Electrochemical formation of nanoporosity in n-InP anodes in KOH

We review our recent work on anodic formation of nanoporosity in n-InP in aqueous KOH. Typically, a nanoporous sub-surface region is formed beneath a thin, dense near-surface layer. Atomic force microscopy (AFM) shows pit formation on the surface in the earlier stages of etching, and transmission electron microscopy (TEM) shows individual nanoporous domains separated from the surface by a thin InP layer. Each domain develops from a surface pit. We developed a model based on propagation of pores along the A directions. The model predicts porous domains with a truncated tetrahedral shape and this was confirmed by scanning electron microscopy (SEM) and TEM. Pores are cylindrical and have well-developed facets only near their tips. No porous layers are observed at a KOH concentration of 1.1 mol dm -2 or lower. Linear sweep voltammograms (LSVs) show a pronounced anodic peak corresponding to the formation of the porous region. We describe a technique to deconvolute the effects of potential and time in LSVs and explain their shape and their relationship to porous layer formation

Deconvolution of the potential and time dependence of electrochemical porous semiconductor formation

A layer of porous InP is grown beneath a thin dense surface layer when n-InP electrodes are anodized to sufficiently high potentials in aqueous KOH solutions. The shape of the linear sweep (LSV) or the cyclic voltammogram (CV) is dependent on carrier concentration. A technique is presented to deconvolute the effects of potential and time on a CV. The results obtained from this technique are used to explain the shape of the anodic current response and its relation to porous layer formation. The accuracy of the deconvolution technique is then tested by comparison to experimental results

Formation of nanoporous InP by electrochemical anodization

Porous InP layers can be formed electrochemically on (100) oriented n- InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains which appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually, the domains meet forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Numerical models of this process have been developed. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value

Numerical simulation of the anodic formation of nanoporous InP

Anodic etching of n-type InP in KOH electrolytes under suitable conditions leads to the formation of a nanoporous region beneath a ~40 nm dense near-surface layer [1]. The early stages of the process involve the formation of square-based pyramidal porous domains [2] and a mechanism is proposed based on directional selectivity of pore growth along the directions. A numerical model of this mechanism is described in this paper. In the algorithm used the growth is limited to the directions and the probability of growth at any pore tip is controlled by the potential and the concentration of electrolyte at the pore tip as well as the suitability of the pore tip to support further growth. The simulated porous structures and their corresponding current versus time curves are in good agreement with experimental data. The results of the simulation also suggest that, after an initial increase in current caused by the spreading out of the porous domains from their origins, growth is limited by the diffusion rate of electrolyte along the pores with the final fall-off in current being caused by irreversible processes such as the formation of a passivating film at the tips or some other modification of the state of the pore tip

Mechanism that dictates pore width and <111>a pore propagation in InP

We report a mechanism for pore growth and propagation based on a three-step charge transfer model. The study is supported by electron microscopy analysis of highly doped n-InP samples anodised in aqueous KOH. The model and experimental data are used to explain propagation of pores of characteristic diameter preferentially along the A directions. We also show evidence for deviation of pore growth from the A directions and explain why such deviations should occur. The model is self-consistent and predicts how carrier concentration affects the internal dimensions of the porous structures

Nanoporous domains in n-InP anodized in KOH

A model of porous structure growth in semiconductors based on propagation of pores along the A directions has been developed. The model predicts that pores originating at a surface pit lead to porous domains with a truncated tetrahedral shape. SEM and TEM were used to examine cross- sections of n-InP electrodes in the early stages of anodization in aqueous KOH and showed that pores propagate along the A directions. Domain outlines observed in both TEM and SEM images are in excellent agreement with the model. The model is further supported by plan-view TEM and surface SEM images. Quantitative measurements of aspect ratios of the observed domains are in excellent agreement with the predicted values

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 directions, contrary to previous suggestions, and that the resulting anoporous domains, initially formed, have triangular cross-sections when viewed in one of the {110} cleavage planes, ‘dove-tail’ crosssections viewed in the orthogonal {110} cleavage plane and square profiles when viewed in the (100) plane of the electrode surface

Preferential <111>A pore propagation mechanism in n-InP anodized in KOH

This paper describes the formation of pores during the anodization of n-InP in aqueous KOH. The pores propagate preferentially along the A crystallographic directions and form truncated tetrahedral domains. A model is presented that explains preferential A pore propagation and the uniform diameters of pores. The model outlines how pores can deviate from the A directions and from their characteristic diameters. It also details the effect of variation of carrier concentration on the dimensions of the porous structures

Modeling information technology impacts in clerical work environments

The industrialization of white collar work via information technology (IT) is a key indicator of the emerging post-industrial economy (Hirschhorn, 1988). A conceptual model linking clerical work environments to effective IT impacts such as IT utilization, IT\u27s perceived ease of use and performance impact was developed and empirically tested. Underlying the model is the premise that clerical job structure is a facilitator of IT impacts (Weber, 1988). The results indicate that clerical job structure factors such as typing, composing or editing, and bookkeeping were significantly related to IT impacts in clerical work settings. A job holder\u27s IT competence and top management commitment to IT were also found to be significantly related to IT impacts in clerical work settings. Antecedent relationships among the IT impacts also contributed to understanding the IT impacts - for example, IT utilization affected perceived ease of use and perceived performance impact, and perceived ease of use affected perceived performance impact