In this issue of the MRS Bulletin we present five papers that involve point defect phenomena in a wide variety of materials—metals, conducting ceramic oxides, semiconductors, amorphous alloys, and high Tc superconducting oxides. The unifying theme of this issue is point defects—zero-dimensional defects. Even for the high Tc oxides, where planar defects are discussed, it is the ordering of oxygen/vacancy chains that ultimately gives rise to twins in the famous YBa2Cu3O7.δ (1:2:3) oxide superconductor.

Hillard Huntington, professor emeritus of physics at Rensselaer Polytechnic Institute, is an early and important pioneer in the study of point defects in metals. A theorist, he has also performed many experiments over the years; for example, he performed key early experiments on electromigration effects. Huntington's article presents a historical review of the research on vacancies and self-interstitial atoms in metals during the period that stretches from the mid-1930s to the mid-1960s. He played a crucial role in this field as a result of his seminal theoretical calculations, with Fred Seitz in 1942, on the enthalpies of formation and migration of vacancies or self-interstitial atoms in pure copper. Huntington's and Seitz's calculations indicated that diffusion occurs predominantly by a monovacancy mechanism since the enthalpy of formation of a self-interstitial atom, the [100] split form, also called the dumbbell form, is too large to be compatible with the activation enthalpy for self-diffusion in copper. It is now well established that the latter is given by the sum of the enthalpy of formation and the enthalpy of migration of a monovacancy in many face-centered-cubic (fcc), body-centered-cubic (bec), and hexagonal-close-packed (hcp) metals.

This article aims to review in rather cursory fashion the ways the concepts of lattice defects have contributed at an early stage to our understanding of solid-state kinetics in metals and insulators.

The importance of point defects in solid-state kinetics was just beginning to be widely recognized in the late 1930s. The crucial experiments of Kirkendall and others were still to come. Many of the leaders in the field of metallurgy believed almost tacitly that diffusion in substitutional alloys occurred by direct interchange or perhaps a ring mechanism.

For the ionic salts, however, basic calculations were further advanced, and it was possible to figure quite confidently the role of Schottky and Frenkel defects in facilitating atom movements. In their seminal paper, Mott and Littleton made specific calculations as to the energies involved in diffusion by the various mechanisms and hence to the relative importance of these mechanisms in the kinetics of these materials. They began by taking over the Born-Mayer short-range formula for ionic repulsion. Next they treated in detail the polarization response of the salt to an extra charge in the lattice, whether interstitial or vacancy. This polarization included the individual polarizabilities of the ions and, for the static case, the ion displacements. Application of this analysis gave good quantitative results for the activation energies to be expected for diffusion and ionic conductivity. For the alkali halides it was made clear that the Schottky defect would dominate and that Frenkel defects would be few.

The topic that I am attempting to cover easily requires a whole book to do it justice. To deal with it in a short article, I will focus on a few basic principles that dominate the behavior of defects in oxides and then present some examples taken from well-studied and relatively well-understood materials.

The study of defects in oxides differs from the study of defects in metals and alloys or in semiconductors in several important ways. First, oxides generally have very high melting points so that intrinsic point defect concentrations at temperatures below ~1500°C are usually negligibly small. Second, an oxide does not always maintain the composition at which it was fabricated, as alloys do, but can exchange oxygen with the ambient atmosphere, according to the ambient partial pressure of oxygen, P(O2). Thus, the stoichiometry, and accordingly the defect concentration, can be controlled by annealing in a specific P(O2) atmosphere. Third, and most important, oxides have a high degree of ionicity so that most point defects possess an effective charge. The consequences of this fact are far reaching; some of them are as follows:

1. In a binary oxide, e.g. a metal oxide (MO), there are two sublattices. The diffusion of cations and of anions proceeds each on its own sublattice, involving different point defects. Accordingly, cation and anion diffusion rates can be very different and, in fact, usually are.

Semiconductor devices generally contain n- and p-doped regions. Doping is accomplished by incorporating certain impurity atoms that are substitutionally dissolved on lattice sites of the semiconductor crystal. In defect terminology, dopant atoms constitute extrinsic point defects. In this sense, the whole semiconductor industry is based on controlled introduction of specific point defects. This article addresses intrinsic point defects, ones that come from the native crystal. These defects govern the diffusion processes of dopants in semiconductors. Diffusion is the most basic process associated with the introduction of dopants into semiconductors. Since silicon and gallium arsenide are the most widely used semiconductors for microelectronic and optoelectronic device applications, this article will concentrate on these two materials and comment only briefly on other semiconductors.

A main technological driving force for dealing with intrinsic point defects stems from the necessity to simulate dopant diffusion processes accurately. Intrinsic point defects also play a role in critical integrated circuit fabrication processes such as ion-implantation or surface oxidation. In these processes, as well as during crystal growth, intrinsic point defects may agglomerate and negatively impact the performance of electronic or photovoltaic devices. If properly controlled, point defects and their agglomerates may also be used to accomplish positive goals such as enhancing device performance or processing yield.

Since 1959 when P. Duwez showed at Caltech that Au-Si alloys could be quenched into a glassy state, there has been much interest in elucidating the nature of these amorphous materials. Certainly part of the motivation for studying amorphous alloys derives from their potential technological value: they are characterized by high hardness, corrosion and oxidation resistance, and high magnetic permeabilites and electrical resistivities but an equally strong motivation is simply that they are interesting materials. Scientific curiosity is stimulated by such fundamental questions as: What is their structure? Can defects be defined within this structure? What are the possible mechanisms of atomic transport? In addition, amorphous alloys provide a paradigm of a dense metastable structure, and how atomic transport can take place in such systems without transforming to more stable configurations is not well understood. Yet, as materials formed by nonequilibrium processing, e.g., nanocrystals, superlattices, rapidly solidified and ion-beam-modified materials, etc., find their way into technology, this question becomes increasingly germane. Ironically, much of the renewed interest in diffusion in amorphous alloys has been stimulated by the discovery, also at Caltech, of the solid-state amorphizing reaction, where multilayers of crystalline metal films transform to amorphous alloys by solid-state diffusion, rather than vice versa.

Characterization of microstructural defects in high critical temperature oxide superconductors has been very valuable in gaining an understanding of their phase transformations and insight into the relationship between defects and some superconducting properties such as critical current densities Jc. For this purpose, transmission electron microscopy (TEM) techniques have been proved powerful. In this article we will briefly summarize typical defects in the perovskite-based layered oxides, i.e., [La(1−x)(Ba,Sr)x]2CuO4, YBa2Cu3O7−δ, and (Bi,Pb)2Sr2Can−1Cun−O4+2n+δ. Then we will discuss the nature of defects in YBa2Cu3O7−δ in detail since this oxide has been most extensively studied. Among the many varieties of microstructural defects in YBa2Cu3O7−δ, we will emphasize only those we consider most important. An account of the field before 1990 is given in an excellent review by Beyers and Shaw and a more recent review of the structure of the grain boundaries in YBa2Cu3O7−δ by Babcock.

Since the types and the nature of the defects depend greatly on the crystal structure of the materials, the basic structures of the three oxides are shown in Figures la to 1c: (a) [La1−x(Sr,Ba)x]2CuO4, (2:1:4), (b) YBa2Cu3O7 (1:2:3:), and (c) Bi2Sr2CaCu2O8 (2:2:1:2). (The structure of Tl cuprates are very similar to the Bicuprate and will not be discussed here.)

The first International Workshop on the Science and Technology of Thin Films for the 21st Century, held this past summer at Northwestern University, assembled the world's leading experts to assess the trends and opportunities in one of the most fruitful areas of materials research. A complex field, thin film science touches virtually all areas of technology and has had a major economic impact as well. Silicon integrated circuits, for example, form the basis of the computing and telecommunications industries, both noted economic cornerstones.

The diverse, highly qualified group of 150 invited speakers and registered participants came to several conclusions:

∎ The scientific, technological, and commercial opportunities in this field have never been more exciting. The panel chairs have articulated their findings, and they are presented here in separate reports.

∎ Although the thin film field is huge, complex, and interdisciplinary, there are common scientific linkages or driving forces in how the field has developed and is practiced. The discovery of new deposition and fabrication techniques has historically led to new thin film materials. Analysis of the basis molecules of these materials has led to the discovery of new physical and chemical phenomena. And the practitioners of thin film science have traditionally had a lively, realistic interest in applying their work.

∎ The funding situation and lack of a coherent national policy in the United States were causes for concern among U.S. workshop participants.