(Invited) Development of Nanoporous Structures and Oscillatory Behavior During Anodization of n-InP in Alkaline Electrolytes
Authors: Buckley D. Noel , Quill Nathan , O'Dwyer Colm , Melly Tom , Harvey Elizabeth , Lynch Robert P. 
Published: December 2020

Electrochemical etching of semiconductors under anodic conditions can lead to the growth of nanoporous structures. There has been extensive research in this area for silicon and III-V electrodes1-7. Porous structures are obtained3-7 when n-InP is anodized in aqueous KOH at concentrations of 2 mol dm-3 or greater; planar etching occurs below 1 mol dm-3. The observation of current oscillations during the anodization of semiconductors has been reported by several authors.8-11 Current oscillations on silicon in fluoride-containing electrolytes have been linked to the cyclic growth and dissolution of thin oxide layers. We have observed10,11 current oscillations on InP both in aqueous KOH and in aqueous (NH4)2S.
A typical current-voltage curve of InP anodized in 5 mol dm-3 KOH is shown in Fig. 1. The anodic peak at 1.9 V (SCE) corresponds to the dissolution of InP leading to the formation of a porous InP layer that extends over 500 nm into the substrate. At higher potentials, sustained oscillations are observed.
Porosity originates from pits in the surface creating domains of pores beneath a thin dense near-surface layer of ~40 nm.4 Each domain is connected to the surface via an individual channel, and eventually these domains merge to form a continuous porous layer. Pore propagation in InP in KOH is usually crystallographically oriented (CO), although it can be current-line-oriented (CLO) under certain conditions. We have proposed5 a three-step model 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.
Current oscillations on n-InP in aqueous (NH4)2S occur above ~1.7 V(SCE). Similarly, in galvanostatic measurements oscillations in potential occur at higher current densities. A remarkable linear relationship is observed between the frequency of oscillation and the average current density. Thus, the oscillations exhibit a characteristic constant charge per cycle. Anodization of highly doped n-InP in aqueous KOH also shows current oscillations at higher potentials. In this case, however, the charge per cycle increases linearly with potential.
The oscillatory current region in the InP/(NH4)2S system corresponds to growth of a thick porous In2S3 layer on the electrode. In contrast, the oscillatory region in the InP/KOH system corresponds to anodic dissolution of InP with the formation of a porous region near the surface. The characteristics of the InP/KOH system are very similar to reported behavior for silicon in HF, where porous silicon formation and anodic current oscillations are also observed.
We will review our work on nanoporosity and oscillatory behavior in these systems and discuss the underlying mechanisms.

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Figure 1