating atomic ledges can meet to form sites that can become new pore tips and this enables branching of pores along any of the <111>A directions, corresponding to experimentally observed results. [10-12] This mechanism can be extended to show how current-line oriented etching can occur under certain conditions. [13,14] Furthermore, we believe that the mechanism is generally applicable to electrochemical pore formation in III-V semiconductors.
In this talk, we will present a numerical model of this three-step mechanism for pore formation. The model shows that competition in kinetics between hole diffusion and electrochemical reaction determines the average diffusion distance of holes along the semiconductor surface and this, in turn, determines whether etching is crystallographic. If the kinetics of reaction are slow relative to diffusion, etching can occur at preferred crystallographic sites within a zone in the vicinity of the pore tip, leading to pore propagation in preferential directions. Symmetrical etching of three {111}A faces forming the pore tip causes it to propagate in the (resultant) <111>A direction.

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