Silicon carbide polytypes, hexagonal

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. 

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). 

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... 

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. 

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.,... 

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. 

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. 

Refractive indices for several polytypes of silicon carbide have been measured [8-12]. The 6H polytype of SiC has been measured in the most detail [8]. For the hexagonal form the c axis is assumed to be perpendicular to the surface. Thus, a normal incidence wave can be used to measure transmission and/or reflection. This normal incidence wave is often called the ordinary ray. A prism with its sides perpendicular to the c-axis is used to determine both n0 and ne in the normal way (n0 = ordinary ray, ne = extraordinary ray). A short summary of the data is presented here for additional information please refer to [8,9]. 

Dean et al [7] measured the Zeeman splitting of a luminescence line involving the 2p donor state, obtaining the electron effective mass m t=(0.24 0.01)mo and m /m, =0.36 0.01 for n-type cubic crystals. Measurements of infrared Faraday rotation due to free carriers were made by Ellis and Moss [8] at room temperature in a number of n-type hexagonal specimens belonging to the 6H and 15R polytypes of silicon carbide. One component of the total density-of-states effective mass was explicitly determined by this method. A value for the... 

As mentioned, the sinterability of pure silicon carbide strongly depends on the properties of the powder used. Powders made by high-temperature routes are predominantly composed of different hexagonal SiC polytypes (e.g., 2H, 4H, and 6H). In contrast, SiC produced by polymeric routes almost exclusively consists of p-SiC. In principle, SiC can be formed by the pyrolysis of either polysilanes or polycarbosilanes. 

Silicon carbide (SiC) is the only solid compound known in this system. It occurs with two modifications, cubic jS-SiC (referred to as low temperature modification) and hexagonal a-SiC with numerous polytypes where 4H, 6H and 15R are the most common ones. The a-/) -SiC transformation temperature is not well known. Kistler-de Coppi (1985) [77] studied in detail the transformation kinetics at temperatures between 1973 K and 2573 K. According to these results (metastable) ) -SiC transforms at temperatures above 2273 K to a-SiC. SiC melts incongruently forming a sihcon-rich liquid phase and graphite. 

Polycrystalline silicon carbide obtained by the Acheson process exhibits a large number of different structures (polytypes), some of which dominate. These can be classified in the cubic, hexagonal, and rhombohedral crystal systems (Table 1). 

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. 

Polycrystalline silicon carbide obtained by the Acheson process exhibits a large number of different polytypes, some of which dominate. More than 200 different polytypes are currently known these can be classified into the cubic, hexagonal, and rhombohedral crystal system, and all have the same density of 3.21 gcm . Written polytype nomenclature [11] indicates the number of layers in the repeating layer pack by a numeral, while the crystal system is denoted by the letters C, H, or R. 

Silicon carbide occurs in two sUghtly different crystal structures the cubic pSiC, and a large number of hexagonal rhombohcdral varieties known collectively as aSiC.1 11 1 The single cubic form, pSiC, is obtained vdien the carbide is synthesized below 2100"C. It is a fiice-centered cubic (fee) structure of the zincblende type shown in Fig. 7.1. Zincblende is a mineral of zinc sulfide also known as sphalerite. In this illustration, the zincblende structure is represented with the cube diagonals vertical and appears as series of identical (although translated) puckered sheets of atoms widi an AA layer sequence. Another view of the pSiC crystal is shown in Fig. 7.2 (the carbon atoms, all located in the 4/ sites, are omitted for clarity] The pSiC structure has no polytype (see Table 7.3 for crystal structure data). 

Polymorphism and polytypism. Silicon carbide has two polymorphs. At temperatures above 2000°C alpha silicon carbide (a-SiC), with a hexagonal crystal structure, is the more stable polymorph with iridescent and twinned crystals with a metallic luster. At temperatures lower than 2000°C, beta silicon carbide ((3-SiC) exhibits a face-centered cubic (fee) crystal structure. 

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. 

Silicon carbide exists in the form of a very large number of crystallographic varieties. The most widespread polytypes are the P-SiC (cubic, low temperature) and a-SiC (hexagonal, high temperatrrre) varieties. SiC powder is densified by reaction sintering reaction, natural or presstue sintering [CHE 80]. 

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