Columnar growth morphology

We now turn to a-Si H. Street and Knights (1981) studied a-Si H with columnar growth morphology. The large internal surface area of these films facilitates the reaction studies of the surface, and we can assume that the same processes occur on the top surface of films that do not have these internal surfaces. The freshly deposited films had a very small spin density because of the efficient bond saturation with hydrogen. During exposure to... 

Sputtered films showed columnar growth of crystallites normal to the substrate surface [% No strong growth morphology was observed in evaporated films f]. 

A special kind of morphology found in polymers crystallizing in contact with a nucleating surface is the trans-crystalline structure. The nucleating object may be a flat surface or a fibril. The densely appearing nucleations at the surface of the nucleating object result in a one-dimensional (columnar) growth in a direction parallel to the normal of the surface. The thickness of the trans-crystalline layer depends on the outcome of the competition between surface nucleation and bulk nucleation. 

The contamination of film surfaces after film deposition but before the next processing step or before use can be a concern. The as-deposited film surface is clean and has a very reactive surface. In addition, it may have a very high surface area and be porous because of the growth of a columnar film morphology. This means that if the film surface is contaminated it will be very difficult to clean. The surface should be protected and stored in a manner commensurate with its subsequent use (Sec. 13.9). 

Fig. 14. Titanium carbide coating on graphite showing the nodular growth with a columnar morphology and the fracturing resulting from a coefficient of...

For the most part, even qualitative relationships between such structural considerations and film properties are lacking. Recently, however, two important correlations have been presented Device quality a-Si contains only monohydride bonding structures (207), and the intrinsic film stress rises drastically as a morphological transition from columnar to noncolumnar growth occurs (210). Both studies point to the importance of defects in a-Si, although the precise role of these defects is still unclear. 

Finally, an area that will most likely see an explosive growth over the next few years is the self-assembly of nanoparticles covered with mesogenic and pro-mesogenic capping agents. A number of different approaches have been summarized in this review, and the formation of nematic, smectic-like, cubic, and columnar phases and/or superstructures have been demonstrated. Once more, the possibilities to produce such metamaterials using nanoparticles and liquid crystal motifs are endless, and future research will surely discover other, in part, more complex phase morphologies as well as uniquely tunable nanoscale properties as a result of liquid crystal phase formation. 

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