In semiconductor materials, electrochemical processes during anodization can cause localized etching to occur.^1,2 This localized etching leads to dissolution of material in a way that creates complex porous structures. Such structures are of considerable interest. With silicon, crystals can be engineered that behave as though they have a direct bandgap, which is of interest in photonics.^2,3 Metals such as aluminium and titanium can also be used to form tubular structures, via localized etching of oxide layers that form at the surface.^4-6 The resulting structures have high surface area and so are of interest for a range of applications.

Under many circumstances, it is possible to etch crystallographically-oriented pores. As the term suggests, these pores align themselves with certain crystallographic directions, such as <100> or <111>. Such pores lead to the formation of very regular porous structures.^3,7-12

The III-V semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP), exhibit pore propagation along <111> crystallographic directions, more specifically <111>A, the directions of bonds from A atoms (gallium or indium) to B atoms (arsenic or phosphorus).^8,9 This crystallographic pore propagation is interesting from a theoretical modelling point of view. The case of InP anodized in potassium hydroxide (KOH) is of particular interest because the etching is purely electrochemical, i.e. simple models can be easily compared with actual experiments.

These regular structures also exhibit intriguing optical properties. In the case of InP, typical pore width is an order of magnitude smaller than the wavelength of visible light, so pores cannot be resolved optically, but the features of the porous structure do cause constructive and destructive interference.¹³ Furthermore, interference patterns are also observed on the surface of anodized InP electrodes.¹⁴

In this presentation, we will investigate computational modelling of pore formation and propagation during anodisation of InP in aqueous KOH solutions and describe the propagation of visible light in such porous structures by modelling of the propagation of waves.

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