Ionic compounds energy bands

Fig. 2-26. Band gap e, and bond energy AH, of binary ionic compounds. [From Vyb, 1970.]...

Some semiconducting compounds can be of the II-VI type, which also has an average valence of four, but these have much more ionic character than Ill-V compounds. Their band gaps are thus larger, and in some cases they may even be viewed as insulators. For example, ZnS, with a band-gap energy of 3.6 eV, is an insulator, whereas ZnSe has an band gap of 2.8 eV, which is closer to a semiconductor. A wide variety of... 

Energy Bands. Electrons make up the chemical bonds between atoms in a solid. In silicon, this bonding is primarily covalent, whereas in compound semiconductors (group II-VI compounds in particular), the bonds also have substantial ionic character. The electrons participating in these bonds are termed valence electrons. Free electrons created by breaking bonds or doping (see Chapter 6) are available for current flow and are known as conduction electrons. 

In a more accurate picture of ionic crystals, the ions are held together in a three-dimensional lattice by a combination of electrostatic attraction and covalent bonding. Although there is a small amount of covalent character in even the most ionic compounds, there are no directional bonds, and each Li ion is surrounded by six F ions, each of which in turn is surrounded by six Li ions. The crystal molecular orbitals form energy bands, described in Chapter 7. 

Conductors, such as the metals, are characterized by a partially filled band, so that the highest filled level and the lowest empty level are essentially at the same energy, the Fermi energy. Insulators have a large residual gap between the valence and conduction bands. Examples are ionic compounds, but also some covalent compounds such as diamond. Semiconductors have a small gap between the bands. Most of the covalent compounds in Table 5.3 fall into this class. 

Note that each atom in the solid state has its own inner electron shells, unique to its own atomic number, plus those that It contributes to the overall structure through bonding by ionic or covalent means (or some combination or both). In addition, when two compounds react in the solid state to form a new compound, the bonding electrons of each compound become redistributed into well-defined energy bands of the new compound (as defined by the ci3rstal lattice structure). Such bands are called BriUouin zones after a French investigator, L6on Brillouin (1931) who first worked on this problem. 

Monovalent Azides. The relation of the cation s first ionization energy to ionic character and band-gap width has already been stressed. The monovalent azides TIN3, AgN, and CuNa have larger values of Ii than the alkali azides and hence less ionic character and smaller band gaps. The conduction band in each is assumed to be primarily formed of neutral cation states. The expected nature of the highest-lying valence band and of possible excitons is discussed below. Comparisons with halide compounds should be qualified by noting that the latter have different structures. 

The perovskite oxides are ionic compounds with an eledrostatic Madelung energy that is large enough to raise most cation outer s and p eledronic energies well above the top of the anion p bands, where they remain essentially unoccupied at ordinary temperatures. The exceptions are some A-site cations with 5s or 6s cores, such as Sn +, T1 +, Pb +, or Bi +. However, the empty 5s or 6s bands of cations Sn" +, Tl +, Pb" + or Bi +, located on the B-sites, are at low enough energies to accept eledrons. 

Fig. 3. AB valence compounds Separation of compounds with the rocksalt structure (CN 6-6) from those with the diamond, sphalerite or wurtzite structures (CN 4-4) on a diagram of the spectroscopically defined covalent and ionic energy band gaps, and C. After Phillips and Van Vechten (see Ref. 14).

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