Vapour condensation techniques
In the gold-silicon system of Figure 5, at compositions far from the cusp, glasses cannot be formed by melt quenching—even by the rapid splat-quench technique ofFigure 4. (This is the reason that the Tg curve of Figure 5 spans only compositions near the cusp.) Amorphous solids can still be prepared by dispensing with the liquid phase completely and constructing a thin solid film in atom-by-atom fashion from the gas phase. Figure 4D shows the simplest of these vapour-condensation techniques. A vapour stream, formed within a vacuum chamber by thermalevaporation of a sample of the material to be deposited, impinges on the surface of a cold substrate. The atoms condense on the cold surface and, under a range of conditions (usually a high rate of deposition and a low substrate temperature), an amorphous solid is formed as a thin film. Pure silicon can be prepared as an amorphous solid in this manner. Variations of the method include using an electron beam to vapourize the source or using the plasma-induced decomposition of a molecular species. The latter technique is used to deposit amorphous silicon from gaseous silane (SiH4). Among the amorphous solids listed in the table, those that normally require vapour-condensation methods for their preparation are silicon (Si), germanium (Ge), water (H2O), and the elemental metallic glasses iron (Fe), cobalt (Co), and bismuth (Bi).
Vapour condensation techniquesIn the gold-silicon system of Figure 5, at compositions far from the cusp, glasses cannot be formed by melt quenching—even by the rapid splat-quench technique ofFigure 4. (This is the reason that the Tg curve of Figure 5 spans only compositions near the cusp.) Amorphous solids can still be prepared by dispensing with the liquid phase completely and constructing a thin solid film in atom-by-atom fashion from the gas phase. Figure 4D shows the simplest of these vapour-condensation techniques. A vapour stream, formed within a vacuum chamber by thermalevaporation of a sample of the material to be deposited, impinges on the surface of a cold substrate. The atoms condense on the cold surface and, under a range of conditions (usually a high rate of deposition and a low substrate temperature), an amorphous solid is formed as a thin film. Pure silicon can be prepared as an amorphous solid in this manner. Variations of the method include using an electron beam to vapourize the source or using the plasma-induced decomposition of a molecular species. The latter technique is used to deposit amorphous silicon from gaseous silane (SiH4). Among the amorphous solids listed in the table, those that normally require vapour-condensation methods for their preparation are silicon (Si), germanium (Ge), water (H2O), and the elemental metallic glasses iron (Fe), cobalt (Co), and bismuth (Bi).
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