Nathan Quill

Deconvolution of the Potential and Time Dependence of Electrochemical Porous Semiconductor Formation

A layer of porous InP is grown beneath a thin dense near-surface layer when n-InP electrodes are anodised to sufficiently high potentials in 2-10 mol dm KOH [1,2]. The cyclic voltammogram (CV) shows a characteristic single anodic peak for samples with carrier concentrations of ~3-6 × 10 cm. For electrodes with a higher carrier concentration (> ~5 × 10 cm), two anodic peaks can be distinctly observed on the forward potential sweep. A novel technique is used to deconvolute the effects of potential and time on a CV and this enables us to interpret observed differences in the linear sweep voltammograms (LSVs) of InP electrodes of different carrier concentrations. Cyclic voltammograms of InP electrodes were acquired after anodization to a series of upper potentials in the potential range for porous layer formation [2]. The slopes of these voltammograms were measured in both the forward and reverse directions at the potential of scan reversal. These slope values can be used to deconvolute the effects of potential and time from the CV at any given potential. The result of this analysis, shown in Fig. 1, is a deconvolution of the contribution of potential and time on the voltammogram as a function of the applied potential during porous layer growth. Analysis of Fig. 1 shows that in the early stages of porous layer formation the increasing potential has little effect, resulting in initially low measured current. In this potential range (0.0-0.2 V in Fig. 1) we typically observe only surface pitting. Above a certain potential (the pore formation potential, Ep = 0.23 V), pore growth occurs, where resulting domains of pores continually create further current paths effectively increasing the current density. Between the first and second current peak, it is the increasing potential that directly influences the rate of current increase. This indicates that the pore growth process has already reached its maximum rate for a given potential; the process is thus self-limiting as individual porous domains have now coalesced into a single porous layer. No new current paths can now be formed, and the further passage of current leads only to a thickening of the porous layer.