MATERIALS SYNTHESIS

The materials in modern cars and airplanes that make them safer, lighter, and more fuel-efficient than their predecessors result from advances in materials synthesis and processing. Progress in the synthesis of materials takes many forms: research aimed at discovering new materials, development of methods for inexpensive and reliable production of such materials, incorporation of well-known materials in new geometries and environments, and continuous improvement of the production and processing of traditional materials. Each of these activities has firm roots in materials physics and chemistry.

“Nonequilibrium” materials processing involves raising the energy of the starting materals (for example, by heating) and guiding them into the desired final state. Such an approach has allowed the creation of new surface alloys that improve the wear characteristics of artificial joint replacements and machine tools. The opposite approach, operating very near equilibrium, is also useful. For example, it makes possible the growth of large, ultrapure, defect-free crystals of silicon for use in the semiconductor industry.

The production of traditional materials also continues to evolve. An object as simple as an aluminum can is a good example.The raw material these days consists increasingly of recycled cans. Can walls are being made thinner and thinner, an achievement made possible by close control of the alloy composition and of the processing of the aluminum sheet. Optimization of these processes increasingly requires integration of computer-based modeling over a large range of length scales: from atomic bonds, motion of dislocations, and deformation and rotation of individual crystallites, to macroscopic behavior.

Another example is the development of alloys for jet aircraft. Alloys in early jets suffered from fatigue that ultimately led to disintegration. Modern alloys are not only stronger and lighter but also more resistant to stress.

FIGURE 2.1 A futuristic high-performance aluminum car. (Courtesy of Ford Motor Company.)

COMPOUND SEMICONDUCTOR ELECTRONICS

Silicon is the material underlying most electronics, but compound semiconductors composed of more than one element, such as gallium arsenide (GaAs) and silicon germanium (SiGe), have advantages that can lead to devices with intrinsically higher speed and lower noise. The worldwide market for compound semiconductors is estimated to be $750 million in 1996, and it is growing at the rate of 40% per year. Discrete components are now widely used in the low-noise receivers of cellular telephone handsets, in addition to the specialized high-speed microwave applications for which they have long been the materials of choice.

FIGURE 2.2 A high electron-mobility transistor (HEMT) such as those used in cellular telephones. The round bonding pads are 100 microns in diameter, roughly the size of a human hair. The gate of the transistor, just 0.05 microns across, appears as the two narrow lines in the center of this scanning electron micrograph. (Courtesy of Sandia National Laboratories.)

Compound semiconductors such as GaAs, SiGe, and gallium nitride (GaN) are key to the development of the next generation of wireless telephones, which will use higher frequency microwaves in order to transmit more information. GaN transistors, for example, are characterized by high breakdown voltage and great robustness. A potential high-volume application for such transistors is in transmitter power amplifiers for wireless base stations.

Pushing the limits of semiconductor materials technology is essential for increasing the speed of transistors and advancing our ability to modulate lasers for high-speed optical information transmission. Because compound semiconductors are composed of more than one element, they promise a vastly increased range of materials from which to select those with desired electronic properties. This promise can be realized with manufacturing techniques such as molecular beam epitaxy, which allows the repeated, controlled, precise growth of one material on another in single atomic layers, producing compound layered materials not seen in nature. In the future, the use of novel forms of microscopy for fabrication and testing will determine our ability to design and build such structures on the atomic scale— a scale on which the motion of electrons is governed by quantum mechanics.



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