In 1916 a group of Russian chemists—Kurnakov and his colleagues—discovered that slowly cooled CuxAu1 − xmetal alloys had anomalously low electrical resistivities at simple compositions of CuAu and Cu3Au. Nine years later in 1925, Swedish physicists Johansson and Linde found by x-ray-diffraction experiments that the alloys had ordered structures on the face-centered-cubic lattice, now called CuAu I-type and AuCu3-type. Two years later, Johansson and Linde discovered CuPt-type ordering in Cu0.5Pt0.5 alloy by noticing a similar anomaly in their electrical-resistivity measurements for CuxPt1 − x. These were among the events at the dawn of physical metallurgy and study on order-disorder transformations. Many years later in 1985, the first observation of ordering in semiconductor alloys was made in AlGaAs grown on GaAs(110); the type was CuAu I. CuPt-type ordering was observed in SiGe grown on Si(001). Since the (110) orientation of the substrate for AlGaAs was clumsy for practical purposes and the degree of ordering in SiGe was extremely low, these phenomena seemed to have little relevance in practical applications. Other observations were made such as famatinite-type ordering in GalnAs and chalcopyrite-type ordering in GaAsSb. These observations however were quite rare, with only isolated cases reported.

Displacement damage in graphite and carbon-based materials can occur when energetic particles, such as neutrons, ions, or electrons impinge on the crystal lattice. The displacement of carbon atoms from their equilibrium positions results in lattice strain, bulk dimensional change, and profound changes in physical properties. This article will discuss the effects of displacement damage in graphites and carbon-based materials. The materials considered here are those whose bonding is sp2—that is, graphites, pyrolytic carbons and graphites, carbon fibers, and carbon-carbon (C/C) composites. Radiation damage in sp3 (diamond) carbon forms is not discussed.

Carbon-based materials and graphites are widely used in nuclear applications. For example, polygranular (manufactured) graphites have been employed as a moderator in nuclear reactors since the 1940s. More recently, pyrolytic graphites, artificial graphites, and C/C composites have been adopted as plasma-facing components in fusion devices. Engineering applications, such as those just cited, have necessitated a full understanding of the basic mechanisms of radiation damage, as well as the effects of radiation damage on the physical properties of carbon-based materials.

Metal-ceramic composite applications range from electronic devices such as electronic packaging, thin-film technology in microwave circuitry, and magnetic storage media to catalyst supports, protective coatings, and high-temperature structural components. These applications rely heavily on the ability to engineer the mechanical and electronic properties of metal-ceramic interfaces. To understand fundamental aspects of these interfaces, a variety of experimental and theoretical studies on “model systems” have been performed, correlating macroscopic material behavior and microscopic characteristics. The results serve as a guide to more complex systems, with a closer relationship to technological applications.

From a single chain in a dilute solution to an entangled polymer melt, from bulk systems to more complex interfacial problems, computer simulations have played a critical role not only in testing the basic assumptions of various theoretical models but also in interpreting experimental results. Early computer simulations of polymers were mostly carried out on a lattice using Monte Carlo methods. This approach has led to significant progress in recent years and will continue to do so in many areas. In some cases however, for example in the study of shear, lattice models have serious limitations. For this reason and also due to the availability of more powerful computers, continuum, off-lattice polymer models have recently become popular. In this article, we review some of the recent progress in studying polymers at surfaces and interfaces using continuum models.

As on-chip device densities increase and active device dimensions shrink, signal delays and noise increase due to capacitive coupling and crosstalk between the metal interconnections. Since delays, noise, and power consumption all depend critically on the dielectric constant of the separating insulator, much attention has focused recently on replacing standard silicon dioxide with new intermetal dielectrics (IMDs) having dielectric constants considerably lower than conventional oxide (k = 3.9–4.2). On-chip silicon dioxide insulators are currently deposited by gas-phase techniques such as chemical vapor deposition or plasma-enhanced chemical vapor deposition. Silicate films may also be formed at lower temperatures by sol-gel procedures. In the sol-gel process, typically an orthosilicate ester is hydrolyzed with water. This often occurs in an organic solvent to form a soluble, partially condensed polymer (sol) that can be spun on a substrate to produce a solvent-containing film. Subsequent solvent removal and curing results in the silicate film. The process involves hydrolysis to generate polyfunctional silanols followed by condensation polymerization to eventually yield a gel. Since both processes involve the substantial loss of volatile materials, considerable shrinkage occurs (75–85% is typical). Inhomogeneity of shrinkage or shrinkage on constraining substrates can often lead to cracking unless the films are very thin (often <1 μm). In the sol-gel process, a variety of techniques are employed to avoid capillary-driven cracking forces, including (1) very slow drying, (2) drying with supercritical fluids, or (3) chemically controlled condensation.

Since the discovery of high electrical conductivity in doped polyacetylene in 1977, π-conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, one must understand the nature of the polymer surface's electronic structure and the interface with metals. For conjugated polymers, photoelectron spectroscopy—especially in connection with quantum-chemical modeling—provides a maximum amount of both chemical and electronic structural information in one (type of) measurement. Some details of the early stages of interface formation with metals on the surfaces of conjugated polymers and model molecular solids in connection with polymer-based light-emitting devices (LEDs) are outlined. Then a chosen set of issues is summarized in a band structure diagram for a polymer LED, based upon a “clean calcium electrode” on the clean surface of a thin film of poly(p-phenylene vinylene) (PPV). This diagram helps to point out the complexity of the systems involved in polymer LEDs. No such thing as “an ideal metal-on-polymer contact” exists. There is always some chemistry occurring at the interface.

Quasicrystals (QCs) exhibit unusual physical properties that are significantly different from those of crystalline materials and are not expected for alloys consisting of normal metallic elements. Opposite to conventional metallic alloys, their thermal conductivity and diffusivity are unusually low (with a positive temperature coefficient)—practically that of an insulator—which is atypical for materials containing about 70-at.% aluminum. Moreover the thermal conductivity decreases when the structural perfection is improved. One observes a low, if any, electronic contribution to the heat capacity and thus a vanishing density of electronic states at the Fermi level.

The origin of this unexpected behavior was first attributed to the existence of a deep pseudogap at the Fermi level with a localization tendency of electrons near the Fermi level.However experimental evidence led to an alternative approach related to the structure of quasicrystals. In QCs, well-defined atomic clusters form self-similar subsets of the structure over which electronic and vibrational states are expected to extend. According to the inflation symmetry of the icosahedral structure, the so-called recurrent localization effects may then explain the conduction behavior and other striking features of quasicrystals (e.g., brittleductile transition at high temperature, corrosion resistance, low friction, high hardness).

In the following, we first present the main thermal properties of quasicrystalline alloys compared to those of conventional materials with an emphasis on the variation of the thermal conductivity with temperature. The combination of such peculiar conduction, mechanical, and tribological properties gives the quasicrystalline alloys a technological interest for applications where superficial thermal and mechanical conditions are of prime importance. This is illustrated with two examples involving a QC coating on a base Al substrate: (1) thermal insulation for which a low conductivity is needed and (2) quenching heat-transfer modification due to a low-effusivity superficial effect. These processes are then explained in the third part of this article in terms of the cluster-modes delocalization mechanism responsible for the low conductivity of the quasicrystals.

Powder metallurgy (P/M) involves a multiple-step process to prepare and consolidate granulated metal and composites into near-net-shaped components. The modern P/M method starts with a precisely defined final geometry and a set of properties for a desired component. Decisions are then made about a variety of processing variables, recognizing that many variables strongly interact counter to the desired properties. Finally preproduction fabrication trials are conducted to optimize key variables until he specifications are achieved, and the processing conditions are established. A critical step in the overall process is the compaction of powder into a sufficiently strong, low-porosity, properly shaped part with uniform microstructure that can be sintered, or alternately heated and pressed to the final dimensions and properties.

The recent advances in the material quality of the group-III-nitride semiconductors (GaN, A1N, and InN) have led to the demonstration of high-brightness light-emitting diodes, blue laser diodes, and high-frequency transistors, much of which is documented in this issue of MRS Bulletin. While further improvements in the material properties can be expected to enhance device operation, further device advances will also require improved processing technology. In this article, we review developments in two critical processing technologies for photonic and electronic devices: ion implantation and plasma etching. Ion implantation is a technology whereby impurity atoms are introduced into the semiconductor with precise control of concentration and profile. It is widely used in mature semiconductor materials systems such as silicon or gallium arsenide for selective area doping or isolation. Plasma etching is employed to define device features in the semiconductor material with controlled profiles and etch depths. Plasma etching is particularly necessary in the III-nitride materials systems due to the lack of suitable wet-etch chemistries, as will be discussed later.

Figure 1 shows a laser-diode structure (after Nakamura) where plasma etching is required to form the laser facets that ideally should be vertical with smooth surfaces. The first III-nitride-based laser diode was fabricated using reactive ion etching (RIE) to form the laser facets but suffered from rough mirror facet surfaces that contributed to scattering loss and a high lasing threshold. This is a prime example of how improved material quality alone will not yield optimum device performance.

Organic semiconductors have been studied since the early 1950s, and the large amount of work devoted to them has allowed a better understanding of their charge-transport properties. However owing to their very poor semiconducting characteristics, they were merely considered as exotic materials with little potential interest for applications until the late 1980s, when two significant steps simultaneously appeared in the literature. Richard Friend's group showed that light-emitting diodes could be made from a conjugated semiconducting polymer, and our laboratory showed that efficient field-effect transistors (FETs) could be realized from short-conjugated oligomers. These two results launched intensive research on these two types of organic-based devices, and the extensive work accomplished since has largely confirmed the technological pertinence of organic semiconductors, showing the promise for applications in flexible and large-area electronics. Two categories of organic semiconductors are actually under development: (1) conjugated polymer-based ones whose amorphous state is favorable to strong luminescence and (2) conjugated oligomer-based ones, in which charge-transport efficiency is directly related to the long-range packing of molecules in the semiconducting film. In fact conjugated oligomers can be said to be forming molecular polycrystals whose electrical properties are essentially controlled by molecular order. Thus performance of sexithiophene-based FETs has been improved by a factor of nearly 50 by controlling the molecular ordering in the evaporated film, from a disordered three-dimensional structure to a well-ordered two-dimensional organization where all the molecules stack along a packing axis nearly parallel to the substrate surface.

There have been many studies (see for instance Reference 1) directed at quantifying the effects of ionizing radiation on polymers. Mainly these studies have focused on assessing the degradation of mechanical properties. In addition to changes in mechanical properties, a mixture of gases results from the irradiation of polymeric compounds. The amount of gas produced is related to the total absorbed dose while the type of gas is independent of dose and is related only to polymer structure. For many polymers, hydrogen is the major component in the gaseous mixture but polyesters and anhydride-cured epoxide resins are a notable exception.

Some problems associated with radiation damage may be further complicated when the irradiation temperature is in the cryogenic range. When materials are irradiated at low temperatures (<150 K), the release of volatile degradation products is delayed and occurs—possibly very rapidly and exceeding normal diffusion rates—only when the polymer is warmed. The rapid release of gaseous degradation products could occur as a result of warming a superconducting magnet that has been resin bonded and irradiated at low temperatures during operation. Radiation damage-related problems may be summarized as follows: (1) changes in short-term mechanical properties, (2) production of gases and low-molecular-weight species, and (3) longer term and geometrical effects (unknown) of trapped gases on electrical and mechanical properties.

This article considers mechanisms of radiation-induced damage and relates the chemical structure of the polymer to the rate of damage accumulation. Changes in electrical properties measured on irradiated polymers are generally small, and mechanical damage is generally more significant than changes in electrical characteristics. However all such reports are a result of electrical measurements on thin films and do not consider the effects of trapped gases on long-term electrical properties in bulk specimens.

The behavior of solids in the nanometer size regime, as their dimensions approach the atomic scale, is of increasing fundamental and applied interest in materials research. Electronic, optical, magnetic, mechanical, or thermodynamic properties all may depend on the size and shape of the solid. As a result, in the nanoscale regime, size and shape may be used as design variables to tailor a material's properties such as giant magnetoresistance in multilayer films, or the optical properties in semiconductor nanocrystals. In most cases, the size dependence of properties is not well-understood. Nanophase materials constitute a new frontier in materials science, and accurate nanoscale characterization is extremely important in exploring this new frontier. In this area, transmission electron microscopy (TEM) plays a key role. Because of its unique ability to provide information on the structure and composition of internal interfaces in solids, TEM is particularly important in cases of buried nanophase structures such as small solid inclusions—that is, solid particles embedded within another solid.

Nanoscale inclusions have recently been shown to exhibit unusual melting behavior that depends strongly on their size and the embedding matrix. For example, small inclusions of Pb in SiO exhibit melting-point depressions of several hundred degrees, whereas similarsized Pb inclusions in aluminum have shown large increases in melting point. Although a full understanding of these effects is still lacking, it appears that they are related not just to inclusion size but also to their shape and interface structure.

The application of III-V semiconductor alloys in device structures is of importance for high-speed microelectronics and optoelectronics. These alloys have allowed the device engineer to tailor material parameters such as the bandgap and carrier mobility to the need of the device by altering the alloy composition. When using ternary or quaternary materials, the device designer presumes that the alloy is completely disordered, without any correlation between the atoms on the cation (anion) sublattice. However the thermodynamics of the alloy system often produce material that has some degree of macroscopic or microscopic ordering. Short-range ordering occurs when atoms adopt correlated neighboring positions over distances of the order of a few lattice spacings. This can be manifested as the preferential association of like atoms, as in clustering, or of unlike atoms, as in chemical ordering (e.g., CuPt ordering). Long-range ordering occurs over many tens of lattice spacings, as in the case of phase separation. In either short-range or long-range ordering, the band structure and the crystal symmetry are greatly altered. Therefore it is absolutely critical that the mechanisms be fully understood to prevent ordering when necessary or to exploit it when possible.

A variety of opportunities to develop technological applications of quasicrystals is being considered. Several of these are discussed in this issue. A common feature of many proposed applications is that they involve surface or near-surface interactions. Information about the surface structure, composition, and chemistry is prerequisite to understanding the origin of the desirable traits of quasicrystals. For instance, basic issues of surface structure—whether quasicrystals are intrinsically rough or flat, quasicrystalline or not quasicrystalline—impact directly on our understanding of their surface thermodynamic, electronic, and chemical properties.

Studying the properties of endanchored polymer layers has been a fashionable occupation for numerous physicists, chemists, and material scientists for more than 10 years. Theoreticians have realized that grafted macromolecules are nice statistical objects wriggling around under thermal motion, which give rise to nontrivial long-range entropic effects. These can be described by elegant scaling laws and analogies with quantum or classical mechanics. For experimenters the area turned out to be a marvelous playground in which both very simple and sophisticated techniques such as x-ray or neutron scattering and reflectivity, nuclear magnetic resonance (NMR), Rutherford backscattering, and optical and atomic force microscopy (AFM) have been used to discover interesting and subtle phenomena. All this effort was also motivated by the importance of grafted layers in applications such as paints, adhesives, lubricants, colloidal stabilizers, and composite materials. By anchoring a thin, soft polymer layer to a solid surface, one can tune the surface properties. In this short article, we will discuss how the wetting and spreading of liquids and polymer melts can be profoundly altered by the presence of such protective layers.

In this article, we review recent progress in GaN-based photodetectors and field-effect transistors (FETs), including optoelectronic FETs, and discuss materials parameters and fabrication technologies that determine the device characteristics of these two device families. Many types of visible-blind photodetectors and nearly all types of FETs have been demonstrated in GaN-based materials systems. However many challenges remain, both in improving the existing devices—the performance of which is still quite far from reaching its full potential—and in developing entirely new devices, which use unique properties of this wide-bandgap materials system.