Facet formation

The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53], The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate-adsorbate and adsorbate-substrate interactions determines the stereochemistry of the facet. 

A number of workers have studied the epitaxial relationships of ZnO on Zn (57-61). The earlier reports of pseudomorphism (58) have not been confirmed by later workers. The most careful study with attention to the surface preparation of the metal was that of Lucas (57) and his results for room temperature oxidation are included in Table IL This work emphasized the importance of surface preparation, and showed conclusively that different orientations could occur on a surface as a result of facet formation. At higher temperatures Raether (59) reported an orientation in which the (0001) planes of the ZnO were normal to the surface rather than parallel as in the case of room temperature oxidation. Yang (61), however, reported the usual parallel orientation at 350°C. 

FIGURE 2.25. Facet formation in the course of catalytic CO oxidation on a Pt(l 10) surface [34] (a) continuous splitting of the (0,1)-LEED beam and sketch of the facets (b) variation of the beam splitting (open symbols) and the steady-state reaction rate (filled symbols) at two temperatures. 

This can easily be achieved with an ordered removal of a 1/4--monolayer of terminating atoms. The pattern also shows the expected 3-fold rotational symmetry. A more complicated diffraction pattern is that of CdS (OOOl), shown in Fig. 6 for six different voltages.These patterns show a 6-fold symmetry. Also, there is evidence of a /J x -30° reconstruction (panels a and c) and of facet formation (the streaks in panels b and d) ... 

Y. Kunii, M. Tabe, K. Kajiyama, Amorphous Si/ crystalhne Si facet formation during Si solid phase epitaxy near Si/Si02 boundary. J. Appl. Phys. 56, 279-285 (1984)... 

Frequently, partial 3D models are applied, which consist typically of (crucible) melt and crystal. Such models are useful to perform basic studies on convection and crystallographic phenomena (facets, stress, strain, dislocations, ghde planes, etc.). In [28] it was shown that a small tilt of the ampoule relative to gravity can have a significant influence on the flow pattern and the flow velocities, fan and Tu [29] presented 3D simulations for the growth of YAG crystals, in which facet formation is coupled to heat flow and segregation. 

Two cases corresponding to small and large convexity of the rounded crystal/melt interface toward the melt were studied in detail. The first can be achieved when the cylindrical side surface is diSuse, and the second, when it is specularly reflecting. Computations showed that the relation between the facet radius and supercooling is very close to a well-known square root law in the first case and it is strongly different in the second. Thus, in the case of large convexity of the rounded interface the effect of internal radiation on the facet formation turns out to be very significant. 

Shibata, M., Nitta, Y, Fujita, K., and Ichikawa, M. (2000). Facets formation of pyramidal Si nanocrystals selectively grown on Si(OOl) windows in ultrathin Si02 films, J. Crystal Growth 220, 449-456. 

Tseng, H.-C., Chang,. C.Y, Pan, F.M., Chen, J.R., and Chen, L.J. (1997). Effects of isolation materials on facet formation for silicon selective epitaxial growth, Appl. Phys. Lett. 71, 2328-2330. 

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