Silicon carbide polytypism

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-... 

Sorokin ND, Tairov YuM, Tsvetkov VF, Chernov MA (1982a) The laws governing the changes of some properties of different silicon carbide polytypes. Dokl Akad Natrk SSSR 262 1380-1383 (in Russian) Sorokin ND, Tairov YuM, Tsvetkov VF, Chernov MA (1982b) Crystal-chemical properties of the polytypes of silicon carbide. Sov Phys Crystallogr 28 539-542... 

Hartman, J. S., Richardson, M. F., Sherriff, B. J., and Winsborrow, B. G., Magic angle spinning NMR studies of silicon carbide Polytypes, impurities, and highly inefficient spin-lattice relaxation, J. Am. Chem. Soc., 109, 6059 (1987). 

Large numbers of experiments have been conducted by various researchers in an attempt to define stability regions for the basic silicon carbide polytypes. Work by both Knippenberg [5] and Inomata et al. [6] resulted in stability diagrams. In both studies, the stability was shown to increase with temperature moving from 3C to 2H to 4H to 15R to 6H. Specifically, the work by Inomata et al. indicates that 3C polytype will convert to a hexagonal polytype above about 1600°C and that near eomplete eonversion, to the 6H polytype, will occur alx>ve approximately 2200°C. Thus, at high process temperatures, the formation of the 6H polytype is predieted. 

Comparing the literature data, the 3C polytype should have a slightly higher specific heat Table I. From the data presented here, several cases stand out where the specific heat values are roughly the same for similar amounts of SiC. With the actual difference of the pure silicon carbide polytypes being 0.002, 1/ g K, differences in the actual data are most likely contained within the experimental error of this measurement. The specific heat should therefore not have a great impact on the thermal conductivity of these materials with respect to their respective polytypes. 

Technically, the thermal expansion of these materials should decrease with increasing SiC content based on the monolithic data of the thermal expansion data for pure silicon and SiC in Table 1. The aggregate data suggests a slight downward trend in thermal expansion with increasing SiC volume percent however, this is not seen as statistically significant. Conclusions can not be drawn as to the eflects of the silicon carbide polytype and its effect on thermal... 

Ramsdell Numbers. A shorthand notation for labelling polytypes (q.v.) of crystal structures The numbers are of the form /iR or nH were n is the number of layers in the unit cell (q.v.) necessary for the arrangement of the layers to repeat itself. R, H stand for rhombohedral or hexagonal crystal form respectively. Originally devised to classify silicon carbide polytypes, the system has been applied to other carbides and nitrides. (L.S. Ramsdell, Amer. Miner 32, 147, p64). 

Tairov, Y.M. and Tsvetkov, V.F. (1981) General prindples of growing large-size single crystals of various silicon carbide polytypes. J. Cryst. Growth, 52, 146-150. 

Powell, J.A., P. Pirouz, and W.J. Choyke, Growth and characterization of silicon carbide polytypes for electronic applications, in Semiconductor Interfaces, Microstructures, and Devices Properties and Applications, Z.C. Feng, ed. 1993 UK Institute of Physics Publishing p. 257. 

Figure 2 Arrangements of silicon (small circles) and carbon (big circles) atoms in (1120) plane of some silicon carbide polytypes.

Table 1 Crystallochemical Parameters of Silicon Carbide Polytype Structures...

Similar equations for vapor pressures over other silicon carbide polytypes can be defined with the help of Table 3. The ratio of silicon and carbon concentrations in the gas phase, NsJ Nc, for most temperature values is greater than unity and can be estimated using the equation... 

There are two types of deviations from the stoichiometric composition in the crystals of silicon carbide polytypes. The first one is characterized by big deviations (Table 1), and it is supposed that it is caused by structural regulation of silicon and carbon vacancies in the crystal lattice. The Si—C layers in cubic and hexagonal positions keep their composition in different... 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

Other stacking sequences than these are also possible, for example AaBpAaCy... or statistical sequences without periodic order. More than 70 stacking varieties are known for silicon carbide, and together they are called a-SiC. Structures that can be considered as stacking variants are called polytypes. We deal with them further in the context of closest-sphere packings (Chapter 14). 

When Acheson found the hexagonal crystals in the voids, he sent some to B.W. Frazier, a professor at Lehigh University. Professor Frazier found that although the crystals were all silicon carbide, they differed in their crystalline structure. He had discovered the polytypism of SiC [18]. Polytypism will be explained in Section 1.3.2. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

Chung, G. Y., et al., Effect of Nitric Oxide Annealing on the Interface Trap Densities Near the Band Edges in the 4H Polytype of Silicon Carbide, Applied Physics Letters, Vol. 76, No. 13, March 27, 2000, p. 1713. 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Daulton, T. L., Bematowicz, T. J., Lewis, R. S. et al. (2003) Polytype distribution in circumstellar silicon carbide Microstructural characterization by transmission electron microscopy. Geochimica et Cosmochimica Acta, 67, 4743-4767. 

The properties of silicon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal silicon carbide. The pressureless-sintered silicon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded silicon carbide refractories. Table 1 lists the properties of the fully compacted, high purity material. 

Crystal Structure. Silicon carbide may crystallize in the cubic, hexagonal, or rhombohedral structure. There is a broad temperature range where these structures may form. The hexagonal and rhombohedral structure designated as the a-form (noncubic) may crystallize in a large number of polytypes. 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of silicon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-silicon carbide formed by transformation from the p-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

Semiconducting Properties. Silicon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stability of its electronic structure, silicon carbide has been studied for uses at high (>500° C) temperature. The Hall mobility in silicon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-type crystals, activation energy for ionization of nitrogen impurity varies with polytype (50,51). 

Silicon carbide whiskers typically have diameters of a few micrometers and lengths up to 5 cm. They may be composed of either P-SiC or CC-SiC, the latter in one or more polytypes, and occur mosdy as hair- or ribbonlike crystals. Despite many attempts to produce SiC whiskers on a large scale at low cost, they have never acquired a wide importance. SiC whiskers have been reviewed (111—120). 

The analysis of silicon carbide involves identification, chemical analysis, and physical testing. For identification, x-ray diffraction, optical microscopy, and electron microscopy are used (136). Refinement of x-ray data by Rietveld analysis allows more precise determination of polytype levels (137). 

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

---