Morphology anodic dissolution

The fundamental and applied electrochemistry of the silicon/electrolyte interface is presented in an authoritative review by Dr. Gregory Zhang, with emphasis in the preparation of porous silicon, a material of significant technological interest, via anodic dissolution of monocrystalline Si. The chapter shows eloquently how fundamental electrokinetic principles can be utilized to obtain the desired product morphology. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF solutions. The formation of PS was first reported in the late 1950s in studies on electropolishing of silicon. Since then, particularly after 1990 when luminescence of PS was discovered, numerous investigations have been undertaken. These investigations have revealed that PS has extremely rich morphological features with properties that are very different from those of silicon and the formation process of PS is a very complex function of many factors such as HF concentration, type of silicon, current density, and illumination intensity. 

In this chapter, the conditions for the formation of PS, the relation between the formation conditions and PS morphology, and the mechanisms for the formation of PS and morphology are discussed. The various aspects of surface condition, nature of reactions, and reaction kinetics that are fundamentally involved in the anodic dissolution of silicon are discussed in Chapters 2-5. 

A quantitative description of the diverse morphological features of PS requires the integration of the aspects discussed above as well as the fundamental reaction processes involved in silicon/electrolyte interface structure, anodic dissolution, and anodic oxide formation and dissolution as detailed in Chapters 2-5. Any mathematical formulation for the mechanisms of PS formation without such a global integration would be limited in the scope of its validity and in the power to explain details. In addition, a globally and microscopically accurate model would also require the full characterization of all of the morphological features of PS in relation to all of the... 

Fig. 17) and identified as (100) plane which confirmed earlier electrochemical work on the BDP process of lead. They reported the time dependent Pb surface reconstruction from (100) plane to (111). The obsen ation of formation of (111) plane has supported Finch and Layton s view on lateral changes of Pb deposits. They reported the same obsciv ation for anodic dissolution inducing remarkable surface morphology changes. 

According to X-ray diffraction and light optical investigations, the anodic dissolution of -brass results in a e — y — a phase transformation with porous product phases [22, 23]. As revealed by a more detailed investigation of the corrosion morphology, the extent of this transformation depends on the overpotential of the zinc dissolution reaction [24]. At a low overpotential of Eh = —0.75 V, the only product phase is y. Similar to the scheme of... 

The effect of the frequency of pulsation on the morphology is also illustrated by Figs. 2.18-2.20. It seems that under deposition at high frequencies (Fig. 2.20) more pronounced anodic dissolution during the pause occurs compared with the deposition at lower frequencies (Figs. 2.18 and 2.19), which leads to a formation of less dendritic deposit. 

Figure 4.10.9 SEM picture of titanium anode (0 0.25mm) after anodic dissolution in (see figure 4.10.8). Inset Morphology of titanium surface after anodic dissolution...

A model accounting for changes in the surface concentration of kinks was developed by Heusler " from quantitative observations of the steady-state morphology for iron surface vicinal to (112) during anodic dissolution. He described the model as follows ... 

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