ABSTRACT
Crystalline silicon forms a covalently bonded diamond-cubic structure, which has the same atomic arrangement as carbon in the diamond form and belongs to the more general zinc-blende classification (in which the lattice positions are not all necessarily occupied by the same atom). Silicon, with its four covalent bonds, coordinates itself tetrahedrally, and these tetrahedrons make up the diamond-cubic structure (Figure 4.1). There are only four elements that come in diamond lattices: C, Si, Ge, and α-Sn. Diamond (carbon), silicon, and germanium all have a valence of four, and all have the same crystal structure. However, diamond is an insulator, whereas the others are semiconductors as a result of the energy bandgap difference. Diamond’s bandgap is 5.5 eV, silicon’s is 1.1 eV, and germanium’s is 0.67 eV. The large bandgap of diamond makes it an insulator. The unit cell in a diamond lattice is cubic with atoms at each corner and in the middle of each face [i.e., face-centered cubic (FCC); see Chapter 2]. In the interior there are four additional atoms located along the cube diagonals exactly one-quarter of the way down the diagonal. This structure can also be described as two interpenetrating FCC lattices, one displaced (1/4, 1/4, 1/4)* times the lattice parameter a with respect to the other, as shown in Figure 4.2. The lattice parameter a for silicon is 5.4309 Å, and silicon’s diamond-cubic lattice is surprisingly wide open, with a packing density of 34% compared with 74% for a regular FCC lattice. The {111} planes present the highest packing density, and the atoms are oriented such that three bonds are below the plane. In addition to the diamond-cubic structure, silicon
is known to have several stable high-pressure crystalline phases and a stress-induced metastable phase with a wurtzite-like structure [wurtzite is a sulfide mineral of zinc and iron with the composition (Zn,Fe)S], referred to as diamond-hexagonal silicon. The latter has been observed after ion implantation and hot indentation.
