Extended X-Ray Absorption Fine Structure (EXAFS) is a technique capable of probing local structure in condensed- matter systems, determining distances, coordination numbers, and species of atoms near an x-ray excited atom. Further, this information is obtained separately for each constituent atomic species. EXAFS can be applied equally well to crystals, alloys, or even systems having no long-range order such as liquids, gases, and amorphous materials.

Here we review EXAFS spectroscopy and its use in studying semiconductors. First, we show EXAFS in the study of impurities to determine the local environment of Fe implanted in Si. Second, we show how EXAFS can be used to determine local structural distortion in diluted magnetic semiconductors. We next demonstrate that EXAFS can determine the nature of the phase transition in ferroelectric Pb1-x Gex Te, where the transition is revealed to have an order-disorder character in contrast to GeTe where the transition is purely displacive. The theme of these examples is that EXAFS can determine the relative positions of atoms in condensed matter and that the short-range order as determinedn by EXAFS often differs from the long-range order or theoretical expectations.

In the EXAFS technique, one measures the x-ray absorption coefficient as a function of x-ray energy. The absorption spectrum shows a number of x-ray edges corresponding to the threshold of excitation of core electrons from the sample's various atomic species. At energies above each of these edges, oscillations in the absorption coefficient are due to backscattering of the photoelectron from the neighboring atoms.

were very exciting but speculative, being technologically feasible only if new classes of materials could be developed The subject of materials in nonlinear optics (NLO) encompasses a wide range of important topics. Today the line between materials and NLO processes has become fuzzy, particularly for newer NLO processes (e.g. photorefrac-tion, and optical bistability, logic and computing). For more established NLO processes (e.g., harmonic generation, parametric processes, linear electro-optic effect, etc.) the subjects are well studied and the importance of various materials properties on the NLO process are known, though these properties are not necessarily predictable, controllable, or optimized in current materials.

A decade ago, having been introduced to NLO phenomena through postdoctoral research, I had an opportunity to define and pursue an NLO research program at Xerox's Webster Research Center. The question was posed: “Are new materials needed for NLO applications?” The answer must start with another question: “Which NLO process … with light of what wavelength, pulse duration, and power… and for what purpose?”

It was clear that important limitations to many of the novel things one might do with optics were: insufficient nonlin-earity magnitude, inability to fabricate reliable device structures, occurrence of deleterious optical properties, and restrictions due to other material properties. The newer NLO phenomena. Use of older NLO processes in new technological applications seemed a more down-to-earth quest.

My introduction in the November MRS BULLETIN to this two-part series on deposition processes discussed the extensive use of thin films in science and technology. That it takes two issues and nine articles to cover this topic — and by no means exhaustively — is testimony to the manifold ways thin films are prepared.

If all deposition processes resulted in the same product, then such extensive coverage would be redundant and unnecessary. Thin films, however, cover a virtual infinity of free energy states — and related crystal structures, micro-structures, defects, defect densities, impurities, compositions, composition modulations, etc. — that are sensitive to the particular deposition process and its conditions. It is this richness of choice that makes thin film science and technology both exciting and, at times, frustrating.

Along with the freedom to extensively vary thin film characteristics, resulting properties and applications comes the difficulty in understanding preparation-characterization-property relations in enough detail to control and reproduce deposition processes.

The November articles covered molecular dynamics computer modeling of nucleation and growth processes, molecular beam epitaxy, organometallic vapor phase epitaxy, and chemical vapor deposition. This month's articles continue the sequence of ways to deposit films, the general direction being toward lower substrate temperatures. Plasmas, which offer both increased flexibility and complexity, are primarily considered. The last article covers thermal plasmas, not to control the vapor deposition but to melt powders which result in a multiple splat-quenched array of particles that form coatings important to industry.

Chemical vapor deposition (CVD) is one of the few deposition processes in which the deposited phase is produced in situ via chemical reaction(s). Thus the vapor source for CVD can consist of high vapor pressure species at moderate temperatures and yet deposit very high-melting phases. For example, pure TiB2, which melts at 3225°C, can be produced at 900°C from TiCl4, BC13, and H2.

Chemical vapor deposition and its variants such as low pressure CVD (LPCVD), plasma-assisted CVD (PACVD), and laser CVD (LCVD) have been active areas of research for many years. Recent review articles have contained extensive lists of the phases deposited by CVD, which include most of the metals and many carbides, nitrides, borides, silicides, and sulfides. The techniques have found increased acceptance as commercial methods for the fabrication of films and coatings which are fundamental to the semiconductor device and the high-performance tool bit industries. They have been used to prepare multiphase-multilayer coatings, stand-alone bodies, and fiber-reinforced composites. As the demand increases for more complex and sophisticated materials, it is expected that CVD will play a still larger role.

In CVD a solid material is deposited from gaseous precursors onto a substrate. The substrate is typically heated to promote the deposition reaction and/or provide sufficient mobility of the adatoms to form the desired structure. Chemical vapor deposition was performed for the first time when early humans inadvertently coated cooking utensils with soot from the campfire. In this CVD process, hydrocarbons generated by the heated wood pyrolyzed on the utensil surface, depositing carbon.

The most intriguing aspect of the new “high Tc” superconductors is that we still have no consensus about the nature of the basic mechanism for superconductivity in these materials, nor about why Tc is so much larger than in conventional superconductors. As is now well known, these materials include compounds of the type La2-xSrxCu2O4-y (where Sr can also be replaced by Ba) in which the maximum Tc attained is between 35 K and 40 K, and of the type YBa2Cu3O7-δ (where Y can also be replaced by most rare-earth atoms) in which the maximum Tc attained is about 98 K. Very recently, new compounds such as Bi2Sr2CaCu2O8+y have been discovered where Tc's of up to 120 K have been observed, but the physical properties of these at the time of writing are much less well known. The interesting thing about all these compounds is that they all result from doping of cupric oxide insulators, and they all contain planes of Cu-O atoms.

Perhaps not entirely by coincidence, high Tc superconductivity thus occurs in a class of materials, namely transition metal oxides, whose electronic ground states are currently least well understood in terms of conventional band theory. It is now generally accepted that the d-shells of transition metals ions (including Cu) have associated with them a fairly large Coulomb repulsion that makes it energetically unfavorable for two holes to be on the same d-shell simultaneously. For Cu, the energy U is estimated to be between 4 and 8 eV. This implies that Cu++ (d9 configuration) is favored, while Cu+++ (d8 configuration) is not. This is in accordance with spectroscopic measurements of these compounds.

From a commercial viewpoint, gallium arsenide (GaAs) is currently the leading member of the III-V compound family. Oriented substrates, cut and polished from single-crystal boules, form the materials foundation for a rapidly maturing technology of high speed and high frequency electronic devices and circuits. The initial thrust of GaAs applications was in high powered lasers and light-emitting diodes (LEDs) fabricated on n-type (Si-doped) GaAs wafers grown by the horizontal Bridgman technique. One of the important benefits of using GaAs is the high electron mobility compared to Si. This property has driven the development of low noise and power field-effect transistors (FETs) for microwave applications. The semi-insulating substrate requirement (>107 Ω-cm) was initially met by chromium doping. Currently, the interest is focused on MMIC (microwave monolithic integrated circuits), MIMIC (millimeter microwave ICs), analog ICs for lightwave transmitters and receivers, and digital ICs. The digital circuits have been realized with ion-implanted FETs, selectively doped heterostructure transistors (SDHTs), and heterostructure bipolar transistors (HBTs). Presently, most of the semi-insulating (SI) material processed by the industry is nominally undoped, and grown by the liquid encapsulated Czochralski (LEC) technique. The SI behavior is attained via a delicate balance of deep EL2 donors and carbon acceptors, avoiding chromium in order to eliminate the anomalous out-diffusion and type-conversion associated with this dopant.

GaAs wafers up to 4 inches in diameter, with electrical properties homogenized by whole ingot annealing, are currently available from U.S. domestic and overseas suppliers. However, the overall quality is compromised by the large dislocation densities, varying 104 – 105/cm2.

There are now theories which predict the effect of texture on elastic properties and hence on the velocities of bulk, surface, and guided ultrasonic waves. Furthermore, there is a relation between texture and formability. These relationships can be potentially exploited to make ultrasonic measurements of formability, and partial pole figures can be made with ultrasonic techniques.

This article presents results of various ultrasonic measurements made to characterize the formability of rolled steel and aluminum alloy sheet. A good correlation generally exists between ultrasonic velocity measurements and formability. Furthermore, there is a good possibility of on-line formability measurement using a noncontacting ultrasonic transducer.

The influence of texture on material properties has been long recognized, and several excellent texts have been wholly or partly devoted to the subject. Texture classically has been measured with either x-rays or neutron diffraction. Lately, there has been interest in characterizing texture with ultrasound (high-frequency sound) as well.

Ultrasound has several potential advantages over x-ray and neutron diffraction techniques. Ultrasonic systems are nondestructive, can rapidly scan large volumes of material, require no radiation sources, and can be devised to measure either surface or bulk textures. There is a distinct possibility that ultrasonic texture monitoring can be implemented in real-time in a production environment such as a rolling mill.

Physical vapor deposition (PVD) technology consists of the basic techniques of evaporation deposition and sputter deposition. PVD is used to deposit films and coatings or self-supported shapes such as sheet, foil, tubing, etc. The thickness of the deposits can vary from angstroms to millimeters.

Applications range widely, from decorative to utilitarian and over significant segments of the engineering, chemical, nuclear, microelectronics, and related industries. They have been increasing rapidly because modern high technology demands multiple and often conflicting sets of properties from engineering materials, e.g., combination of two or more of the following: high temperature strength, impact strength, specific optical, electrical or magnetic properties, wear resistance, fabricability into complex shapes, biocompatibility, cost, etc. A single or monolithic material cannot meet such demands. The solution is a composite material, a core material and a coating each having the requisite properties to meet the specifications.

This article will review evaporation-based deposition technologies, theory and mechanisms, processes, deposition of various types of materials, and also the evolution of the microstructure and its relationship to the properties of the deposits.

The first evaporated thin films were probably prepared by Faraday in 1857 when he exploded metal wires in a vacuum. The deposition of thin metal films in vacuum by Joule heating was discovered in 1887 by Nahrwold and was used by Kundt in 1888 to measure refractive indices of such films. In the ensuing period, the work was primarily of academic interest, concerned with optical phenomena associated with thin layer of metals, research into kinetics and diffusion of gases, and gas-metal reactions.

From the time of their conception, holography and holograms have evolved as a metaphor for human memory. Holograms can be made so that the information they contain is distributed throughout the holographic medium—destroy part of the hologram and the stored information remains wholly intact, except for a loss of detail. In this property holograms evidently have something in common with human memory, which is to some extent resilient against physical damage to the brain. There is much more to the metaphor than simply that information is stored in a distributed manner.

Research in the optics community is now looking to holography, in particular dynamic holography, not only for information storage, but for information processing as well. The ideas are based upon neural network models. Neural networks are models for processing that are inspired by the apparent architecture of the brain. This is a processing paradigm that is new to optics. From within this network paradigm we look to build machines that can store and recall information associatively, play back a chain of recorded events, undergo learning and possibly forgetting, make decisions, adapt to a particular environment, and self-organize to evolve some desirable behavior. We hope that neural network models will give rise to optical machines for memory, speech processing, visual processing, language acquisition, motor control, and so on.

The 1988 Spring Meeting of the Materials Research Society will be held at Bally's in Reno, Nevada, with events spanning April 4-10. Program Chairs David Clark, Clif Draper, and Chain T. Liu have planned the most diversified topical symposia coverage to date. In addition to a thorough examination of the popular and “hot news” areas, the meeting will also offer a selection of intriguing specialty topics never before offered at an MRS meeting. This year's Spring meeting will feature 16 topical symposia, a program of 23 short courses, and an equipment show. Highlights of the symposia are described below. The names of the short courses and equipment exhibitors are listed elsewhere in this issue. For details see the 1988 MRS Spring Meeting Preliminary Program mailed to all MRS members.

During the Plenary and Student Awards session, Raymond D. Tuminaro of AT&T Bell Laboratories will present the Plenary Address on “Materials Aspects of the SL Undersea Optical Cable Design.” Tuminaro will focus on the fiber and cable materials, review their vulnerabilities to degradation mechanisms, and explore methods currently being used to assure acceptable performance levels for the projected 25-year service life of these systems.

A special feature is being planned for the 1988 MRS Spring Meeting. A major photomicrography exhibition—Microscapes: The Hidden Art of High Technology—will focus on the seldom-seen world of advanced developments in microelectronics and lightwave communications.

Nondestructive evaluation has historically been used almost exclusively for detecting macroscopic defects after materials have been made or put into service. However, the role of NDE is now changing to include in-process control in an effort to increase yields and performance of materials. This includes materials stability during transport, storage, and fabrication, as well as degradation behavior during in-service life. The NDE community has implied that applying NDE in these ways is crucial to solving the economic problems of U.S. manufacturing industries. So called “intelligent manufacturing” is impossible without integrating modern NDE techniques into the production of today's advanced materials.

The Johns Hopkins University Center for Nondestructive Evaluation (CNDE) was established in 1984 as an interdisciplinary center for research and instruction, drawing on the resources and talent of the School of Engineering, Applied Physics Laboratory, School of Medicine, and School of Arts and Sciences. Currently 31 faculty or senior staff work with the Center. The techniques being developed are aimed at reliable in-process control of materials and processes. Another important purpose is the education of talented students who will enter the NDE field; 150 students are associated with the Center, a third of whom are graduate students. The CNDE also provides both research collaborators and industrial sponsors with access to all NDE research and instruction at Johns Hopkins University. Twenty-six research institutions have formal cooperative research programs and 19 organizations are corporate sponsors (see Figure 1).

Epitaxial growth of magnetic metals actually pre-dates epitaxial growth of semiconductors. The earliest work (1936), which reported single-crystal Fe growth on NaCl, exploited the fact that single-crystal substrates of NaCl were easy to obtain, readily cleaved, and could be cleaned in a vacuum by heating. The good lattice match between the two systems and lack of interfacial disruption upon growth permitted excellent quality single-crystal films of Fe to be grown in relatively modest vacuum. Improved vacuum techniques broadened the range of materials which could be studied, with respect to both the films and substrates. The most recent ultrahigh vacuum (UHV) techniques developed for molecular beam epitaxial growth of semiconductors, including the large array of electron-based analytical tools, have also been exploited to grow and characterize magnetic metal films.

Some requirements for these magnetic materials, such as the need for higher temperature effusion sources to generate useful fluxes of Fe, Co and Ni, and high vacuum in the presence of e-beam sources in order to avoid oxidation of the rare earths, served to stimulate new technical developments for the field in general. It is now possible to control growth to a fraction of a monolayer (ML) and even to know when one ML coverage is complete and another is beginning. The techniques have become so successful that a whole new subfield of magnetism has emerged — surface and interfacial magnetism — in which the work would be largely meaningless if one could not grow precisely characterized epitaxial magnetic metal films. Recent work has shown that it is now possible to grow single-crystal magnetic metal films on a wide variety of substrates, including insulators (oxides and salts), semiconductors, and metals.