Nucleation amorphous silicon

There is little information regarding the surface chemistry involved in the nucleation of amorphous silicon by photo-induced chemical vapor deposition (photo-CVD). The reason seems to be that effective chemical and physical means of detecting a small amount of silicon are hardly available at present. In our laboratory, the initial process of amorphous silicon (a-Si) formation from silanes or disilanes on Si02 substrate by photo-CVD has been studied by a new technique of chemical... 

The study of small and intermediate-sized clusters has become an important research field because of the role clusters play in the explanation of the chemical and physical properties of matter on the way from molecules to solids/ Depending on their size, clusters can show reactivity and optical properties very different from those of molecules or solids. The great interest in silicon clusters stems mainly from the importance of silicon in microelectronics, but is also due in part to the photoluminescence properties of silicon clusters, which show some resemblance to the bright photoluminescence of porous silicon. Silicon clusters are mainly produced in silicon-containing plasma as used in chemical vapor deposition processes. In these processes, gas-phase nucleation can lead to amorphous silicon films of poor quality and should be avoided.On the other hand, controlled production of silicon clusters seems very suitable for the fabrication of nanostructured materials with a fine control on their structure, morphological, and functional properties. ... 

In the sections that follow, we consider the kinematics and kinetics of plastic flow by repeated nucleation of STs, primarily in metallic glasses and amorphous silicon as key examples. Similar corresponding applications to glassy polymers are covered in Chapter 8. 

Hyper nucleating agents have been developed recently and offer the combined benefits of high crystallization rates and isotropic shrinkage control, which leads to improved production and part quality performance, in addition to the traditional mechanical property-related enhancements. Hyper nucleating agents are formulated compounds. For example. Hyperform HPN-68L is a blend of fixe disodium salt of bicyclo[2.2.1]heptane-23-dicarboxylic acid, Eruc-amide and amorphous silicon dioxide (11). 

As for the effect of hydroxyl ion, it is not possible to see how it could catalyze the dissolution of stishovite in which silicon has already reached its maximum coordination number. No data seem to be available on the effect of pH on the dissolution rate of stishovite, but it is interesting that at pH 8.4 it dissolves about as fast as vitreous silica when compared on the basis of equal areas of surface being exposed to the solution. Furthermore, it continues to dissolve past the saturation level for vitreous or amorphous silica. The concentration of soluble silica can reach as high as 190 ppm, at which point colloidal particles are nucleated (139). It is likely that stishovite is hydrolytically unstable and would eventually decompose completely to amorphous silica. Whether or not" pH has an effect on the rate of hydrolysis is not known. 

The inclusion of nucleation inhibitors such as silicon dioxide can modulate nu-cleation process, thus prolonging the suspension stability. Particle size control of amorphous formulations is essential for homogeneity and withdrawability for dosing accuracy. 

Most of the work has been done in silicon, in a comparison [ 23 ] with metals one must note that usually interface velocities are higher due to the greater thermal conductivity. Supersaturated alloys have been obtained by ion implantation in combination with electron or laser beam pulsed heating. In some cases the coupling between thermal and matter transport, i.e. the Soret effect, has been evidenced [24] Precipitation in the liquid phase has been shown in Sb-implanted A1 system together with the measurement of submicrosecond nucleation times. Amorphous phase formation requires usually alloys. 

The control of the second critical step, crystallization of the amorphous alloy, is the focus of the following discussion. Iron-silicon and iron-aluminum systems are discussed. The critical lengthscales in the iron-aluminum and iron-silicon systems are much larger than that observed in the molybdenum-selenium systems. By critical lengthscale, we refer to the thickness of the repeat unit in the multilayer below which the multilayer evolves completely into an amorphous material without the nucleation of any crystalline phase. The samples discussed are all layered on a lengthscale which is less than this critical value. That is, they evolve from a layered initial state, through a distinct amorphous intermediate, to a crystalline compound. 

Stoichiometry has long been used by solid-state chemists to control the final products of a reaction. However, traditional synthetic techniques do not have the ability to control reaction intermediates and all stable phases will form as illustrated in Figure 2. For example, in the iron-silicon system, thin film diffusion couples have been used to determine the sequence of phase formation (79). FeSi was always found to nucleate first, followed by the crystallization of FeSi2 at the FeSi-Si interface and FeSi3 at the FeSi-Fe interface. The following paragraphs provide evidence that stoichiometry of the amorphous intermediates can be used to control nucleation to obtain the desired crystalline compounds directly. Thus, we use stoichiometry to control the mechanism of the reaction. 

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