Photo anodic Etching

In order to discuss the correlation between etching kinetics and etching morphology, let us first reconsider the current-potential characteristics of III-V semiconductors, as depicted in Fig. 1. For the sake of clarity, the behavior in alkaline solutions (Fig. 1 (b)) is treated first. As announced, the discussion is mainly based on experimental results obtained from GaP single crystals. 

This table clearly shows that for all three crystal faces studied, etching at a diffusion-limited rate leads to a flat surface (Table 2, cases A, B, C, and G, H, I). This is explained by the fact that for a diffusion-limited process, the etching rate is the same over the entire surface and is hence unaffected by solid-state factors. 

Identification letter Type Face Etching conditions Rate-determining step Surface morphology 

A P (ITT) anodic current plateau (fig. 1 (p) (b) region II) OH diffusion homogeneously flat 

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As stated in the introduction, wet etching processes may proceed either with or without external current flow. In the former case, the semiconductor crystal is incorporated as an electrode in an electrochemical cell and polarized anodically under illumination or in darkness for n- and p-type samples, respectively, leading to dissolution of the sample (see Sec. 2). This is referred to as the (photo)anodic etching process. 

Table 2. Survey of results on (photo)anodic etching of GaP in aqueous 0.1 mol 1 KOH solutions.

In summary, the interpretation of the morphology of electroless etching processes at GaP is analogous to the proposed interpretation of (photo)anodic etching. In electroless etching, though, the potential is not applied by an external source, but it is imposed as a rest potential through addition of Fe(CN) to the solution. 

Highlights of research results from the chemical derivatization of n-type semiconductors with (1,1 -ferrocenediyl)dimethylsilane, , and its dichloro analogue, II, and from the derivatization of p-type semiconductors with N,N -bis[3-trimethoxysilyl)-propyl]-4,4 -bipyridinium dibromide, III are presented. Research shows that molecular derivatization with II can be used to suppress photo-anodic corrosion of n-type Si derivatization of p-type Si with III can be used to improve photoreduction kinetics for horseheart ferricyto-chrome c derivatization of p-type Si with III followed by incorporation of Pt(0) improves photoelectrochemical H2 production efficiency. Strongly interacting reagents can alter semicon-ductor/electrolyte interface energetics and surface state distributions as illustrated by n-type WS2/I-interactions and by differing etch procedures for n-type CdTe. 

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