The following is an edited version of the Von Hippel Award address, given by recipient Sir Alan Cottrell at the 1996 MRS Fall Meeting. Cottrell received the Materials Research Society's highest honor for “converting crystal dislocations from a hand-waving hypothesis to a rigorous discipline, transforming the understanding of brittle fracture, making varied and crucial advances in the theory of radiation damage, and for transforming the teaching of materials science throughout the academic world through his pioneering textbooks.” Cottrell is an honorable distinguished research Fellow in the Department of Materials Science and Metallurgy, University of Cambridge.

The science of materials as a specific discipline is a relatively modern development. Within this rather modern field, the study of semiconductor materials is an even more recent development. Modern textbooks on materials science focus primarily on the properties of metals and second on the properties of ceramics, reflecting the commercial importance of these materials 50 years ago when the transistor was demonstrated using germanium. Today the commercial importance of semiconductor materials is comparable to that of metals and ceramics, and the advances in semiconductormaterials technology are driving rapidly growing business sectors in communications, software, and biotechnology.

For decades scientists have accepted the premise that solid matter can only order in two ways: amorphous (or glassy) like window glass or crystalline with atoms arranged according to translational symmetry. The science of crystallography, now two centuries old, was able to relate in a simple and efficient way all atomic positions within a crystal to a frame of reference in which a single unit cell was defined. Positions within the crystal could all be deduced from the restricted number of positions in the unit cell by translations along vectors formed by a combination of integer numbers of unit vectors of the reference frame. Of course disorder, which is always present in solids, could be understood as some form of disturbance with respect to this rule of construction. Also amorphous solids were naturally referred to as a full breakdown of translational symmetry yet preserving most of the short-range order around atoms. Incommensurate structures, or more simply modulated crystals, could be understood as the overlap of various ordering potentials not necessarily with commensurate periodicities.

For so many years, no exception to the canonical rule of crystallography was discovered. Any crystal could be completely described using one unit cell and its set of three basis vectors. In 1848 the French crystallographer Bravais demonstrated that only 14 different ways of arranging atoms exist in three-dimensional space according to translational symmetry. This led to the well-known cubic, hexagonal, tetragonal, and associated structures. Furthermore the dihedral angle between pairs of faces of the unit cell cannot assume just any number since an integer number of unit cells must completely fill space around an edge.

Particle packing and compaction are important in many aspects of materials science and technology, including the flow and storage of slurries and of granular agricultural and fertilizer products; the forming of ceramic components, powder metallurgy (P/M) parts, and pharmaceutical tablets; road and dam construction; landfill and waste-remediation-site design and development; shoreline and earthquake stabilization; predicting and controlling mud slides, rock slides, and avalanches; and mine and oil-well positioning and stability. On the less technical side but perhaps of more practical interest are how packing and compaction produce the “apparently” half-empty packages of cereal and potato chips, and how to make better sand and snow castles. There is a great need for engineers and scientists to understand and control particle packing and powder compaction.

Near-field scanning optical microscopy (NSOM) provides a means to study optical and optoelectronic properties of materials at the nanometer scale. The key to achieving resolution higher than the diffraction limit is to place a subwavelength-sized light source—e.g., an aperture—within the near-field zone of the sample. In this case, the area of the sample illuminated is determined by the aperture size and not by the wavelength (see Figure 1). An image can then be formed by moving the sample and light source with respect to each other. While the principle of near-field optics is straightforward, its realization at visible-light wavelengths was not achieved until the invention of scanning-probe techniques in the 1980s. Since Betzig et al. demonstrated in 1991 that bright subwavelength apertures can be made by tapering and metal-coating single-mode optical fibers, research activities involving NSOM have increased tremendously. The later incorporation of shear-force feedback to regulate tip-sample separation adds another strength to NSOM. Using this distance regulation, a topographic image similar to that obtained by a conventional scanning force microscope is acquired simultaneously with the optical image. This provides a way to correlate structural and physical properties at the same sample positions and greatly simplifies interpretation of the NSOM data.

A1N, GaN, and InN are very promising materials for use in optoelectronic and high-temperature electronic devices. These materials and their ternary and quaternary alloys cover an energy bandgap range of 1.9–6.2 eV, suitable for band-to-band light generation with colors ranging from red to ultraviolet (uv), with wavelengths ranging from 650 to 200 nm. On the device front, they are suitable for example for negative electron-affinity cold cathodes, electronic devices, surface acoustic wave devices, uv detectors, Bragg reflectors and waveguides, uv and visible light-emitting diodes (LEDs), and laser diodes (LDs) for digital data read-write applications. Stifled by the absence of native substrates, growth and doping of high-quality III-V-nitride thin films, particularly p-type, have been major obstacles for developing GaN-based devices. Development of electronic devices such as modulation-doped field-effect transistors (MODFETs) and opto-electronic devices such as LEDs and LDs has also proven challenging.

An important part of materials science is the study of the behavior of materials under difficult environmental conditions. Some operating conditions have been traditional objects of study—for example, high temperatures and corrosive atmospheres. The study of materials in radiation environments gained particular significance after the advent of nuclear reactors, and radiation has joined the list of insults to which materials are subjected. Technological applications that involve significant levels of radiation include but are not limited to fission reactors, proposed fusion reactors, nuclear waste, ion accelerators, and spacecraft, with types of radiation encompassing thermal and fast neutrons, light and heavy ions, and energetic photons. More detailed information on applications and accompanying radiation fields appears in the articles published in this issue of MRS Bulletin and in the references listed under Further Reading.

The four theme articles in this issue focus on radiation effects in four families of materials commonly used in radiation environments: metals, ceramics, graphite and carbon-based materials, and polymers.

SiC electronics research has been driven by the continued successful development of SiC technology for high-power and high-frequency semiconductor devices, and for service in high-temperature, corrosive, and high-radiation environments. The development of this technology has been accelerated by the introduction of commercially available SiC wafers, which have decreased in cost with time. The most recently demonstrated commercial SiC-based products include ultraviolet (uv)-flame sensors for terrestrial turbine engines and high-definition-television transmitter systems utilizing SiC-based transistors. Prototype microelectronic SiC devices include high-voltage Schottky rectifiers and power metal-oxide-semiconductor field-effect transistors, microwave and millimeter-wave devices, and high-temperature, radiation-resistant junction FETs (JFETs). These advancements in SiC-based device technology are attributable to both the successful development of commercially available, bulk SiC substrates and the recent advancements in SiC epitaxial layer growth technologies.

The discovery in 1977–78 that trans-polyacetylene — (CH)x, the prototype conducting polymer (Figure 1)—could be chemically p-doped (partly oxidized) or n-doped (partly reduced) with a concomitant increase of its conductivity through the semiconducting to the metallic regime introduced new concepts of considerable theoretical and possible technological importance to condensed matter science. In 1979 it was discovered that p- or n-doping of trans-(CH)x could be accomplished electrochemically and that these processes were electrochemically reversible. Polyacetylene is the simplest example of a conjugated polymer, a polymer in which the “backbone” atoms are joined alternately by single and double bonds. All conducting polymers, “synthetic metals,” are conjugated polymers, at least in their doped forms. Other conducting polymers, including for example, poly(paraphenylene), polypyrrole, polythiophene, and polyaniline, have since been examined as electrochemically active materials. These findings have stimulated much industrial and academic interest in the electro-chemistry of conducting polymers and their possible technological applications in for example, energy storage, electrochromic displays, electrochemical drug-delivery systems, electromechanical devices, and light-emitting devices.

This article will show the relationship between the doping of a conjugated polymer, the reduction potential of the polymer, and the role of “dopant” ions. These interrelationships have frequently caused considerable confusion in understanding electrochemical doping. Electrochemical synthesis of conjugated polymers and the role of cyclic voltammetry in elucidating the mechanism of electrochemical redox processes involving conjugated organic polymers will also be discussed. This article will also summarize a few selected applications involving electro-chemical properties of conjugated polymers. The coverage is intended to be

exemplary rather than exhaustive. Furthermore since the electrochemistry of (CH), the “prototype” conducting polymer, has been extensively studied and comprises a relatively simple, reversible electrochemical system, it will be used to exemplify the basic concepts involved. These basic concepts can then be applied with appropriate modification as necessary to the electrochemistry of other conjugated polymers. Polyaniline will then be used to illustrate a more complex conjugated polymer electrochemical system.

For devices with feature sizes below 0.18 μm, it is desirable to have materials with a dielectric constant below 2.5 as interlayer dielectrics. Polymeric materials are possible candidates. There are two main strategies to grow polymeric films. The most widely used method is the spin-on technique. The other method is by vapor deposition. Although vapor deposition is less common, it has several attractive features that look quite promising, especially when the wafer size becomes very large.

There are several advantages to vapor-deposited polymers:

(1) The deposition of the polymers is a dry process. It is solvent-free and does not produce waste. No remedial measures are necessary to take care of the waste. The process is attractive from both energy-conservation and environmental considerations.

(2) They can provide an extremely uniform coating over a very large area. For 200-mm wafers, for example, one can achieve better than 2% uniformity for vapor-deposited parylene (a type of polymer to be described later) films. Similar uniformity can be expected for future 300-mm wafers.

(3) Many vapor-deposited polymers possess superior gap-filling capability. Small vias and trenches of very high aspect ratios can be filled without voids.

There are some shortcomings in vapor deposition of polymeric thin films. First of all, except for some special cases, processing issues for these materials are not well-studied. Manufacturing equipment is not well-developed.

The articles in this issue illustrate how various theoretical and computer models have been used to probe behavior of polymers at penetrable and impenetrable interfaces. Interest in polymer interfaces stems from the control interfaces commonly have over macroscopic properties—such as the strength or biocompatibility of the material. Consider the “alloying” or blending of existing polymers. This technique provides an inexpensive means of fabricating new materials that can display the desirable properties of the individual components. Most polymer pairs however are immiscible, and the mixture segregates into macroscopic domains separated by relatively weak interfaces. To enhance the structural integrity of the blend, copolymer “compatibilizers” are added to the mixture. These chains localize at the interface between the immiscible polymers, enhance the adhesion between the phase-separated regions, and thereby improve the mechanical properties of the blend.

On an impenetrable interface, polymer films or coatings are commonly used to modify the properties of the underlying substrate. For example the polymer layer can be utilized to tailor biocompatibility, wettability, or roughness of the surface. Polymers anchored to solid surfaces can also be used to control the interaction between these surfaces. Thus the tethered layer can promote the adhesion between dissimilar solids or prevent the aggregation of colloidal particles.

Polymer films that contain well-defined patterns can be used in a variety of novel applications. For example such films can serve as the scaffolding in fabricating organic/inorganic composites with controlled architectures. One means of forming patterned films is to anchor the ends of homopolymers onto a substrate (so that the ends are fixed and cannot move) and immerse the system in a poor solvent. The incompatibility between the polymer and solvent drives the system to phase-separate. Since the ends are immobilized however, the polymers can only escape the unfavorable solvent by clustering with neighboring chain s into distinct aggregates or “pinned micelles.” These micelles have a uniform size and spacing, and form a regular array on the surface. In this article, we use theoretical models to extend this concept and show that, by tethering copolymers—chains that contain more than one type of monomer—we can drive the system to form more complicated surface patterns. These copolymer patterns provide a handle for engineering the interaction between surfaces and thus facilitate the fabrication of novel optical devices. If the copolymer films are composed of both hydrophilic and hydrophobic domains, the surface can also be used as a template for growing biological cells with tailored shapes and sizes.

Heteroepitaxial pseudomorphic thin-film growth allows the stabilization of solids in structures—that is, with crystalline symmetries and lattice constants that are far from those of the corresponding bulk material. This facilitates the tailoring of materials properties that do not exist in nature. Usually a strong correlation is found between the atomic, electronic, and magnetic structure of thin films, which leads to a drastic dependence of their physical properties on the details of growth conditions such as substrate temperature, evaporation rate, and background pressure. To learn more about the fundamental relationship between structural, electronic, and magnetic properties of thin films as well as to improve the reproducibility of their physical properties in view of technical applications, it is mandatory to apply experimental techniques with sufficient, ultimately atomic spatial resolution combined with high sensitivity. The class of scanning-probe microscopies and spectroscopies is ideally suited for that purpose as schematically illustrated in Figure 1. Scanning tunneling spectroscopy (STS) establishes the correlation between atomic structure and local electronic properties, while spin-polarized scanning tunneling spectroscopy (SPSTS) relates local electronic and magnetic properties. Additionally the correlation

between topographical and magnetic structure can be addressed by magnetic force microscopy (MFM) though the spatial resolution is limited to about 10–50 nm. Since magnetic thin films are usually sensitive to oxidation, their preparation and characterization have to be performed in situ under ultrahigh vacuum (UHV) conditions in order to achieve reproducible experimental conditions. While STS is now routinely applied in UHV, it has only been recently that the first MFM studies under UHV conditions have been reported. In the following, representative examples of the application of STS, SPSTS, and MFM in ultrathin Fe, Gd, and Co films will be presented.

Particle packing is directly controlled by the particle-size distribution of a material being processed. For this reason, particle packing is important to all particulate/fluid systems. After the solids fraction of a body is defined, interparticle chemistry controls how the body will pack and flow. A system of powders can never pack better than the maximum possible level defined by the particle-size distribution alone. Proper control of interparticle chemistry however can help achieve maximum packing, can be used to open the structure, and/or can be used to modify rheological or other process properties.

The main goals of particle-packing research have been to determine how systems of particles pack, to develop algorithms for calculating packing densities and porosities for any distribution of particles (spherical or nonspherical, rough or smooth, wet or dry), and to determine how packing and its properties affect the variety of industrial operations that utilize particulate/fluid systems.

For many years, it was believed that when two isovalent semiconductors are mixed, they will phase-separate (like oil and water) at low temperature, they will form a solid solution (like gin and tonic) at high temperatures, but they will never produce ordered atomic arrangements. This view was based on the analysis of the solid-liquid equilibria at high temperatures and on empirical observation of phase separation at low temperatures. These observations were further rationalized and legitimized by applying the classic (Hildebrand) solution models, which predicted just this type of behavior. These models showed that the observed behavior of the AxB1 − x alloys implied a positive excess enthalpy ΔH(x) = E(x) − xE(A) − (1 − x)E(B) (where E is the total energy) and that this positiveness (“repulsive A-B interactions”) resulted from the strain energy attendant upon packing two solids with dissimilar lattice constants. The larger the lattice mismatch, the more difficult it was to form the alloy. Common to these approaches (“regular solution theory,” “quasiregular solution theory,” “delta lattice-parameter model,” etc.) was the assumption that the enthalpy ΔH(x) of an alloy depends on its global composition x but not on the microscopic arrangement of atoms (e.g., ordered versus disordered). Thus, ordered and disordered configurations at the same compo sition x were tacitly assumed to have the same excess enthalpy ΔH(x). Clearly the option for ordering was eliminated at the outset. While these theories served to produce very useful depictions of the immiscibility of many semiconductor alloys (and continue to guide strategies of crystal growth), they also cemented the paradigm that semiconductor alloys don't order, they just phase-separate. This was true, at the time.