Nanotechnology is an area of growing industrial importance and there is considerable interest in the fabrication and characterisation of nano-scale structures for various applications. Although considerable progress has been made in developing technology for such applications, the underlying fundamentals are not always well understood and many important scientific questions need to be investigated if technological progress is to be maintained. Fundamental studies of key processes such as nanopore formation therefore provide important pathways to understanding and controlling nanofabrication techniques.

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. Many different pore morphologies have been observed and several models have been proposed to explain them. In this paper, we review our work^1-5 on nanopore formation during anodization of n-InP electrodes in aqueous KOH.

When the potential of an InP electrode in a KOH solution (2 – 5 mol dm^-3) is swept in the anodic direction, a current peak is obtained after which the current decreases to a low value. Cross-sectional electron microscopy of electrodes shows the formation of a porous sub-surface region of fairly uniform thickness separated from the surface by a thin non-porous region, ~40 nm in thickness. Porous InP structures can be formed both under potentiodynamic and potentiostatic conditions. Remarkably, however, porous InP layers are not formed in KOH solutions more dilute than 1.5 mol dm^-3.

Detailed scanning and transmission electron microscopy (SEM and TEM) shows that pores emanating from surface pits propagate along the <111>A crystallographic directions to form, in the early stages of anodization, porous domains (Fig. 1) with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the (100) surface of the electrode. This was confirmed by comparing the predictions of a detailed model of pore propagation with SEM and TEM observations. The model considered pores originating from a pit at the (100) surface and propagating along <111>A directions at rates equal at any instant in time. It showed in detail how this leads to domains with the shape of a tetrahedron truncated by a (100) plane. Observed cross sections correspond in detail and with good precision to those predicted by the model. SEM and TEM showed that cross sections are trapezoidal and triangular, respectively, in the two cleavage planes of the wafer, and TEM showed that they are rectangular in the surface plane, as predicted. Aspect ratios and angles calculated from observed cross sections were in good agreement with predicted values. The pore patterns observed were also in good agreement with those predicted and SEM observations of the surface further confirmed details of the model.

We have proposed 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. The model explains the observed uniform width of pores and its variation with temperature, carrier concentration and electrolyte concentration. It also explains pore wall thickness, and deviations of pore propagation from the <111>A directions. We believe that the model is generally applicable to electrochemical pore formation in III-V semiconductors.

REFERENCES

1. R. P. Lynch, N. Quill, C. O’Dwyer, S. Nakahara and D. N. Buckley, Phys. Chem. Chem. Phys., 15, 15135-15145 (2013)
2. R. P. Lynch, C. O’Dwyer, N. Quill, S. Nakahara, S.B. Newcomb and D. N. Buckley, J. Electrochem. Soc., 2013, 160, D260
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