Crystal structures of semiconductors

If a crystal breaks along a plane related to the lattice structure, the plane is called a cleavage plane. 

Other compounds crystallizing with a Cdl2 structure type include MgBr2, Mgl2, Cal2, iodides of many tf-block metals, and many metal hydroxides including Mg(OH)2 (the mineral brucite) in which the [OH] ions are treated as spheres for the purposes of structural description. 

The structure of CdCl2 is related to the Cdl2 layer structure but with the Cl ions in a cubic close-packed arrangement. Examples of compounds adopting this structure are FeCl2 and C0CI2. Other layer structures include talc and mica (see Section 14.9). 

The perovskite (CaTi03) structure type a double oxide 

The stmetures of some high-temperature superconductors are also related to that of perovskite (see Figs. 28.12 and 28.13). Another mixed oxide stracture type is that of spinel, MgAl204 (see Box 13.7). 

This section draws attention to some of the common structure types adopted by semiconductors. The diamond-type network (often referred to an adamantine solid structure) is adopted by Si and Ge the addition of dopants occurs without structural change. Related to this network is the zinc blende lattice and among compounds adopting this structure are GaAs, InAs, GaP, ZnSe, ZnTe, CdS, CdSe, 

HgSe and HgTe. Each binary compound (including zinc blende) is an intrinsic semiconductor. The wurtzite lattice is also important in semiconducting materials ZnO, CdSe and InN are examples of compounds adopting this structure. 

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W. B. Pearson, The crystal structures of semiconductors and a general valence rule. Acta Crystallogr. 17 (1964) 1. 

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. 

Figure 3. Crystal structures of semiconductors (part I). For general structures ABX, A atoms are represented as spheres and BX units are shown as polyhedra.

The empirical pseiidopotential method can be illustrated by considering a specific semiconductor such as silicon. The crystal structure of Si is diamond. The structure is shown in figure Al.3.4. The lattice vectors and basis for a primitive cell have been defined in the section on crystal structures (ATS.4.1). In Cartesian coordinates, one can write G for the diamond structure as... 

One of palladiums unique characteristics is its abihty to absorb 900 times its own volume of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)... 

II. ELECTRONIC AND CRYSTAL STRUCTURE OF M0S2-TYPE SEMICONDUCTORS... 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

To probe the electronic structures of the materials in the solid state, band structure calculations on the crystal structure of compound 22 were carried out. The results obtained by using the linear muffin-tin orbital (LMTO) self-consistent field (SCF) method support the interpretation that compounds 22 (R1 = Me, Et R2 = H) are small-band-gap semiconductors. 

The X-ray crystal structure of the molecular complex of 9,9 -trans-bis-telluraxanthenyl 25 and fullerene has been reported <1997CPH407>. These types of fullerene complexes act as organic semiconductors (see Section 7.11.8.2). 

One may expect that future work on the electrochemistry of diamond should take two paths, namely, an extensive investigation (search for new processes and applications of the carbon allotropes in the electrochemical science and engineering) and intensive one (elucidation of the reaction mechanisms, revealing the effects of crystal structure and semiconductor properties on the electrochemical behavior of diamond and related materials). It is expected that better insight into these effects will result in the development of standard procedures for thin-film-electrodes growth, their characterization, and surface preparation. 

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... 

Temperature dependent conductivity studies reveal a metallic character of the salt and the remarkably high conductivity of 500000 S/cm at 3.5 K. It should be noted that the TCNQ copper salt is a semiconductor with a room-temperature conductivity of 2x 10 S/cm. The crystal structure of the salt TCNQI 2Cu reveals segregated columns formed from the quinone and the copper ion whereby the copper chains are surrounded by 4 quinone stacks [344]. 

In this chapter, we have discussed the unique interactions of electromagnetic radiation with semiconductor NWs that lead to resonant absorption and scattering, the importance of Raman selection rules and phonon confinement in determining the crystal structure of NWs, and the ways in which Raman spectroscopy can be used to measure composition, strain, and temperature quantitatively with submicron resolution. These qualities of Raman spectroscopy are already commonly employed in the characterization of semiconductor NWs, and one may anticipate Raman spectroscopy to be used even more widely as the applications to NW... 

Figure 15-14 (a) A simplified representation of a side view of a perfect crystal of a polar substance of 0 K. Note the perfect alignment of the dipoles in all molecules in a perfect crystal. This causes its entropy to be zero at 0 K. There are no perfect crystals, however, because even the purest substances that scientists have prepared are contaminated by traces of impurities that occupy a few of the positions in the crystal stracmre. Additionally, there are some vacancies in the crystal structures of even very highly purified substances such as those used in semiconductors (see Section 13-17). (b) A simplified representation of the same perfect crystal at a temperature above 0 K. Vibrations of the individual molecules within the crystal cause some dipoles to be oriented in directions other than those in a perfect arrangement. The entropy of such a crystalline solid is greater than zero, because there is disorder in the crystal. 

Chemical vapor deposition (CVD) finds its primary use in semiconductors and insulators, especially where epitaxial growth is required. (Epitaxy is the growth of a single-crystal film with a fixed orientation with respect to the crystal structure of the underlying substrate.) It employs reactive gas-phase precursors that decompose over the hot substrate, leaving solid behind. Fig. 6 shows an example of a tool commonly used for epitaxial deposition. Because of the surface reactions involved, films can sometimes be grown selectively on some... 

Optical Band Gaps and Crystal Structures of Some Insulators and Semiconductors... 

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