Abstract
Anodization of n-InP electrodes was carried out over a range of temperatures and KOH concentrations. Scanning electron microscopy showed <111>A aligned pore growth with pore width decreasing as the temperature was increased. This variation is explained in terms of the relative rates of electrochemical reaction and hole diffusion and supports the three-step model proposed earlier. As temperature is increased, both the areal density and width of surface pits decrease resulting in a large increase in the current density through the pits. This explains an observed decrease in porous layer thickness: pits sustain mass transport at the necessary rate for a shorter time before precipitation of etch products blocks the pores. As the concentration of KOH is increased, both pore width and layer thickness decrease to minima at ∼9 mol dm−3 after which they again increase. This variation of pore width is also explained by the three-step model and the variation in layer thickness is explained by mass transport effects. Layer porosity follows a similar trend to pore width, further supporting the three-step model. A transition from porous layer formation to planar etching is observed below 2 mol dm−3 KOH, and this is also explained by the three-step model.
Highlights
Anodic etching often leads to the formation of porous structures in many materials and can be technologically important in nanofabrication processes
We have previously shown that anodization of an n-InP electrode in >2 mol dm−3 KOH results in the formation of a nanoporous InP layer[3,48,49,50,51] of finite thickness.[52,53,54,55,56]
The variation in linear sweep voltammograms (LSVs) with temperature will be discussed later but it is first necessary to consider the results of cross-sectional and surface microscopy on anodized electrodes
Summary
Anodic etching often leads to the formation of porous structures in many materials and can be technologically important in nanofabrication processes. The large increase in pit current density (Fig. 7) explains the observed decrease in layer thickness (Fig. 8) and shorter etch time (Fig. 2a) as temperature is increased.
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