Silicon seed crystals

Recently, afluidized-bed approach has been developed wherein SiHa and H2 gases are fed into the bottom of a vertical reactor held at a temperature >600°C. Silicon seed crystals are suspended in the chamber due to the gas flow, and decomposition of the gaseous precursor causes the nucleation/growth of silicon on the surface of the seeds. When grown to large sizes, the particles no longer remain suspended and are collected at the bottom of the reactor. 

In the process shown in Fig. 11, hydrogen and silane are fed to the bottom of a fluidized-bed reactor, while high purity silicon seed crystals are charged to the top. Hydrogen utilization is quite low so that most of the hydrogen is recycled. The reactor operates at 600-800°C and slightly above atmospheric pressure. The desired decomposition reaction occurs on the surface of the seed crystals and results in particle... 

The nature of the deposit and the rate of nucleation at the very beginning of the deposition are affected, among other factors, by the nature of the substrate. A specific case is that of epitaxy where the structure of the substrate essentially controls the structure of the deposit.Plb lP ] Epitaxy can be defined as the growth of a crystalline film on a crystalline substrate, with the substrate acting as a seed crystal. When both substrate and deposit are of the same material (for instance silicon on silicon) or when their crystalline structures (lattice parameters) are identical or close, the phenomena is known as homoepitaxy. When the lattice parameters are different, it is heteroepitaxy. Epitaxial growth cannot occur if these stmctural differences are too great. 

The substrate materials are metals (W, Mo, Ti), silicon (e.g. mirror-polished wafers used in the production of semiconductor devices), glassy carbon, graphite [15], etc., depending on manufacturer or user preferences. Diamond nanocrystals are used as seed-crystals on the substrate surface to enhance the nucleation and make the film growth more uniform. The silicon substrate can be then etched off, and a freestanding diamond film is thus produced. 

If a seed crystal is introduced at the start of the zone melting, a single crystal can be grown. This was widely applied for germanium, the first technically important semiconductor. For silicon, which became increasingly important, this method failed, since the silicon melt reacts with any boat or crucible material and the crystallized material sticks tightly at the container wall, so that both will break when cooling down. 

Very pure, low-defect crystals are produced by the alternative float zone process in which a cylindrical polysilicon rod has a seed crystal melted into its lower end with an encircling inductive heating coil. The heater is then raised along the rod length, entraining a molten zone and a monocrystalline solidified region below. Impurities have higher solubility in the molten silicon and are carried with it to the top. The process may be repeated to enhance the purity and crystallinity. 

One of the most difficult aspects of this method is the attachment of the seed crystal to the rod. This can be accomplished by heating a glass rod tip and attaching the crystal to the molten glass. Other methods include using a heated metal rod and inserting the heated metal directly into the crystal. The use of epoxy and silicon rubber to attach the crystal has also been reported. 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

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