Metals and Insulators

Doping can have profound effects upon the electronic properties of a crystal. In this section three examples of changes from insulating to metallic properties are described. 

Acceptor doping of La2Cu04 is simply carried out by replacing the La3+ ion with a similar sized divalent ion, typically an alkaline earth ion such as Ca2+, Sr2+, or 

In this process, small changes in the oxygen content also occur, which will be ignored in the following discussion. 

Following substitution, charge neutrality can be maintained in one of the three ways, as described above, that is, by the introduction of cation interstitials, anion vacancies, or holes. Replacement of some of the La3+ by Sr2+ can be written in terms of La2C 3 alone (Section 1.11.7)  

This last equation is equivalent to filling of the anion vacancies by atmospheric oxygen  

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Figure C2.16.1. A nomogram comparing electrical resistivity of pure (intrinsic) and doped Si witli metals and insulators.

Several striking examples demonstrating the atomically precise control exercised by the STM have been reported. A "quantum corral" of Fe atoms has been fabricated by placing 48 atoms in a circle on a flat Cu(lll) surface at 4K (Fig. 4) (94). Both STM (under ultrahigh vacuum) and atomic force microscopy (AFM, under ambient conditions) have been employed to fabricate nanoscale magnetic mounds of Fe, Co, Ni, and CoCr on metal and insulator substrates (95). The AFM has also been used to deposit organic material, such as octadecanethiol onto the surface of mica (96). New appHcations of this type of nanofabrication ate being reported at an ever-faster rate (97—99). 

Catalysis. Ion implantation and sputtering in general are useful methods for preparing catalysts on metal and insulator substrates. This has been demonstrated for reactions at gas—soHd and Hquid—soHd interfaces. Ion implantation should be considered in cases where good adhesion of the active metal to the substrate is needed or production of novel materials with catalytic properties different from either the substrate or the pure active metal is wanted (129—131). Ion beam mixing of deposited films also promises interesting prospects for the preparation of catalysts (132). 

Deposition of Thin Films. Laser photochemical deposition has been extensively studied, especially with respect to fabrication of microelectronic stmctures (see Integrated circuits). This procedure could be used in integrated circuit fabrication for the direct generation of patterns. Laser-aided chemical vapor deposition, which can be used to deposit layers of semiconductors, metals, and insulators, could define the circuit features. The deposits can have dimensions in the micrometer regime and they can be produced in specific patterns. Laser chemical vapor deposition can use either of two approaches. 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

In the microelectronics industry, powdered metals and insulating materials that consist of noimoble metals and oxides are deposited by screen printing in order to form coatings with high resistivities and low temperature coefficients of resistance. This technique may be useful in depositing oxide—metal refractory coatings. 

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

Removal of special components Loose fittings are mostly removed. They include plastic accessories and trays, steel racks, glass shelves and doors made of plastic, metal, and insulation material. 

In metals, valence electrons are conduction electrons, so they are free to move along the solid. On the contrary, valence electrons in insulators are located around fixed sites for instance, in an ionic solid they are bound to specific ions. Semiconductors can be regarded as an intermediate case between metals and insulators valence electrons can be of both types, free or bound. 

Fig. 2. Schematic illustrations of two back-end surface configurations after CMP. For the conventional technology, a simple oxide surface must be cleaned up, whereas for damascene structures, both outcropping metals and insulator must be processed together. The difficulty is to preserve the metal lines and the diffusion barriers.

Torrance JB (1979) The difference between metallic and insulating salts of tetracyanoqui-nodimethone (TCNQ) how to design an organic metal. Acc Chem Res 12 79-86... 

Temperature changes do not appreciably affect ultraviolet optical properties of both metals and insulators, although at low temperatures absorption bands associated with excitons and electron band transitions are usually sharper, and frequencies of peak absorption may shift slightly. In the soft x-ray region, transitions of core electrons buried in the interior of atoms hardly notice temperature changes. 

Figure 6.51 Phase diagram for transition-metal compounds in the f//t and A/t space showing the parameter ranges for metallic and insulating states (after Sarma et al., 1992).

Figure 7.24 Resistivity data of cuprates in comparison with those of traditional metals and insulators.

Bulk crystalline or amorphous solid-state materials whose conductivity is intermediate between metals and insulators and whose resistance decreases with increasing temperature. The valance band of an undoped semiconductor is completely filled, whereas its conduction band is empty. The energy difference between the valence and conduction bands (the band-gap) defines a semiconductor (see Fig. 95). 

Construction of electronic devices usually involves deposition of thin layers of semiconducting, metallic, and insulating materials onto a suitable substrate (which might be a wafer cut from a Czochralski silicon boule with a diamond saw).15-24 In some cases, it is possible to grow crystalline layers onto a substrate such that the crystallographic order of the atoms in the film is related to that of the surface of the substrate this is known as epitaxial growth. 

Before the discovery of antiferromagnetism, it was pointed out that, according to the Wilson theory of metals and insulators (Chapter 1), nickel oxide should be a metal—whereas it is a transparent insulator. The nickel ions should have eight d-electrons, and the only splitting of the d-band to be expected is into the eg and t2g bands, with four and six electrons. Peierls explained in 1938 how this... 

At temperatures when interstitial atoms and ions become mobile (typically, 20-30 K in both metals and insulators), they perform thermally-activated incoherent hops, the frequencies of which are exponentially dependent on reciprocal temperature ... 

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