Propagation of Nanopores and Formation of Nanoporous Domains during Anodization of n-InP in KOH

Nanopore formation due to electrochemical etching occurs in a range of semiconductors from Si to InP and 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 recent work^1-4 on nanopore formation during anodization of n-InP electrodes in aqueous KOH.

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 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 propose¹ 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 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 (remaining) <111>A direction.  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. 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

3.  N. Quill, R. P. Lynch, C. O’Dwyer and D. N. Buckley, ECS Trans., 2013, 50 (37), 143

4.  C. O’Dwyer, D. N. Buckley, D. Sutton, M. Serantoni and S. B. Newcomb, J. Electrochem. Soc., 2007, 154, H78

5.  C. O’Dwyer, D. N. Buckley, D. Sutton and S. B. Newcomb, J. Electrochem. Soc., 2006, 153, G1039