Hexagonal polytypes

Table 1 Hsts the properties of several semiconductors relevant to device design and epitaxy. The properties are appropriate to the 2incblende crystal stmcture in those cases where hexagonal polytypes exist, ie, ZnS and ZnSe. This first group of crystal parameters appHes to the growth of epitaxial heterostmctures the cubic lattice constant, a the elastic constants, congment sublimation temperature, T. Eor growth of defect-free...

A very striking and beautiful feature of polytypism is the behavior of impurity atoms. In Figure 1.6, it may be seen that the sites are not equivalent in the hexagonal polytypes 6H-SiC and 4H-SiC. The difference is in the second-nearest neighbors. 

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. 

The hexagonal polytypes are obtained as mixtures, although impurities encourage the formation of certain structures e.g., boron produces the 6H structure, A1 gives 4H and La encourages the formation of 2H structures. 

Table 2. Magnetic parameters in some hexagonal polytypes AMX3...

Rgure 30 a) Synchrotron radiation diffractogram and b) microgr h of the hexagonal polytype of faujasite... 

Using a commercially available finite element analysis program, ABAQUS, models representing SiC particulates in a ZrBa matrix were created. The SiC phase was modeled as a round particle in a two dimension (2D) ZrB matrix. Material properties were assumed to be isotropic for both the ZrB and the SiC after initial modeling efforts indicated only small changes in stress fields as a result of the anisotropic properties of the a-SiC (hexagonal polytype). The material properties used in the models, as well as other key model input variables, are included as Table II. 

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. 

The solubility of boron nitride in (3-SiC is small (0.5 to 1 mol%) under conventional hot-pressing conditions (35 MPa, below 2250°C) a-BN stabilizes 3-SiC and low hexagonality polytypes [242, 243]. The microstructure of BN-SiC composites as studied by transmission... 

There are, in addition, numerous polymorphs or polytypes of many MX2 structures due to the possibility of different layer-stacking orientations. These include the hexagonal polytype with two layers per unit cell (2H), rhombohedral poly type with three layers per unit cell (3R), and the trigonal polytype (IT) with one layer per unit cell. Figure 3 illustrates the difference in layer stacking between the 2H and 3R polytypes of M0S2. 

The word carbon Is derived from the Latin carbo , which to the Romans meant charcoal (or ember). In the modern world, carbon is, of course, much more than charcoal. From carbon come the highest strength fibers, one of the best lubricants (graphite), the strongest crystal and hardest material (diamond), ein essentially non-ciystalline product (vitreous carbon), one of the best gas adsorbers (activated charcoal), and one of the best helium gas barriers (vitreous carbon). A great deal is yet to be learned and new forms of carbon are still being discovered such as the fullerene molecules and the hexagonal polytypes of diamond. 

The AMO3 structure accommodates a t > 1 by the formation of hexagonal polytypes [4] the principal interest of these structures for the present discussion is the demonstration they provide for a dt/dP < 0 as the normal... 

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