The morphology of film growth

Plasma deposition tends to be intermediate between the CVD and PVD extremes. When highly reactive gas species are formed in the plasma and the deposition rate is high, then a PVD mechanism is favored, whereas less reactive species favor CVD. The gas dilution 

One way to observe and quantify the type of growth is to deposit material into a deep trench cut into the substrate. In the PVD process the total deposit in the trench is proportional to the area of the top opening because it is flux-limited, and when the trench is deep and narrow, the film will be thin. In contrast, the deposit in a CVD process is proportional to the substrate surface area, so that the film in the trench is the same thickness as on the top surface. These two situations are illustrated in Fig. 2.7. 

The flat, conformal, CVD-like growth of a-Si H is important both for the material properties and for the device technology. Invariably, PVD conditions result in films with a high density of electronic defects, which are associated with the internal surfaces of voids. The columns also oxidize rapidly when exposed to air, because their open structure allows the diffusion of oxygen from the atmosphere. CVD films have a much lower defect density and no oxidation except for a thin layer on 

The structure and morphology of the a-Si H films obviously depend greatly on the form of the growing surface. In situ ellipsometry is a powerful experimental tool which gives information about the surface layer during the deposition, by measuring the optical constants of the film. The effective dielectric fimction of the material is obtained from 

The surface roughness of the films is also obtained from ellipsometry. Typical roughness values of 10 A are found for films with optimum CVD growth conditions, while PVD growth conditions lead to a much greater roughness. 

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Due to the variation of the structure and surface orientation, the diffusion barrier of the adatoms differs along a certain direction (Brune 1998). In addition, the diffusion barrier is also influenced by the presence of other adatoms, nuclei, and impurities on the surface. Thus, according to Equation 18.2, small modifications in the diffusion barriers cause an exponential increase of changes in the diffusion rates, which is an important aspect in the morphology of film growth. 

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