Semiconductive solids

Semiconducting solid sulfide -do- Zinc sulfide in oxygenated water ZnS (s) + 2 02 (aq) —> ZnS04 (aq)... 

The physical properties of impurities in semiconducting solids are governed by the nature and strength of the interaction between the impurity and its environment in much the same way as molecular characteristics are determined by the interactions among constituent atoms. From such a chemical perspective, the hydrogen atom is unique. Since it lacks a core, its one electron resides in a state that is bound as tightly as the active electrons... 

The counterparts to electrons in semiconducting solids are holes, represented by the symbol h. Each hole will bear an effective positive charge, qe, of +1, which is represented by the superscript to emphasize that it is considered relative to the surrounding structure. The concentration of holes that are free to carry current through a crystal is often given the symbol p in semiconductor physics. 

For the semiconducting solids, the band gap decreases down a group, for example, GaP> GaAs>GaSb AlAs>GaAs>InAs. 

III. The Mechanism of Simple Reactions on the Surface OF Semiconducting Solids... 

V. Kinetic Energy Distribution of Photoelectrona from Aromatic Semiconducting Solids. 407... 

Figure 16-5. Basic in-situ experiment to detect Pj( ,t) with the help of a miniaturized solid electrolyte (emf probe), (i, k) is a metallic or semiconducting solid solution which forms from the components i and k. (if) = reference electrode, tX = electrolyte.

Photovoltaic Devices. For many inorganic semiconductors, absorption of light can be used to create free electrons and holes. In an organic semiconducting solid, however, absorption of a photon leads to the formation of a bound electron—hole pair. Separation of this pair in an electric field can... 

Silicon is a hard, gray, semiconducting solid that melts at 1410°C. It crystallizes in a diamondlike structure but does not form a graphitelike allotrope because of the relatively poor overlap of silicon it orbitals. In nature, silicon is generally found combined with oxygen in Si02 and in various silicate minerals. It is obtained in... 

The group 6A elements are oxygen, sulfur, selenium, tellurium, and polonium. As shown in Table 19.7, their properties exhibit the usual periodic trends. Both oxygen and sulfur are typical nonmetals. Selenium and tellurium are primarily non-metallic in character, though the most stable allotrope of selenium, gray selenium, is a lustrous semiconducting solid. Tellurium is also a semiconductor and is usually classified as a semimetal. Polonium, a radioactive element that occurs in trace amounts in uranium ores, is a silvery white metal. 

To discuss the properties of semiconductors from a chemical perspective, it is important to first understand the structures of semiconducting solids. Semiconductors comprise a diverse group of inorganic materials and exhibit a variety of different Crystal Structures. The most basic semiconductor structure is based on the interpenetration of two face-centered cubic (fee) lattices. A familiar, nonsemiconducting solid that adopts this structure is NaCl, where the Na+ cations constitute one fee lattice and the Cl anions constitute the other (Figure 1(a)). Many specific crystal structures of semiconductors are related to this basic face-centered cubic lattice. 

The structure of semiconducting solids provides a convenient basis for understanding the important electronic properties of these materials. The important optical and electrical characteristics of semiconducting solids arise from the delocalized electronic properties of these materials. To understand the origin of this electronic delocalization, we must consider the nature of the bonding within semiconductor crystals. The basic model that has been successfully used to describe the electronic structure of semiconductors is derived from the Band Theory of solids. Our treatment of band theory will be qualitative, and the interested reader is encouraged to supplement our discussion with the excellent reviews by... 

We will not describe this process in detail for semiconducting solids, because there are complications that result from the different symmetry types of varions unit cells, as well as from spin-orbit coupling considerations. These factors tend to increase the complexity of the MO description of inorganic solids relative to the simple MO treatment of polyenes. However, the principles of the procednre are the same as those outlined for the polyene example, with each individual atom in the lattice contributing a set of atomic orbitals toward the formation of crystal orbitals. These crystal orbitals will extend thronghont the sohd, and will yield discrete energy bands... 

This effective mass, m, describes the movement of an electron under the influence of the periodic potential in the lattice. The value of can be calculated from the band structure of the semiconducting solid, and can also be measured experimentally using cyclotron methods. ... 

In general, semiconductors can have different types of valence band and conduction band structures. These differences can affect the chemical reactivity of the various types of semiconducting solids. For example, in a covalent solid such as Si, the valence and conduction bands can be considered as crystal orbitals that are either bonding or antibonding combinations of hybridized Si atomic orbitals. This situation is closely related to our polyene example, where the valence band consisted of bonding tt-orbitals and the conduction band consisted of antibonding 7r -orbitals. However, in an ionic crystal such as TiOi, the valence band is composed of crystal orbitals that are derived from the filled O 2p orbitals, while the conduction band is composed of crystal orbitals that are... 

With the above description of the band structure and optical properties of semiconductors, it is now possible to describe the remaining key characteristic of semiconductors electrical conductivity. The electrical conductivity of semiconductors forms the basis for most of the modem electronics industry. Without precise control over the electrical conductivity of semiconductors, many modem electronic devices would not perform satisfactorily. The goal of this section is to understand the chemical basis for the electrical properties of semiconducting solids. 

To this point, we have seen how the structural, electronic, optical, and electrical properties of semiconductors can be treated within a common framework. The bonding in the lattice determines the structure of the solid, and the structure of the lattice in turn affects the band structure. This band structure then can be used to describe the chemical, optical, and electrical properties of the semiconducting solid. Thus, chemical control over the electronic properties of semiconductors is an important component of modem research in solid-state chemistry and solid-state physics. The concepts described above enable this process to be understood from a relatively qualitative, chemically based viewpoint. Further... 

This technique was originally applied by the pioneers of IR spectroscopy and is stiU perhaps the most widely used today. However, the application of this technique to solids involves problems of sample form and preparation. This technique can be successfully applied to powdered insulating or semiconducting solids, although problems arise from radiation scattering. 

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