Diamond-type crystal structur

Intrinsic semiconductors. The group fourteen elements carbon, silicon, germanium, and tin can be found to adopt the diamond-type crystal structure shown in Figure 3 a. Other crystalline structures are also found for example, graphite and diamond are different crystal structures of the same element, carbon. Because of its size and orbital energies, carbon forms very... 

Thus we have shown that when s and p orbitals are available and s—p quantization is broken an atom can form four (or fewer) equivalent bonds which are directed towards tetrahedron corners. To the approximation involved in these calculations the strength of a bond is independent of the nature of other bonds. This result gives us at once the justification for the tetrahedral carbon atom and other tetrahedral atoms, such as silicon, germanium, and tin in the diamond-type crystals of the elements and, in general, all atoms in tetrahedral structures. 

Fig. 1. Structure of the ill, 110 and 100 surfaces of the diamond-type crystals (Ge or Si for example) or zinc blende crystals. In the zinc blende crystals such as the III-V compounds (InSb) the plain and shaded atoms correspond to the group V and group III atoms respectively. Electron configuration shown is for group IV el cm era s.

Cubic boron nitride (cBN) has a zinc blende-type crystal structure with a lattice constant of 3.615 A, which is very close to that of diamond (3.567 A). The difference is only about 1.3%. According to RHEED measurements with the electron beam parallel to the 111 layer of cBN, a growth of diamond by DC plasma CVD on cBN(lll) [150] using c = 0.5%CH4/H2, T = 900°C, and F=180Torr led to a result that a smooth (111) layer of diamond was epitaxially deposited in such a way that the [110] direction of diamond was parallel to that of cBN. Namely, D 111 //cBN(lll and D[110]//cBN[110]. In the RHEED pattern, however, extra spots were observed, which were presumably due to the twinnings of (111 diamond layers. In the Raman spectra, there were two lines due to cBN at 1054.5 and... 

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm. ... 

The most common—and perhaps most important—hybrid orbitals are the tetrahdral ones formed by adding one s-, and three p- type orbitals. These can be arranged to form various crystal structures diamond, zincblende, and wurtzite. Combinations of the s-, p-, and d- orbitals allow 48 possible symmetries (Kimball, 1940). 

We shall now discuss the method of crystal growth and the electronic properties of GaAs, a typical example of a III-V compound which is expected to become more useful than Si and Ge in the near future, concentrating on the relation between non-stoichiometry and physical properties. GaAs has a zinc blende type structure, which can be regarded as an interpenetration of two structures with face centred cubic lattices, as shown in Fig. 3.29. Disregarding the atomic species, the structure is the same as a diamond-type... 

The cubic form resembles diamond in its crystal structure and is almost as hard. The theoretical density is 3.48 g/mL. It is colodess and a good electrical insulator when pure traces of impurities add color and make it semiconducting, eg, a few ppm of Be make it blue and />-type whereas small amounts of S, Si, or CN favor yellow, -type crystals. It is possible to makep—n junctions by growing -type material on j -type seed crystals (12). If this is done carefully in an alkaline-earth nitride bath using a temperature difference technique, as with large diamond crystals (see Diamond, SYNTHETIC), the resulting diodes are several mm in size and emit blue light when forward-biased (13,14). 

The NaTl-type structure is the prototype for Zintl phases, which are inter-metallic compounds which crystallize in typical non-metal crystal structures. Binary AB compounds LiAl, LiGa, Liln and Naln are both isoelectronic (isovalent) and isostructural with NaTl. In the Li2AlSi ternary compound, A1 and Si form a diamond-like framework, in which the octahedral vacant sites of the A1 sublattice are filled by Li atoms, as shown in Fig. 13.7.2(b). 

FIGURE 1.16 Tetrahedral bonding of atoms in a diamond-type structure of C, Si, and Ge crystals. 

The quantities produced with the laser vaporization method were however not even sufficient for doing experiments to verify the proposed structure. This was solved by Kratschmer, Huffman and their students who had as early as in 1982, [144-146] i.e. three years before the discovery of Ceo in 1985, produced Ceo without knowing it. They used an electric arc in a helium atmosphere of 150 torr and produced a special kind of soot with a unique type of optical absorption known as the camel hump smoke in the UV region. Their recorded spectrum fitted however very nicely to some predictions of the present author [147]. After a number of trials, they found in 1990 [148] that the special carbon soot could be dissolved in benzene, which provided the possibility to separate Ceo from the carbon particles [149], record a UV visible spectrum and even fabricate crystals of Ceo and C70 and determine the crystal structure. Suddenly a new kind of carbon material had been found in addition to the commonly known diamond and graphite. 

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