Structure and Morphology Control of Porous Silicon

In was also found that porous layers formed can have a great variation in microstructure. One can see in Fig. 26.2 how big the variation could be in microstructure of PSi layers formed under different conditions. Schematically possible structures of porous Si are shown in Fig. 26.3. 

According to Zhang (2005), microstructures of PSi layer can have the following peculiarities  

Pores can be either straight with smooth walls (Fig. 26.3Aa) or can be branched (Fig. 26.3Ab-Af). Research has shown that straight large pores with smooth walls can be formed by backside illumination of n-Si of the (100) orientation (Lehmann 1993). 

PSi can have a surface transition layer. It was found that the pores at the surface usually have smaller diameter than those in the bulk of PSi (Fig. 26.3Bb) (Zhang 1991 Smith and Collins 1992). Such an increase in pore diameter from the surface to bulk is due to the transition from pore initiation to steady growth. The thickness of this transition layer is related to the size of pores the smaller the pores, the thinner the surface transition layer. 

PSi can have a layer structure (Fig. 26.3Bc). Two-layer PSi, with a micro-PSi layer on the top of a macro-PSi layer is formed only under certain conditions. For p-Si, a two-layer structure was observed only on lowly doped substrates. For moderately or highly doped p-Si or for n-Si in the dark, the formation of a two-layer PSi has not been observed. For n-Si, the formation of a two-layer 

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The study of small and intermediate-sized clusters has become an important research field because of the role clusters play in the explanation of the chemical and physical properties of matter on the way from molecules to solids/ Depending on their size, clusters can show reactivity and optical properties very different from those of molecules or solids. The great interest in silicon clusters stems mainly from the importance of silicon in microelectronics, but is also due in part to the photoluminescence properties of silicon clusters, which show some resemblance to the bright photoluminescence of porous silicon. Silicon clusters are mainly produced in silicon-containing plasma as used in chemical vapor deposition processes. In these processes, gas-phase nucleation can lead to amorphous silicon films of poor quality and should be avoided.On the other hand, controlled production of silicon clusters seems very suitable for the fabrication of nanostructured materials with a fine control on their structure, morphological, and functional properties. ... 

Electrochemical deposition enables wide applications of porous silicon in many fields such as optics, sensing, microfabrication, and catalysis. Fine tuning in morphology of deposits, which is crucial for applications, has been desired. As reviewed in this chapter, control of metal electrodeposition has greatly improved in the recent decades. However, there still exist many open questions, such as nucleation and growth and mass transfer for 3D structure formation. They are doubtlessly important and seem to be future issues. 

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