Columnar grain growth

Fig. 6.26. Directional heat flow during alloy casting to induce columnar grain growth (schematic).

UOg structure Grain size near periphery 11 yum Columnar grain growth region 0.22 in (5.6 mm) dla. ... 

Temperature and temperature gradient too low for columnar grain growth... 

The stmcture of the polysihcon depends on the dopants, impurities, deposition temperature, and post-deposition heat annealing. Deposition at less than 575°C produces an amorphous stmcture deposition higher than 625°C results in a polycrystalline, columnar stmcture. Heating after deposition induces crystallization and grain growth. Deposition between 600 and 650°C yields a columnar stmcture having reasonable grain size and (llO)-preferred orientation. 

At higher temperatures, deposits tend to be columnar (Fig. 2.13 a and b) as a result of uninterrupted grain growth toward the reactant source. The structure is also often dependent on the thickness of the deposit. For instance, grain size will increase as the thickness increases. A columnar-grain structure develops, which becomes more pronounced as the film becomes thicker. 

After the silicon is completely reduced. Reaction (3) stops and Reaction (1) is resumed. The temporary deposition of sacrificial silicon interrupts the tungsten grain growth as new nucleation sites are created. Larger grain and columnar growth are essentially eliminated... 

CVD graphite can have several structural forms columnar, laminar, or isotropic. The columnar form is shown in Fig. 7.2. The crystallites are deposited with the basal planes (ab directions) essentially parallel to the deposition surface. Their structure tends to be columnar (cone-like) as a result of uninterrupted grain growth toward the reactant source. 

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