Intrinsic semiconductors crystal structures

Figure 9A Two-dimensional representation of crystal structure of an intrinsic semiconductor such as Si crystal. The original tetrahedral structure is oversimplified to a square one for clarity. Dark spots in the sketch illustrate shared valence electrons among Si atoms and form covalent bonds, (a) Simation at 0 K where no ionization takes place, (b) At higher temperature, valence electrons gain sufficient energy and are delocalized, which form the holes in the VB. (c) Energy diagram for the intrinsic semiconductor crystal. Mobility of holes in VB and the electrons in CB imparts conductivity to the intrinsic semiconductor crystal.

Most of the early organic conductors were based on the TCNQ acceptor molecule [1]. Many of these are radical-ion salts having nonuniform stacks composed of dimers, trimers, or tetramers and are thus semiconductors at room temperature (see, e.g., Ref. 8 or Chapter 8 in this volume). From the crystal structures and molecular arrangements [8], it is clear that for the vast majority of these salts, this nonuniformity is driven by Coulomb attraction between the positive donor ions and the ir-electron density on the TCNQ molecules, which is larger in the regions where they are more closely spaced. A few materials with uniform stacks were known before 1972, such as Qn(TCNQ)2 and TTF Br0 7, and these were the best organic conductors known until the discovery of TTF-TCNQ, but their conductivity was limited by intrinsic disorder. 

Eg between the valence band and the conduction band. The band structure of a direct II-VI intrinsic semiconductor like CdSe can be represented reasonably well by a parabolic band model like that shown schematically in Fig. 2. Here, k = 7r/ris the wave vector and r is the radial distance from an arbitrary origin in the center of the crystal. The kinetic energy of the electron is proportional to E- and the energy minimum of the conduction band and the maxima of the valence bands occur at k = 0 (corresponding to r = co in a bulk sample). 

As a probe of lattice vibrations, Raman spectroscopy is very sensitive to intrinsic crystal properties and extrinsic stimuli, especially in semiconductors. It may be employed to study crystal structure and quality, crystal orientation, optical interactions, chemical composition, phases, dopant concentration, surface and interface chemistry, and local temperatme or strain. As an optical technique, important sample information may be obtained rapidly and nondestructively with minimal sample preparation. Submicron lateral resolution is possible with the use of confo-cal lenses. These features have made it a vital tool for research labs studying semiconductor-based technologies. They also are increasingly important for the study of semiconductor NWs fabricated by both top-down and bottom-up approaches since many of the common characterization methods used with bulk crystals or thin films cannot be applied to NWs in a direct manner. 

Silicon is a semiconductor with an intrinsic conductivity of 4.3 x 10" Q" cm and a band gap of I.I2eV at 300K. It has a diamond crystal structure characteristic of the elements with four covalently bonded atoms. As shown in Fig. 2.1, the lattice constant, a, is 5.43 A for the diamond lattice of silicon crystal structure. The distance between the nearest two neighbors is V3a/4, that is, 2.35 A, and the radius of the silicon atom is 1.18 A if a hard sphere model is used. Some physical parameters of silicon are listed in Table 2.1. 

Semi-conductors have conducting properties between those of insulators and conductors. The band structures of conductors, intrinsic semiconductors and insulators are represented schematically in Figure 1.4. The widths and the separations of the bands are dependent upon the intemuclear spacings of the constituents, so that the band structure may be modified in the vicinity of the surfaces of the crystal, by the occurrence of surface reactions, or by interactions with gases, liquids or other solids. 

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

Intrinsic semiconductors. The group fourteen elements carbon, silicon, germanium, and tin can be found to adopt the diamond-type crystal structure shown in Figure 3 a. Other crystalline structures are also found for example, graphite and diamond are different crystal structures of the same element, carbon. Because of its size and orbital energies, carbon forms very... 

Several different types of metal isocyanide complex exhibit solid state optical properties indicative of metal-metal interactions, but most exhibit low electrical conductivities. One series of salts [Rh(CNR)4]X , where R = phenyl, vinyl, ethyl or methyl and X = Cl, PFg, BF4 or CIO4, exhibit much hi er conductivities and Rh—Rh separations as short as 2.94 A. Single crystals of [Rh(CNCHCH2)4]C104 have a room temperature conductivity of 2 Q cm with an activation energy of 100 meV. An indication of a metallic state was observed below the decomposition temperature of 330 K but it was not possible to conclude whether the high conductivity was the result of partial oxidation, small band gap intrinsic semiconductor behaviour or extrinsic semiconductor behaviour due to Rh impurities . More recent studies indicate the existence of polynuclear species but a full structural study of an infinite metal atom chain compound wth isocyanide ligands has not been made, ... 

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