Hardness semiconductors

Silicon carbide (SiC) is a hard semiconductor, and is an inert and inexpensive ceramic used in sandpaper and in grindstones. It is made in bulk on an industrial scale by heating flint with coal tar up to very high temperatures or by means of chemical vapor deposition. SiC fibers are used in technical ceramics as a toughening aid. Like many other III-V semiconductors, it occurs in two modifications, the sphalerite (cubic) and the Wiirtzite (hexagonal) structures. In both structures carbon and silicon are tetrahedrally surrounded by silicon and carbon. The two structures do not differ much in material properties. 

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

Boron is an extremely hard refractory soHd having a hardness of 9.3 on Mohs scale and a very low (1.5 x 10 ohm cm ) room temperature electrical conductivity so that boron is classified as a metalloid or semiconductor. These values are for the a-rhombohedral form. 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

The main use of elemental As is in alloys with Pb and to a lesser extent Cu. Addition of small concentrations of As improves die properties of Pb/Sb for storage batteries (see below ), up to 0.75% improves the hardness and castabilily of type metal, and 0 5-2.0% improves the sphericity of Pb ammunition. Automotive body solder is Pb (92%),, Sb (5 0%), Sn (2.5%) and As (0.5%). Intcrnxitallic compounds with Al, Ga and In give the 111-V semiconductors (p 255) of which GaAs and InAs are of particular value for light-emitting diodes (LEDs), tunnel diodes, infrared emitters, laser windows and Hall-effect devices (p. 258). 

Metallo-organic CVD (MOCVD) and plasma CVD are developing rapidly, not only in the semiconductor-microelectronic area but also in hard coatingsfor erosion andwearapplicationssincethelower deposition temperature now permits the use of a broader spectrum of substrates. Special emphasis hasbeen given to these two areas in this second edition of the CVD Handbook (see Ch. 4 and 5). 

The two covalent carbides have low density, low atomic weight, and useful semiconductor properties. They are extremely hard and strong materials which exhibit typical ceramic characteristics. 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

Sputtering is an important thin-film process used extensively in the semiconductor and hard-coating industries and for decorative and jewelry coatings. PlP] Excellent coatings of refractory compounds and metals can be readily produced with good adhesion and composition control without the high temperature requirements of CVD. 

The importance of materials science to U.S. competitiveness can hardly be overstated. Key materials science areas underlie virtually every facet of modem life. Semiconductors underpin our electronics industry. Optical fibers are essential for communications. Superconducting materials will probably affect many areas ceramics, composites, and thin films are having a big impact now in transportation, construction, manufacturing, and even in sports—tennis rackets are an example. 

The silver white, shiny, metal-like semiconductor is considered a semimetal. The atomic weight is greater than that of the following neighbor (iodine), because tellurium isotopes are neutron-rich (compare Ar/K). Its main use is in alloys, as the addition of small amounts considerably improves properties such as hardness and corrosion resistance. New applications of tellurium include optoelectronics (lasers), electrical resistors, thermoelectric elements (a current gives rise to a temperature gradient), photocopier drums, infrared cameras, and solar cells. Tellurium accelerates the vulcanization of rubber. 

In dielectric materials (oxides, semiconductors, halides, polymers, and he like), polarizability correlates with hardness. For metals, this is not the case. However, the frequencies of the collective polarizations known as plasmons are related to mechanical hardness. 

Crystal dislocations were invented (circa. 1930) by Orowan, Prandtl, and Taylor to explain why pure metal crystals are soft compared with homogeneous shear strengths calculated from atomic theory. They do this very well. However, roughly 15 years later (circa 1945) it was found that pure semiconductor crystals (e.g., Ge and Si) have hardnesses at room temperature comparable with calculated homogeneous shear strengths. Furthermore, it was known... 

Other semiconductor crystals for which photoplasticity has been observed during hardness measurements include III-V compounds (such as GaAs— Koubaiti et al., 1997), and II-VI compounds (such as ZnS and ZnO—Klopfs-tein et al., 2003). Flowever, since the effect declined in these studies with the depth of indentation, it is likely that the observations are artifacts associated with changes of the indenter/specimen friction coefficients. An extensive review of photoplastic effects in II-VI compounds is given by Osip yan et al. (1986). 

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