Local epitaxy

A local epitaxy of diamond crystallites has been reported in Ref. [200], where Si(lOO) substrate was pretreated in microwave plasma under conditions of c —0.1%CH4/H2, P= 100-120Torr, and 7 = 1040-1050°C for lOh, and successively, diamond growth was done under conditions of c= 1%CH4/H2, 7 = 50-60 Torr, and 7s = 850-880 °C for 40 h. On the Si substrate, there were two mutually oriented large diamond crystals of 120 x 150 pm in size and a number of small oriented... 

Y Sato, H Fujita, T Ando, T Tanaka, M Kamo. Local epitaxial growth of diamond on nickel from the vapor phase. In A Lettington, JW Seeds, eds. Thin Diamond Films. London Chapman Hall, 1994, p 56. 

Obviously, only parts of the defects created on the surface are paramagnetic, and thus EPR active. Nonetheless subsequent XiCl4 deposition on samples covered with a non-epitaxial MgCl2 film quenches the EPR signal by 40% [21], This can be taken as a clear indication that some of the defects are localized on the surface of the MgCl2 film, while most of the defects are bulk defects not influenced by adsorbed TiCl4. 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

The use of UPD layers can in principle generate deposits with composition modulated on the atomic scale, and Pauling et al. have produced what they call hetero-structured ultra-thin films containing Ag, Pd and T1 by this method [158], Stickney and coworkers have assembled multilayered deposits of CdTe and GaAs by addition of one atomic layer of the individual components at a time, a process they call electrochemical atomic-layer epitaxy [159 162], The essential controlling feature in the UPD mechanism is that the deposited layers are allowed to reach equilibrium. Hence, the process represents an extreme of local, reversible control. 

For surface area determinations the ideal adsorbate should exhibit BET C values sufficiently low to preclude localized adsorption. When the adsorbate is so strongly tied to the surface as to be constrained to specific adsorption sites, the adsorbate cross-sectional area will be determined more by the adsorbent lattice structure than by the adsorbate dimensions. This type of epitaxial adsorption will lead to decreasing measured surface areas relative to the true BET value as the surface sites become more widely spaced. 

Woemer et al. 373) produced polyacetylene with locally oriented regions and an optical anisotropy of 2 x by polymerization on crystals of biphenyl. Yamashita and co-workers 374,375) have recently reported epitaxial polymerization of acetylene on crystals of anthracene, naphthalene and biphenyl where fibrils of cis- or trans-polymer formed, crystallographically aligned with the substrate. Fincher et al. 376) produced a 3 x extension which gave a 4 x optical anisotropy. 

Figure 18. Fitting of the epitaxial layers planar defect (left) and local configuration of molecules along the planar defect (right)...

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