Effect of electrolyte concentration on anodic nanoporous layer growth for n-InP in aqueous KOH

The surface morphology and sub-surface porous structure of (100) n-InP following anodization in 1 - 10 mol dm^-3 aqueous KOH were studied using linear sweep voltammetry (LSV) in combination with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). LSV of n-InP in 10 mol dm^-3 KOH showed a single anodic current peak at 0.41 V. As the concentration of electrolyte was decreased, the peak increased in current density and charge and shifted to more positive potentials; eventually individual peaks were no longer discernable. Porous layers were observed in SEM cross-sections following linear potential sweeps and the porous layer thickness increased significantly with decreasing KOH concentration, reaching a maximum value at ∼2.2 mol dm^-3. At concentrations less than 1.8 mol dm^-3 the layer thickness decreased sharply, pore diameters became wider and pore walls became narrower until eventually, at 1.1 mol dm ^-2 or lower, no porous layers were observed. It was also observed that the pore width increased and the inter-pore spacing decreased with decreasing concentration. It is proposed that preferential pore propagation occurs along 〈111〉 directions, contrary to previous suggestions, and that the resulting nanoporous domains, initially formed, have triangular cross-sections when viewed in one of the {110} cleavage planes, 'dove-tail' cross-sections viewed in the orthogonal {110} cleavage plane and square profiles when viewed in the (100) plane of the electrode surface. copyright The Electrochemical Society.