Dr. D. Allan Bromley, President Bush's science adviser, is firmly committed to the administration's gaining a better understanding of materials science and engineering and then going forth with a cohesive program from there. The MRS BULLETIN interviewed him on July 17 in his office in the Old Executive Office Building. Following is an excerpt of that interview.

BULLETIN: The Department of Commerce recently issued a report that identified advanced materials as by far the leading emerging technology. Is special consideration being previded to spur this critical technology sector?

BROMLEY: First of ail, we are preparing so that next year we will work with the Office of Management and Budget to develop a coherent federal program that spans ail the agencies together to get a coherent look for the first time at federal activity in materials science and engineering. I have long thought that was an orphan area, not only in government but in many of the universities, because it doesn't fit into department or agency boundaries.

At the same time, the President's Council of Advisers on Science and Technology, chaired by Ralph Gomory, the former chief scientist at IBM and now president of the Sloan Foundation, will be focusing on advanced materials and manufacturing as two very important areas that bear on economie competitiveness.

Third, the new National Critical Materials Council will be focusing specifically on advanced materials and will be comparing the program planned for research in that area next fall.

Some of the most important properties of materials in high-technology applications are strongly influenced or even controlled by the presence of solid interfaces. For example, interfaces are the critical element in fiber-reinforced structural ceramics with mechanical properties not even imagined a decade or two ago. The entire electronics industry is based on the fascinating electrical properties of semiconductor interfaces, with ceramic-semiconductor, metal-semiconductor and metal-ceramic interfaces playing critical roles as well. Other examples are surface modification techniques, designed to enhance the corrosion resistance of materials in hostile environments or tailored for tribological or catalytic applications. In contrast to their enormous technological importance, our basic understanding of even the simplest interfaces, such as free surfaces and grain boundaries, is rudimentary at best. It is increasingly recognized, however, that truly significant technological advances can be achieved by a better understanding and control of interfacial processes.

To draw attention to the tremendous opportunites that lie ahead in this lively area of materials research, some recent promising developments in the atomic-level understanding of solid interfaces are highlighted in this issue and in the October issue of the MRS BULLETIN. The theme common to ail the articles in the September and October issues lies in the atomic-level insights that have been gained. While this issue of the BULLETIN focuses on structure, chemistry and some electronic properties of crystalline interfaces, the October issue will be devoted to their mechanical and high-temperature behavior.

The importance of interfaces in determining the physical properties of technologically important materials — addressed in the Guest Editors' introduction to the articles in this issue — has stimulated theoretical efforts to determine from first principles a detailed understanding of their chemical, electronic, and mechanical properties. Fortunately, this has now been made possible as a result of the dramatic advances in condensed matter theory made in the last decade, driven in large part by new and sophisticated experiments on high-purity materials that have been well and carefully characterized. Particularly in electronic structure, these advances may be attributable directly to the close collaboration of theoretical and experimental researchers. Indeed, the new found ability to apply fundamental theoretical concepts to real materials (rather than to simple model systems) made possible by using the continued rapid development of computer power, has served to fill the increasingly urgent demand of experimentalists for theoretical interpretation of their data. Also, in some cases, these computational efforts can be used to provide data that would be currently impossible or impractical to obtain experimentally. This development has been an essential element in the phenomenal growth in this area of materials science.

More specifically, the advent of accurate self-consistent (local) spin density functional (LSDF) calculations for surfaces, interfaces, and multilayers means that theory is no longer limited to simple, parameter-dependent models. These complex systems are of growing interest because the reduced symmetry, lower coordination number, and availability and role of highly localized surface and interface states offers the possibility of inducing new and exotic phenomena and promotes the possibility of new device applications.

The ability to study, and to some extent control, the magnetic properties of surfaces and interfaces is an extremely interesting development in the field of magnetism. Along with experimental advances in techniques to accurately prepare magnetic structures by such methods as sputtering and molecular beam epitaxy (MBE) have been theoretical and computational advances in the ability to calculate properties of realistic model systems. At the same time, increasingly sophisticated characterization techniques have become available (e.g., intense synchrotron sources, pulsed sources for magnetic neutron scattering, polarized electron microscopy, scanning tunneling microscopy, etc.), which enable sample characterization at a level not before possible.

In recent years, the study of magnetism near surfaces and interfaces has been driven by the observation of significant differences from bulk behavior.1 An understanding of these phenomena is fundamentally important to the field of magnetism and provides the potential for technological applications.2 Although the real situation is more complicated, for the purposes of the discussion here, it is convenient to think about the causes of the observed physical effects at interfaces and in superlattices as falling into two categories: “dimensionality effects,” due to the reduced coordination of the magnetic atoms; and “substrate effects,” due to interactions between the differing materials at an interface. An ultrathin film on an insulator, or the free surface of a bulk crystal, are examples of the first category. However, the magnetic atoms at an interface between two different materials are affected both by the reduced number of similar neighboring atoms (i.e., the dimensionality) and by the crystal and electronic structure of the neighboring layer (i.e., the substrate).

An interface is an internal surface, the boundary between two media which may be crystalline, amorphous solid, or liquid. Its close similarity with a surface, a solid-vacuum boundary, suggests that many of the powerful techniques available for studying surfaces might be applied to the interface structure problem. The extent to which this is possible is the subject of this article.

The techniques to be discussed in this article include low energy electron diffraction (LEED), medium energy ion scattering (MEIS), x-ray diffraction, and x-ray reflectivity. (The most widely used method, transmission electron microscopy (TEM), is the subject of a separate article in this issue of the MRS BULLETIN.) To summarize what we will find, surface methods were developed to be nonpenetrating in order to have surface sensitivity. This works against us in the interface situation by requiring the use of extremely thin samples, at least on one side of the interface. This means special handling of samples in some cases and raises the possibility of artifac-tual results. Of the three methods, x-ray diffraction is the most penetrating and least surface sensitive; it probably has the greatest potential for widespread use in interface science.

This article defines structure as “atomic structure” for this purpose: we are interested in the coordinates of atoms at the interface and their relation to bulk structures on one or both sides. For this reason, we will consider only interfaces that are crystalline on at least one side. Since crystals are by far our strongest structural reference point, much less can be said about other interfaces. We will also consider the morphology of an interface, defined as the boundary of the crystal(s) that demarcates the interface, also at the atomic level. This is most apparent in the form of interface roughness. The roles of strain and misfit dislocations in interface formation, also studied by these techniques, are outside the scope of this article.

The investigation of structure-property correlations is a rather complex endeavor not only because interfacial Systems are intrinsically inhomogeneous, with chemical composition and physical properties differing from the surrounding bulk material, but also since three different aspects of the geometrical structure are involved — namely the macroscopic, microscopic, and atomic structures. As outlined in the Guest Editors' introduction, in addition to the choice of the materials which form the interface, five macroscopic and three microscopic degrees of freedom (DOFs) are needed to characterize a single bicrystalline interface. The importance of the atomic structure at the interface as well as the local interfacial chemistry, extrinsic (i.e., impurity segregation) or intrinsic (for example, via interfacial reactions or space-charge phenomena), greatly add to the task's complexity.

Grain boundaries (GBs) in pure metals represent ideal model Systems for investigating the strictly geometrical aspects of structure-property correlations for the following three reasons. First, the complexity due to the myriad of possible choices of materials combinations forming the interface is avoided, enabling a focus on the different roles of the three distinct geometrical aspects of the structure. Second, because GBs are bulk interfaces, dimensional interface parameters (such as the modulation wavelength in strained-layer superlattices, or the thickness of epitaxial layers) do not enter into the problem. Finally, the GB energy is thought to play a central role in various GB properties, such as impurity segregation, GB mobility and fracture, GB diffusion and cavitation, to name a few. A better understanding of the correlation between the structure and energy of GBs, therefore, promises to offer insights into more complex structure-property correlations, as well.

In an alloy, the equilibrium composition in the vicinity of an inhomogeneity, such as a surface or a grain boundary, will generally differ from the composition in the bulk. This was first recognized by Gibbs. This change in local composition can potentially affect the structure and mechanical properties of the boundary. As an example, experiments by Sass and co-workers have shown that the Burgers vector of the dislocations present in Fe-Au twist boundaries depends on the Au concentration at the boundary. This demonstrates that the structure does, in fact, depend on the local composition of the alloy at the interface.

This article presents computer simulation studies of equilibrium segregation in binary alloys at grain boundaries, and two experimental studies of interfaces using atom-probe field-ion-microscopy (APFIM). These studies address two fundamental questions: What is the composition of the boundary region as a function of the bulk composition and temperature? What is the spatial distribution of the two species in the boundary? Note, that the comparison of experiment and calculations, where possible, is crucial. Such comparisons can provide guidance in the experimental analysis, and the comparison also provides a stringent test of the reliability of the simulation methods.

This article first describes simulation and atom-probe field-ion microscope techniques and next discusses the segregation in Ni-Cu alloys to isolated edge dislocations. This is followed by a study of segregation at twist boundaries in Pt-Au alloys. Then, the segregation at an asymmetric mixed tilt-twist boundary in a Pt-Ni alloy is computed, and the results are compared with experimental atom-probe held-ion-microscopy results. Finally, the results of an atom-probe study of two-dimensional Re segregation at a twist boundary with a small tilt component in a W-Re alloy are presented.

Any finite system is delimited by interfaces. In this trivial sense interfaces are ubiquitous. However, modern epitaxial techniques seek to modify the properties of materials by the creation of interfaces. “Band gap engineering,” the attempt to tailor the electronic properties of semiconductors by interleaving many dissimilar layers, is an example of this approach. Interfaces between lattice-matched, isostructural, crystalline systems, (differing only in composition) are technologically the most advanced and thus most widely used. These “chemical” interfaces are formed by the introduction of dopant impurities, or by the stacking of dissimilar materials. Chemical interfaces formed by the epitaxial growth of dissimilar materials are the subject of this article.

A fundamental tenet of modern epitaxy is the tailoring of properties through control of structure. We will therefore outline how the “structure” of a chemical interface may be defined and experimentally determined. Determining the interfacial structure is a major experimental challenge, inextricably bound with our understanding of how structure affects other properties. The article will thus briefly discuss this vital link. We will conclude with an outline of the way chemical interfaces, in reality systems far from equilibrium, can relax, and how their relaxation mechanisms shed light on the fundamental properties of solids. Because the GaAs/AlGaAs system is the most technologically advanced, and to be concrete, we will illustrate the discussion by reference primarily to this system. Many of the concepts and experimental results, however, are more generally valid.