Semiconductors Ionic-Covalent Bonding

The chemistry of metal oxides can be understood only when their crystal structure is understood. Knowledge of the geometric structure is thus a prerequisite to understanding the properties of metal oxides. The bulk structure of polycrystalline solids can usually be determined by x-ray 

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

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Ionic and covalent materials can combine in any group of valence compounds forming a class centered on simple or complex phases with four electrons per atom. Valence compounds with three and five electrons per atom are the nearest nel bors. In each of these subgroups we find that an increase in the atomic weight tends to increase the metallic interaction between the atoms and to alter the structure, However, most of the valence compounds are substances with mixed (ionic-covalent) bonds and with a gap in the electron energy spectrum, i.e., they are semiconductors. 

The above data are correct to about 20 kJ mole but it will be seen that the general trend among these more covalent bonds does appear to be a decrease in stability from carbon to silicon, i.e. the same way as was found for more ionic bonds in the halides. Thermodynamic data for metallorganic methyl compounds used in the produchon of semiconductor systems are shown in Table 2.3. 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Fig. 2-30. Surface dangling states and surface ion-induced states (a) surface dangling donor (DL-B) and acceptor (DL-AB) leveb on covalent bonding semiconductors, (b) surface cation-induced acceptor (SCL) and surface anion-induced donor (SAL) levels on ionic bonding semiconductors.

The dangling and the surface ion-induced states are intrinsic surface states that are characteristic of individual semiconductors. In addition, there are extrinsic surface states produced by adsorbed particles and siuface films that depend on the enviromnent in which the siuface is exposed. In general, adsorbed particles in the covalently bonded state on the semiconductor surface introduce the danglinglike surface states and those in the ionically bonded state introduce the adsorption ion-induced surface states. In electrochemistiy, the adsorption-induced surface states are important. 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

As early as in 1937, Nyrop (17) suggested that electron transfer may occur during chemisorption. Dowden (18) clarified the situation by classifying the possible reactions with respect to the type of bond (ionic, covalent, or mixed) and the type of adsorbent (metal, semiconductor, or insulator). He attempted to indicate some probable criteria to be used in the choice of the best adsorbent for use with a given adsorbate. 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

Unraveling the relationship between the atomic surface structure and other physical and chemical properties is probably one of the most important achievements of surface science. Because of the mixed ionic and covalent bonding in metal oxide systems, the surface structure has an even stronger influence on local surface chemistry as compared to metals or elemental semiconductors [1]. A vast amount of work has been performed on Ti02 over the years, and this is certainly the best-understood surface of all the metal oxide systems. 

The purpose of this section is to add the DFT-based concepts of fi and y, as defined in equation (5.28), to the existing treatment of solid-state insulators and semiconductors. Also we will use the very simple theory of bonding developed earlier for ionic and covalent bonding to predict, or rationalize, certain properties of importance in solid-state physics. 

The second chemical contribution to the total interaction energy is present if an ionic or a covalent chemical bond between the adsorbed molecule and the surface can be formed. Since covalent bonds also depend on the overlap between the wave functions of the subsystems, their distance dependence is exponential, see Table 1, as is that of the Pauli repulsion. In general, covalent bonds are only possible if at least one of the two partners possesses partially occupied valence orbitals. In contrast to the adsorption at metal or semiconductor surfaces, such a situation is rarely encountered at insulator and in particular at oxide surfaces. In most cases, the ions at the surface of an insulator try to adopt a closed shell electronic structure as they do in the bulk, as for instance the Na+ and Ck ions in NaCl or the Mg + and ions in MgO. Counterexamples are transition metal oxides in which the metal cations possess partially occupied d-shells which might form chemical bonds with the adsorbed molecule. One famous example is the interaction between NO and the NiO(lOO) surface where both the Ni + cations (d configuration with a A2g ground state) and the NO radical ( 11 ground state) have partially filled valence shells (see below). 

Materials with a GaAs structure are usually semiconductors this property is a direct consequence of the covalent bonding. In the III-Vs the band gap inaeases as the ionic component to the bonding increases, as shown in Table 6.2. If we replace all the Ga and all the As by C, Si, or Ge, we have the diamond-cubic (dc) structure of diamond. Si and Ge. Now the bonding is entirely covalent (and Pauling s rules would not work). We consider the GaAs structure again in comparison to AIN. 

Place a point on the chart for sodium chloride, a compound with ionic bonding for sodium, a metal for methane, CH4, a compound with covalent bonding for Si, a semiconductor. 

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