Morphology of Microporous Silicon

Even under magnifications of the order of 105, micro PS looks rather homogeneous under the transmission electron microscope, as shown in Fig. 7.2a. This is in contrast to the microstructure of meso PS, which can be revealed at this magnification, as shown in Fig. 7.2 b. It requires HREM techniques to reveal the struc- 

Raman measurements of free-standing PS films have indicated crystallite dimensions as small as 2 nm [Ka3]. Investigations of PS films grown on substrates of different doping density showed a broad size distribution (2-10 nm) for p+-type substrates, while the size distribution for PS films on low doped p-type substrates 

PS films of 65% porosity formed on moderately (1 Q. cm, micro PS) and highly (0.01 J)cm, meso PS) doped substrates, 

SAXS is sensitive to variations in the electronic density in a material, and so provides information about the shape and size of clusters in micro PS. In contrast to the quantum wire structure proposed in early publications [Cal, Lei], the crystallites in micro PS are found to be almost spherical. There has been some evidence that the dimensions in the growth direction are somewhat smaller than those parallel to the surface [Fr2], The latter result, however, is still controversial because investigations by spectroscopic techniques indicate an opposite elongation [Mi4], A combination of grazing incidence diffraction (GID) and SAXS measurements on various freestanding micro PS films showed crystallite diameters from about 1.5 to 4 nm, depending on formation conditions. A good correlation between size reduction and blue shift of the PL peak position has been observed [Bi3], 

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