Polytype defined

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... 

The process of crystal nucleation and growth is equivalent to a phase transition. The initial phase might be a gas, liquid, solution or solid (e.g. glass or another crystal) and the final phase need not be a crystal as traditionally defined. It could be a liquid crystal, a quasi-crystal, a polytype or some other defect solid. The phase transition proceeds via a critical state, which is intermediate between the two phases of the transition and holds the key to the understanding of crystal growth. 

Several authors used symbols and orientations that differ from convention to describe geometric arrangements of the layer and the stacking sequence of mica polytypes (e.g., Radoslovich 1961 Durovic 1994 Dornberger-Schiff et al. 1982). To make inter-structure comparisons of features easier, however, it is advantageous to define briefly the site nomenclature adopted and the parameters used to describe and characterize layer geometry. The direction defined by the stacking of 2 1 units defines the... 

If the position of a layer is uniquely defined by the position of the adjacent layers and by the so-called vicinity condition VCf, which states the geometrical equivalence of layer pairs, the resulting structure is fully ordered. If, on the other hand, more than one position is possible that obeys the VC, the resulting structure is an OD structure and the layers are OD layers. VC structures may thus be either fully ordered structures or OD structures (Dornberger-Schiff 1964, 1966, 1979 see also Durovic 1999). All OD structures are polytypic the reverse may or may not be true (see the arguments in Zvyagin 1993). Equivalency depends on the choice of OD layers and also on the definition of polytypism (see below). 

The two 7,-symmetries of the Tet layer correspond to 2 x60° and (2 +l)x60° rotations, respectively, of successive M layers about c. Each family of polytypes is defined in terms of the Tt-symmetry of the Oc layer, which is the same as that of the O... 

A) DA symbols. The first symbolic description is from Dekeyser and Amelinckx (1953), who used a set of vectors and numerical symbols to indicate the complete stagger of the layer, defined as the (001) projection of the vector connecting two (OH/F) sites on the two sides of the octahedral sheet. Six characters n = 1,2,3,1,2,3 represent the stagger of the layer with respect to a space-fixed reference (Fig. 8). These symbols apply to the homo-octahedral approximation only and therefore cannot correctly describe polytypes containing M2 layers. 

The number and features of the orthogonal planes (as defined above) depend both on the Class (lattice features) and on the subfamily (OD character). These are easily obtained by taking into account that polytypes in subfamily B and in subfamily A Series > 0 never belong to Class a, whereas polytypes in subfamily A Series 0 always belong to... 

Table 15. Limiting symmetry defining the nnmber of independent lattiee orientations. The (idealized) symmetries of the lattiee and of the family structure are given. The limiting symmetry corresponds to the lower of the two. For mixed-rotation polytypes the family stmcture is defined only within the Pauling model and the limiting symmetry by definition coincides with the symmetry of the lattice.

For these reflections, Gy = Go and Equation (24) holds again. PID is thus defined in a subspace of the reciprocal space, which narrows from subfamily A polytypes to mixed-rotation polytypes, but always includes at least the three r.p. Okl, hhl, hhl. 

For all OD polytypes (both subfamilies A and B) of Series 0, the PID has also a translational symmetry reminiscent of that relation between pairs of translationally equivalent rows defining a minimal rhombus ... 

From our particular perspective, the goals of petrologic studies are determination of how, in the context of a given rock bulk composition, the intensive variables interrelate with the chemistry, polytypism, textural, and physical aspects of the minerals (primarily the white micas) in metamorphic rocks. In principle, if the minerals are in chemical equilibrium, or at least close thereto, interrelationships among mineral chemistry, bulk composition and intensive variables should be well defined inasmuch as they are constrained by the laws of chemistry. The interrelationships involving the intensive parameters and polytypism, textures, and physical aspects of the minerals may be less well defined because the laws controlling them are less well understood. Our emphasis is primarily on the mineral chemistry and secondarily on polytypism and some physical aspects like the elastic constants. 

This Datareview covers the state of diodes in SiC polytypes both made from p-n junctions and using Schottky barriers. After defining the necessary theoretical background to junction diodes the various device structures used and the performances obtained are detailed. A range of metals has been used to produce Schottky barriers on three of the SiC polytypes and the performances achieved in this type of device are listed. 

It has already been mentioned that this band gap predestines SiC for electronic devices at high temperatures, but also for high power and high frequency operations and devices operating in harsh environments [197]. Therefore, it may be of interest to tailor this band gap for special applications. That is feasible by the formation of so-called superlattices, i.e., by forming exactly defined sequences of monolayers of different polytypes. 

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