Wurtzite silicon carbides

Silicon combines with carbon to form silicon carbide or carborundum, SiC, and forms are known that have the wurtzite and zinc blende structures. It is a very hard, tough material that is used as an abrasive and a refractory. The powdered material is crushed after mixing with clay and heated in molds to make grinding wheels. It is prepared by the reaction... 

There are various polymorphs of silicon carbide made by high temperature interaction some have wurtzite (ZnS) or diamond structures. It is exceedingly hard and inert it finds uses in polishing products, furnace linings, and semiconductor technology. 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosUanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

As mentioned, elemental silicon has the diamond structure. Silicon carbide, SiC, occurs in many crystalline forms, some based on the diamond structure and some on the wurtzite structure (see Figures 7-6 and 7-8(b)). It can be made from the elements at high temperature. Carborundum, one form of silicon carbide, is widely used as an abrasive, with a hardness nearly as great as diamond and a low chemical reactivity. SiC has now garnered interest as a high-temperature semiconductor. 

From an optical viewpoint, on the other hand, the difference between semiconductors and insulators lies in the value of Eg. The admitted boundary is usually set at 3 eV (see Appendix A for the energy units) and materials with Eg below this value are categorized as semiconductors, but crystals considered as semiconductors like the wurtzite forms of silicon carbide and gallium nitride have band gaps larger than 3 eV, and this value is somewhat arbitrary. The translation into the electrical resistivity domain depends on the value of Eg, and also on the effective mass of the electrons and holes, and on their mobilities. The solution is not unique moreover, the boundary is not clearly defined. Semi-insulating silicon carbide 4H polytype samples with reported room temperature resistivities of the order of 1010flcm could constitute the... 

Silicon carbide, carborundum, also crystallises in two forms, of which /(-SiC has the cubic zinc blende (sphalerite) structure (Figure 8.8a). When viewed along the cube face-diagonal [110] direction, the layers of both silicon and carbon are packed in the cubic closest packing arrangement. .. aAbBcCaAbBcC. .., where the uppercase and lowercase letters stand for layers of Si and C. The other form of silicon carbide, a-SiC, is a collective name for the various silicon carbide polytypes, which consist of complex arrangements of zinc blende and wurtzite slabs. Some of these are known by names such as carborundum I, carborundum II, carborundum III, and so on. One of the simplest structures is that of carbo-... 

Silicon carbide (carborundum, SiC) is of especial interest on account of its rich polymorphism, no fewer than six structures being known. As is to be expected, each carbon and silicon atom is tetra-hedrally co-ordinated by four atoms of the other kind, and two of the forms of carborundum have the zincblende and wurtzite structures. The close relationship between these two structures has already been discussed ( 4.13), and is emphasized by the many AX compounds (including ZnS itself) in which both are found. It is illustrated in fig. 8.03, where the cubic zincblende structure has been drawn with one of the cube diagonals vertical and parallel to the principal axis of the wurtzite structure. When viewed in this way it will be seen that both structures can be visualized as formed by the superposition of a series of puckered sheets of atoms, but that in zincblende successive sheets are identical (albeit translated) whereas in wurtzite they differ and are related by a rotation through 180° about the principal axis. In the two structures the sequence of sheets can therefore be symbolized as... 

Silicon carbide carborundum) has several polymorphs the 3-form adopts the wurtzite structure Figure 5.20). It is extremely hard, resists wear, withstands very high temperatures, has a high thermal conductivity and a low coefficient of thermal expansion, and has long been used as a refractory material and abrasive powder. Recent development of suitable CVD... 

The layer sequences can repeat themselves in the cycles ABC, ABC. .. (zinc blende, type 3C) or AB, AB. .. (wurtzite, type 2H), according to cubic or hexagonal close packing. In addition, numerous others stack sequences are formed in the case of silicon carbide, resulting in many similar polytypes. 

Silicon carbide exists in two crystallographically distinct modifications (i) hexagonal a-SiC, with a wurtzite-type lattice and a huge number of polytypic variants, depending on the stacking order of the Si-C lattice planes and (ii) cubic 3-SiC, with a zincblende-type lattice (a = 0.437nm). 

Silicon carbide exists in a large number of structural forms called polytypes (more than 140), which represent modifications of hexagonal (wurtzite) and cubic (sphalerite) close-packed crystal structures. 

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