Diamond, crystal structure energy bands

SoUd ice forms a crystal of diamond structure, in which one water molecule is hydrogen-bonded with four adjacent water molecules. Most (85%) of the hydrogen bonds remain even after solid ice melts into liquid water. The structure of electron energy bands of liquid water (hydrogen oxide) is basically similar to that of metal oxides, 6dthough the band edges are indefinite due to its amorphous structure. 

FIGURE 4.8 Energy bands formed from ns and np atomic orbitals for (a) a body-centred cubic crystal and (b) a crystal of diamond structure, depicting filled levels for 4iVelectrons. 

Silicon crystallizes in the diamond structure,16 which consists of two interpenetrating face-centered cubic lattices displaced from each other by one quarter of the body diagonal. In zinc blende semiconductors such as GaAs, the Ga and As atoms lie on separate sublattices, and thus the inversion symmetry of Si is lost in III-V binary compounds. This difference in their crystal structures underlies the disparate electronic properties of Si and GaAs. The energy band structure in... 

Pure silicon is an insulator, similar to diamond in both crystal structure and electronic structure. The electronic structure in pure silicon can be represented by the filled and empty bands shown in Fig. 28.6(a). Suppose that we remove a few of the silicon atoms and replace them by phosphorus atoms, each of which has one more electron than the silicon atom. The energy levels of the phosphorus atoms, impurity levels, are superposed on the band system of the silicon these levels do not match those in silicon exactly. (Since there are so few phosphorus atoms, the levels are not split into bands.) It is found that the extra electrons introduced by the phosphorus atoms occupy the impurity levels shown in Fig. 28.6(b), which are located slightly below the empty band of the silicon lattice. In these levels the electrons are bound to the phosphorus atoms and cannot conduct a current since the... 

Fig. 2.13. Density of states N E) versus energy from band calculations for mercury in a series of crystal structures with constant interatomic separations (Mattheiss and Warren, 1977). The lower (s-) band just begins to overlap the upper (p-) band in the diamond structure (a). In the denser structures, the gap is closed the BCC structure exhibits a well-developed pseudogap.

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