Abstract
In the early stages of anodization of n-InP in KOH, isolated nanoporous domains form and eventually merge into a continuous layer. SEM and TEM cross-sections show clear evidence that these have the shape of truncated tetrahedrons and result from preferential pore progagation along the <111>A directions. We propose a three-step model of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process occurs only at pore tips. The rate-determining Step 1 is the generation of holes at pore tips. If Step 2 (hole diffusion) is faster than Step 3 (electrochemical reaction) then etching can occur at preferred crystallographic sites, such as phosphorus dangling bonds, leading to pore propagation in preferential directions. Well-defined oscillations are observed under certain conditions when n-InP is anodized in either KOH or (NH4)2S. The oscillations in (NH4)2S, have a symmetric profile and the charge per cycle remains constant over a wide range of conditions: we propose a mechanism based on cyclical changes in the morphology of the InP surface beneath an In2S3 film. The exact mechanism of the oscillatory behavior in KOH is still unclear.
Published Version
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