Etching Mechanisms in III-V Semiconductors: Electrochemical Etching of Indium Phosphide
Authors: Buckley D. Noel , Quill Nathan , O'Dwyer Colm , Lynch Robert P. 
Published: February 2020

Etching of III-V semiconductors is often observed to be anisotropic. For example both chemical and electrochemical etching of III-V semiconductors show preferential etching of {111}B planes (i.e. group-V-terminated planes).1,2 The slowest-etching plane is usually {111}A and so these facets are revealed during etching of InP, GaAs and GaP. Due to the differing etch rates of crystal planes, the formation of tetrahedral etch pits (seen as dove-tailed and v-groove voids, respectively, in (011) and (01$\bar{1}$) cross sections) is observed on the (100) surface of III-V semiconductors.
The anodic etching of many materials leads to the formation of porous structures. When semiconducting materials are anodized, localized etching can occur leading to selective removal of material such that the remaining material forms a skeletal structure that encompasses a network of pores. It is quite common for such pores to propagate along preferential directions. These directions and the pore morphology have been shown to be affected by a range of parameters including temperature, composition and concentration of electrolyte, and type, orientation and doping density of the substrate.
We have investigated in detail the early stages of pore formation in InP during anodization in aqueous electrolytes (KOH, KCl etc.). Pores originate from pits in the surface and propagate along <111>A directions to form porous domains beneath a thin near-surface layer penetrated only by one surface pit per domain of pores.3-5 Eventually these domains merge to create a continuous porous layer. Although pore propagation in InP in KOH is usually crystallographically oriented (CO), it can be current-line-oriented (CLO) under certain conditions. Indeed, variation of potential, electrolyte concentration and temperature causes a gradual change in the porous layer structure and a transition from pore formation to planar etching is observed as the KOH concentration is decreased to below 2 mol dm-3.

We have proposed a three-step model6,7 of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process (hole generation) occurs only at pore tips. 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 is 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. We believe that the model is generally applicable to electrochemical etching in III-V semiconductors.
In this paper, we will examine the effect of various parameters including temperature, electrolyte composition, potential and current density on electrochemical etching and nanopore formation in InP. We will discuss the processes leading to crystallographic etching, the factors which affect pore morphology and the transition from porous to planar etching and show that the three-step mechanism can comprehensively explain these various features.

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