Existing 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...

In addition to this cubic Laves phase, a variant with magnesium atoms arranged as in hexagonal diamond exists in the MgZn2 type, and further polytypes are known. 

The familiar diamond structure, with each atom covalently bonded in a perfect tetrahedral fashion to its four neighbors, is adopted not only by C but also by Si and Ge. Silicon can also adopt a wurtzite structure (see below), an example of a polytype (one of several crystal structures possible for a substance having an identical chemical composition but differing in the stacking of layers, and which may exist in a metastable state after its formation at some different temperature or pressure). 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

The structure of a green rust LDH-type material with the formula [Fe 4Fe 2(OH)i2]S04-ca. 8H2O has been determined by Rietveld analysis [157]. The material exists as a one-layer polytype with the interlayers containing two planes of sulfate and water molecules giving a basal spacing of 1.1011 nm. The sulfate ions are oriented with their C3 axes perpendicular to the layers and alternate anions point up and down (as shown earlier in Fig. 14) and form a superlattice with parameter a = UoV5 = 0.5524 nm (Fig. 39). 

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.,... 

High pressure and temperature are required to produce the compounds MCr03, e.g. BaCr03, which exists as several polytypes.13octahedrally coordinated in these compounds,1306 the structure consisting of pairs of face-sharing octahedra linked to other pairs by sharing comers. The Cr—Cr distances are ca. 2.6 A. 

Polytypism The existence of different forms of a crystal that differ in the manner that layers of atoms are stacked. 

---