bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis of Silver Nanoparticles Using a Microfluidic Impinging Jet Reactor
Authors: Baber Razwan , Mazzei Luca , Thanh Nguyen T. K. , Gavriilidis Asterios
Published: August 2016

Synthesis of silver nanoparticles (NPs) in an impinging jet reactor (IJR) was investigated due to its unique properties of efficient mixing and lack of channel walls which avoid fouling. Silver NPs were formed at room temperature by reducing silver nitrate with sodium borohydride in the presence of sodium hydroxide. Two types of ligand were used to stabilize the NPs, trisodium citrate, and polyvinyl alcohol (PVA). Weber number, the ratio between inertial forces and surface tension forces, is used to characterize flow in impinging jets. Flow regimes were investigated forWeber numbers in the range of 13–176. A liquid sheet/chain regime was identified at lowerWeber numbers (<90), and an unstable rim structure was identified at higherWeber numbers (>90). Mixing time was found to be in the range 12–7ms, using theVillermaux-Dushman reaction system and interaction by exchange with the mean mixing (IEM) model. Fastest mixing occurred at Weber number ca. 90. Using trisodium citrate as a ligand, NP size decreased from 7.9 ± 5.8 nm to 3.4 ± 1.4 nm when flow rate was increased from 32 mL/min to 72 mL/min using 0.5 mm jets, and from 6.4 ± 3.4 nm to 5.1 ± 4.6 nm when flow rate was increased from 20 mL/min to 32 mL/min using 0.25 mm jets. Using PVA as a ligand, NP size decreased from 5.4 ± 1.6 nm to 4.2 ± 1.1 nm using 0.5 mm jets and stayed relatively constant between 4.3 ± 1 nm and 4.7 ± 1.3 nm using 0.25 mm jets. In general, the size of the NPs decreased when mixing was faster.
Molecular differentiation of Shigella spp. from enteroinvasive E. coli
Authors: Løbersli I. , Wester A. L. , Kristiansen Å. , Brandal L. T.
Published: August 2016

A real-time polymerase chain reaction (PCR) assay, amplifying the genes encoding lactose permease (lacY) and invasion plasmid antigen H (ipaH), was run on 121 isolates phenotypically classified as Shigella spp., enteroinvasive Escherichia coli (EIEC), or EIEC O nontypable (ONT). The results were compared with data from a generic E. coli multiple-locus variable-number of tandem repeat analysis (MLVA) and a Shigella MLVA.

The real-time PCR verified all Shigella spp. (n = 53) as Shigella (lacY negative) and all EIEC O121 (n = 15) and EIEC O124 (n = 2) as EIEC (lacY positive). However, the real-time PCR typed EIEC O164 as either EIEC (n = 2) or Shigella (n = 2) and, thus, was not suited for classifying this group of isolates. Interestingly, the majority (42/47, 89.4%) of the EIEC ONT were classified as Shigella (lacY negative) by the real-time PCR, and in nearly all cases, (92.9%, 39/42) data from both MLVA assays supported these findings. Overall, in 94.7% (114/121) of the isolates, the results from the real-time PCR were substantiated by the results from the MLVA assays.

In conclusion, the real-time PCR assay was fast and accurate in differentiating Shigella spp. from EIEC, with the exception of the EIEC O164 group. This molecular assay was particularly pragmatic for the challenging EIEC ONT group.
Light Emission From Er-Doped Si: Materials Properties, Mechanisms, and Device Performance
Authors: Coffa Salvatore , Franzo Giorgia , Priolo Francesco
Published: August 2016

The achievement of efficient room-temperature light emission from crystalline Si is a crucial step toward the achievement of fully Si-based optoelec-tronics. However Si, the leading semiconductor in microelectronic applications, is unable to perform as well in the optical arena. In fact due to its indirect bandgap, Si does not exhibit efficient light emission and has been considered unsuitable for optoelectronic applications. Several efforts have been dedicated to overcoming this limitation. Among them, luminescence through the incorporation of rare-earth impurities has been considered In particular, erbium doping has been demonstrated as a valid approach toward achievement of efficient light emission from Si.1−43 Erbium is a rare-earth ion that, in its 3+ state, can emit photons at 1.54 μm because of an intra-4f shell transition between the first excited state (4I13/2) and the ground state (4I15/2). This emission is particularly attractive because its wavelength falls inside a window of maximum transmission for the silica optical fibers. When Er ions are inserted within a Si matrix, the excitation (4I15/2 → 4I13/2) can be achieved through the carriers provided by the host, whereas the subsequent deexcitation (4I13/2 → 4I15/2) can result in a sharp, atomlike light emission.
Adhesion and Friction Issues Associated With Reliable Operation of MEMS
Authors: Maboudian Roya
Published: August 2016

A growing interest exists in developing technologies that use silicon and other electronic materials as mechanical materials. Using standard processes of the integrated-circuit industry, researchers have successfully fabricated miniature mechanical components (micromachines) such as membranes, gears, motors, pumps, and valves. The integration of miniaturized mechanical components with microelectronic components has spawned a new technology known as microelectromechanical systems (MEMS). It promises to extend the benefits of microelectronic fabrication to sensing and actuating functions. Early applications of this technology include the digital mirror display, which has of the order of 106 aluminum thin-film micromirrors fabricated on top of a complementary-metal-oxide-semiconductor static random-access-memory integrated circuit. Other applications include integrated accelerometers for tasks such as air-bag deployment.

A number of fabrication techniques have been developed for this technology and have been reviewed elsewhere. In this review, I focus on surface-micromachining technology and adhesion and friction problems in surface-micromachined polycrystalline silicon (polysilicon) structures, though many of the principles discussed will also apply both to single-crystalline silicon and nonsilicon-based structures. Surface micromachining, defined as the fabrication of micromechanical structures from deposited thin films, is one of the core technological processes underlying MEMS. Surface microstructures have lateral dimensions of 50-500 μm with thicknesses of 0.1–2.5 μm and are offset 0.1–2 μm from the substrate. The basic steps in a surface-micromachining process appear in Figure 1. First the substrate is typically coated with an isolation layer (Figure la) that protects it during subsequent etching steps. A sacrificial layer is then deposited on the substrate and patterned. For simplicity, Figure 1b shows that the opening of the sacrificial layer is terminated on the isolation layer. The microstructural thin film is then deposited and etched (Figure 1c). Finally selective etching of the sacrificial layer creates the freestanding micromechanical structures such as the cantilever beam shown in cross section in Figure 1d. The technique can be extended to make multiple-layer microstructures.
Electrical Studies of Semiconductor-Nanocrystal Colloids
Authors: Alivisatos A.P.
Published: August 2016

The study of nanometer-sized semiconductor crystals has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical and chemical properties can be systematically tuned by variation of the size, according to increasingly well-established scaling laws. This article describes colloidal semiconductor nanocrystals belonging to the II-VI and III-V families, and outlines strategies for obtaining electrical access to such dots.

If an inorganic cluster exceeds a certain size—generally in the 10s of unit cells—then it will likely possess a bonding geometry characteristic of a bulk phase. Above this critical size, the nature of the chemical bonds in the cluster remains fixed as a function of the size, but the total number of atoms—or the surface to volume ratio—changes smoothly. This leads to a slow extrapolation of the properties for ideal nanocrystals toward bulk values with increasing size, according to the scaling laws. The ability to control systematically the properties of inorganic materials by variation of size and shape is an important development with many implications for how materials should be processed and assembled.

Many scaling laws have been investigated, including the size variation of bandgap, charging energy, magnetization reversal, and melting. Study of the scaling laws reveals lessons for how to make nanocrystals. This article focuses on the properties of colloidal semiconductor nanocrystals, how to make them, and ways of gaining electrical access to them. Advances in metal, magnetic, and structural nanomaterials are also occurring. Semiconductor dots produced by other processing techniques in the articles by Bimberg, Gammon, and Tarucha in this issue.
Growth, Spectroscopy, and Laser Application of Self-Ordered III-V Quantum Dots
Authors: Bimberg D. , Grundmann M. , Ledentsov N.N.
Published: August 2016

The development and application of semiconductor light-emitting and laser diodes has been a huge success during the last 30 years in key areas of modern technology like communications, recording, and printing. Still there is ample room for improvement through combination of the atomlike properties for zero-dimensionally localized carriers in quantum dots (QDs) with state-of-the-art semiconductor-laser technology. Low, temperature-insensitive threshold current; high gain; and differential gain have been predicted since the early 1980s.

In the past two decades, the fabrication of QDs has been attempted using colloidal techniques (see the article by Nozik and Mićić in this issue), patterning, etching, and layer fluctuations (see the article by Gammon in this issue). However a break-through occurred recently through the employment of self-ordering mechanisms during epitaxy of lattice-mismatched materials (see the next section) for the creation of high-density arrays of QDs that exhibit excellent optical properties, particularly high quantum efficiency, up to room temperature. The zero-dimensional carrier confinement and subsequent atomlike electronic properties have a drastic impact on optical properties (see the section on Spectroscopy). Also intimately connected is the applicability of QDs as a novel gain medium in state-of-the-art laser diodes with superior properties (see the section on Lasers).
Pottery and Pigments in Arizona: Salado Polychrome
Authors: Simon Arleyn W.
Published: August 2016

Salado-polychrome ceramics, marked by distinctive black on white with red designs (Figure 1), coincided with the development of platform-mound communities and were the result of an amalgamation of technological traditions that occurred during a time of population movements and cultural changes in the prehistoric American Southwest. Saladopolychrome ceramics were the most abundant decorated ware of the Classic period (A.D. 1275–1450) and have been recovered from sites in central Arizona, southwestern New Mexico, and northern Chihuahua, including the site of Casas Grandes. Several archaeologists have interpreted Salado-polychrome designs as symbols of a regional cult (Adams, Crown, Rice) that eased the integration of diverse populations in times of migration and social stress. The color scheme and designs of Salado polychrome are distinctive compared to other contemporary and earlier ceramics, making definition of its development difficult.
Defects and Interfaces in GaN Epitaxy
Authors: Ponce F.A.
Published: August 2016

The recent developments in III-V-nitride thin-film technology has produced significant advances in high-performance devices operating in the blue and green range of the visible spectrum. These materials are grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. Highly specular surfaces are possible by use of low-temperature buffer layers following the method developed by Akasaki et al. The thin films thus grown have an interesting microstructure, quite different from other known semiconductors. In particular, epilayers with high optoelectronic performance are characterized by high dislocation densities, several orders of magnitude above those found in other optoelectronic semiconductor films. The lattice mismatch between sapphire and GaN is ∼14%, and the thermal-expansion difference is close to 80%. In spite of these large differences, little thermal strain is measurable at room temperature in epilayers grown at temperatures above 1000°C. Epitaxy on other systems, like SiC, with much better similarity in lattice parameter and thermal-expansion characteristics, has failed to produce better performance than films grown on sapphire. The origin of these puzzling properties of nitrides on sapphire rests in its microstructure. This article presents a survey of the microstructure associated with epitaxy of nitrides by MOCVD.
Applications of Electron Microscopy in Collaborative Industrial Research
Authors: Ross F.M. , Krishnan K.M. , Thangaraj N. , Farrow R.F.C. , Marks R.F. , Cebollada A. , Parkin S.S.P. , Toney M.F. , Huffman M. , De Araujo C.A. Paz , McMillan L.D. , Cuchiaro J. , Scott M.C. , Echer C. , Ponce F. , O'Keefe M.A. , Nelson E.C.
Published: August 2016

The transmission electron microscope (TEM) is one of the most useful tools available to the materials scientist. Yet both the complexity and expense of the equipment, and the huge investment in time necessary to become proficient in specimen preparation and image acquisition and analysis, mean that it is difficult for most industrial institutions to maintain a state-of-the-art TEM facility. How can industry overcome this problem? One solution is to set up a collaboration with a university, an industrial partner, or a government research laboratory. Such collaborations can be extremely valuable to the company, which gains access to microscopes, specimen-preparation equipment and the expertise of professional microscopists, and to the research laboratory, which benefits from the industrial perspective and the private sector's proficiency in materials preparation and processing.

Such collaborations exist, and they can produce excellent results. In this article, we present three case studies in which successful collaboration has occurred between industry and one of the Department of Energy's scientific user facilities, the National Center for Electron Microscopy (NCEM-see sidebar). Our aim is not only to describe results that we hope will be of scientific interest but also to encourage industrial researchers to consider collaborations with institutes such as NCEM.
High-Permittivity Perovskite Thin Films for Dynamic Random-Access Memories
Authors: Kingon A.I. , Streiffer S.K. , Basceri C. , Summerfelt S.R.
Published: August 2016

An important application of ferroelectric films is their incorporation into dynamic random-access memories (DRAMs) as the storage node capacitor dielectric. Dynamic random-access memories represent a large market that is experiencing strong growth, but they are particularly significant as the technology leader for semiconductor devices. As products move to higher and higher integration density, new developments are first introduced in DRAMs. The steady trend toward higher density has placed severe demands on the device designs, particularly with respect to “squeezing” the capacitor into the available space.
Materials Used in Electroluminescent Displays
Authors: Rack P.D. , Naman A. , Holloway P.H. , Sun S-S. , Tuenge R.T.
Published: August 2016

The flat-panel-display (FPD) market is experiencing rapid growth due to increased demand for portable computers, communication equipment, and consumer electronic products. In all of these applications, the display is the primary human interface that conveys information. The size of the flat-panel-display market is presently estimated to be $10 billion/year and is projected to grow to over $18 billion/year by 1998. Although most current FPDs utilize either passive- or active-matrix liquid-crystal-display (LCD) technology, electroluminescent (EL) displays and light sources, because of their solid-state construction and self-emissive characteristics, can provide improved performance for many demanding display applications. Thin-film electroluminescent (TFEL) technology has been demonstrated over a broad range of display sizes from 1-in. to 18-in. diagonal with resolutions from 50 to 1,000 lines per inch. Also, because of its unique solid-state characteristic, TFEL technology is well-suited to provide a fully integrated display with the light-emitting element and electronics fabricated on the same substrate. An example of a full-color TFEL display is shown in Figure 1.

Thin-film electroluminescent display panels are finding increasing applications in the FPD marketplace due to several fundamental performance advantages over LCDs. These include wide viewing angle, high contrast, wide operating-temperature range, ruggedness, and long lifetime. Alternating-current (ac)-driven monochrome TFEL displays (ACTFEL displays) have become the most reliable, longest running devices on the market. Commercial ACTFEL display panels have operated for more than 50,000 hours with less than 10% luminance change, the equivalent of 25 working years.
In Situ SEM Observations of Electromigration Voids in Al Lines under Passivation
Authors: Flinn Paul A. , Madden Michael C. , Marieb Thomas N.
Published: August 2016

Since the early work of Ilan Blech and Gene Meieran more than 20 years ago, electromigration (mass transport produced in a metallic conductor by current flow) has been recognized as an important reliability hazard for solid-state electronic devices. The continuing shrinkage of dimensions in VLSI devices has resulted in a continuing increase in the current density in the interconnection lines and a corresponding increase in the potential electromigration hazard. Although there have been improvements in the materials and structures used for interconnections, notably the addition of copper to aluminum and the use of layered structures, the provision of an adequate margin of safety has become increasingly difficult and is an important constraint on both device design and process development.
Surface Absorption of Monolayers
Authors: Ulman Abraham
Published: August 2016

When the first paper describing the formation of self-assembled monolayers (SAMs) of octadecyltrichlorosilane [CH3(CH2)17SCl3, or OTS] by adsorption on SiO2 was published, it could not have been predicted that this area of research would become so important in only one decade. Although Zisman was the first to discover that monolayers can be prepared by adsorption of a surfactant onto a clean metal surface, the real revolution in the field occurred when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions. A decrease in the use of moisture-sensitive alkyl trichlorosilanes and the increased use of crystalline gold surfaces were two important reasons for the success of these SAMs. Indeed, monolayers of alkanethiolates on gold are the most studied SAMs to date and thus deserve the most detailed discussion.

SAMs have been intensively studied in the past few years because of their relevance to science and technology. Due to their dense and stable structure, SAMs have potential applications in corrosion prevention, wear protection, and biosensing, for example. The ability to tailor both head and tail groups of the constituent molecules makes them ideal for gaining a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrate, and molecule-solvent interactions like ordering and growth, wetting adhesion, lubrication, and corrosion.
Beneficial Aspects of the Environmental Instability of Materials
Authors: Ambrose John R. , Bendale Priya R.
Published: August 2016

The environmental degradation of materials poses a serious limitation in the utility of engineering materials. The corrosion of metals, for example, has been estimated to represent a 4–5% decrease in the Gross National Product each year. To this, add losses involved in the replacement or restoration of ceramic structures such as buildings and transportation systems, i.e., the “infrastructure,” and the result is a significant sacrifice of economic strength.

Most of us are familiar with the consequences of exposing materials to environments in which the materials are chemically unstable and convert into substances that are unable to perform the function for which the original material was selected. The corrosion of metals into soluble or insoluble oxidation reaction products, chain scission or molecular mutation in polymers, even hydrolysis and leaching of silicious ceramic compounds represent behavior which ultimately limits the service applications of most engineering materials. For example, aluminum and its alloys are unsuitable for use in environments where oxide formation rates are high enough to represent a problem with respect to useful service life.
Mathematical Modeling in Materials Science and Engineering
Authors: Szekely Julian
Published: August 2016

During the past two decades, mathematical modeling has been gaining acceptance as a legitimate part of materials science and engineering. However, as common to all relatively new disciplines, we still lack a realistic perspective regarding the uses, limitations, and even the optimal methodologies of mathematical modeling techniques.

The term “mathematical modeling” covers a broad range of activities, including molecular dynamics, other atomistic scale systems, continuum fluid and solid mechanics, deformation processing, systems analysis, input-output models, and lifecycle analyses. The common point is that we use algebraic expressions or differential equations to represent physical systems to varying degrees of approximation and then manipulate these equations, using computers, to obtain graphical output.

While it is becoming an accepted fact that some kind of mathematical modeling will be needed to make most research programs complete, there is still considerable ambiguity as to what form this should take and what might be the actual usefulness of such an effort.

Among the more seasoned and successful practitioners of this art, clear guidelines have emerged regarding the uses and limitations of the mathematical modeling approach. We seek to illustrate these uses through the successful modeling examples presented by some leading practitioners. Some general principles may be worth repeating as an introduction to this interesting collection of articles.
Emulsion-Derived Foams Preparation, Properties, and Application
Authors: Even William R. , Gregory Donald P.
Published: August 2016

Porous polymeric foams are well-known materials and have been produced by a wide variety of techniques ranging from leaching soluble fillers through gas-blowing to phase separation. Although these materials are widely used, their structure is often irregular and difficult to control. However, a novel method for producing porous materials with a more regular structure has been developed based on high internal phase emulsions (HIPE). These foams are called emulsion-derived foams and are commercially known as Polyhipe® (Unilever) polymers.

If one takes a liquid oil, introduces water and a suitable emulsifier with agitation, a dilute emulsion is produced with small water droplets dispersed within the oil phase, about the consistency of milk. If the amount of the water (i.e., the internal or pore forming) phase is increased, a concentrated emulsion will be formed and the consistency of the mixture changes into a more viscous fluid. As further water is introduced, a limit is reached where the droplets are so prevalent that they approximate a close-packed structure (see cover of this issue); beyond this point the droplets must distort to accommodate any additional internal phase. For a collection of monodispersed spheres this packing limit corresponds to 74% by volume and is the accepted definition of a high internal phase emulsion, i.e., an emulsion with an internal phase greater than 74%.4 The structure of the emulsion is now analogous to soap bubbles, with thin films surrounding and separating the drops. Note also that, compared to “standard” emulsion polymerization, this is the exact reverse or “inverse” of that process. In the standard route, the dispersed oil phase is polymerized to yield a collection of polymeric spheres; with inverse emulsion, the continuous phase is polymerized, yielding a rigid matrix.
Industrial Ecology: The Materials Scientist in an Environmentally Constrained World
Authors: Allenby Braden R.
Published: August 2016

For most materials scientists, as for most people, John Donne's lines “and therefore never send to know for whom the bell tolls. It tolls for thee.” are simply inspired verse. Increasing recognition of the interplay between human economic activity and global environmental perturbations, however, are beginning to add a cold air of prescience to John's words: “Ask not for whom the CFCs are emitted. They are emitted for thee.”

The need for greater understanding, and new, more systematic approaches to these intertwined economic/environmental issues is becoming more apparent. What is perhaps not so clear is the critical role that materials scientists and engineers must play in this effort. An analysis of the symbiotic relationships among economic development, human population growth, and the uses and flows of materials throughout the economy clearly demonstrates this role.

The intuitive feeling held by many technologists—that they are part of the solution, not part of the problem—certainly appears to be correct. But it will require growth on our part, and a more systems-oriented and comprehensive view of our role, if this promise is to be fulfilled.
Control of Directional Properties in Aluminum Can Stock
Authors: Pate Mark A.
Published: August 2016

Aluminum alloy 3004 (Al-1.0 Mn-1.0 Mg) is the predominant material of choice for use in rigid beverage containers. The mechanical property requirements of the draw and iron forming operation dictate careful control of the aluminum sheet production parameters. The demand for ever lighter weight beverage containers has also continued to place increased demands on the strength requirements of the aluminum can stock. To achieve the strength requirements, the sheet must be produced in a highly cold-worked state. A practical consequence of this required level of cold work is pronounced mechanical and crystallographic anisotropy. Without careful control of can stock production parameters, pronounced rolling textures can lead to an undesirable directionality of properties that produce ears, formed during the deep drawing operation of can making, located 45° to the rolling direction. Figure 1 shows the rolling texture ears in a drawn and ironed beverage can.

This article illustrates the minimization and control of rolling texture earing in cold rolled 3004 alloy beverage can stock through modification of the fabricating process, application of metallurgical theory, optimization of hot rolling practice through empirical experimentation, and process control practices.
The National Critical Materials Act
Authors: Leamy Harry
Published: August 2016

Profound acknowledgment of the importance of materials research to the well-being of society has recently been provided by the government of the United States of America in the form of Public Law 98-373: National Critical Materials Act of 1984. This law provides for the establishment of a National Critical Materials Council within the Executive Office of the President that is responsible for coordination of the government's materials-related policies, programs, and research and technology activities. This new legislation, the major features of which are reproduced on the next page in this issue of the BULLETIN, promises to touch all of us in some way during the coming years.

The law establishes a National Critical Materials Council consisting of three presidential appointees who are qualified in materials policy or materials science and engineering. The Council is primarily responsible for the formulation of national materials policies consistent with other Federal policies. This activity includes establishing responsibilities for programs and priorities for materials activities in each Federal department or agency. The Council will also review materials programs and activities of the government for consistency with the National Materials and Minerals Policy Research and Development Act of 1980. It will monitor the critical needs of both industry and government, and it will advise the President of world trends and their implications for national and world economy and national security.
Some Uses of Crystallographic Databases and Bibliographies
Authors: Phillips J.C. , Siegrist T.
Published: August 2016

Some twenty years ago, it appeared that the crystallographic database had grown so large that it could no longer be described by conventional methods (see, for example, Structure Reports). Today we have available both computerized databases (CRYSTDAT, JCPDS diffraction data, etc.) and excellent bibliographies which enable us to search much larger databases rapidly and far more systematically, resulting in qualitative changes in methods and results. Following are five examples illustrating this point.
Fundamental Properties of Intermetallic Compounds
Authors: Nakamura Morihiko
Published: August 2016

More than 25 years have passed since Intermetallic Compounds, edited by Westbrook, was published. Since that time, enormous advances have been made in the understanding and usage of intermetallic compounds. It is known that intermetallic compounds are generally brittle. Thus, alloys that contain intermetallics may also be brittle. However, many intermetallic compounds are known to have extraordinary functions and characteristics that are not observed in ordinary metals and alloys. Thus, they function as magnetic materials, superconductors, semiconductors, hydrogen absorbing alloys, shape memory alloys, and so on.

Many high-strength structural alloys like maraging steels and duralumins are strengthened by fine precipitates of intermetallic phases. Nickel-based superalloys, which are used for airplane-engine parts, contain 60-70% of Ni3Al-based intermetallics by volume fraction and exhibit high strength at high temperatures. Hard metals, which are used for cutting tools, are composed of a large amount of hard but brittle intermetallics like WC and a small amount of ductile cobalt. Intermetallic compounds like TiAl are also investigated for their applications as structural materials where high strength at high temperatures is required.

In a strict sense, intermetallic compounds are composed of two or more metallic elements. In a wider sense, they are composed of metallic and/or semimetallic elements. Each is characterized by an ordered arrangement of two or more kinds of atoms, that is, the formation of a superlattice, and have various kinds of interatomic bonding, ranging from metallic to covalent or ionic bonding. The ordering of atoms and the strong interatomic bonding result in many attractive properties for intermetallic compounds.
SPLEEM of Magnetic Surfaces and Layered Structures
Authors: Poppa Helmut , Bauer Ernst , Pinkvos Heiko
Published: August 2016

Spin-polarized low-energy electron microscopy (SPLEEM) is a new approach to magnetic-surface imaging. Historically, it is an outgrowth of LEEM (low-energy electron microscopy), which has been successfully applied to the high-speed in situ imaging of microscopic surface structures under clean ultrahigh-vacuum (UHV) conditions. In terms of basic instrument design, in SPLEEM a spin-polarized electron source of the Pierce type replaces the LaB6 field-emission electron source used in all dedicated LEEM systems. A new, fully electrostatic version of LEEM has recently been developed, resulting in a more compact, flange-on-type instrument component, which can be integrated with standard UHV surface analytical systems. The main advantage of the new SPLEEM technique is an advanced electron-source design that allows the spin direction to be arbitrarily adjusted with respect to the sample surface (see the following).
X-Ray Magnetic Circular Dichroism Spectroscopy and Microscopy
Authors: Smith N.V. , Padmore H.A.
Published: August 2016

The advent of electron accelerators dedicated to the production of high-intensity x-rays has revitalized experimental techniques based on x-ray absorption. A recent variant is to use circularly polarized x-rays generated either by use of out-of-plane radiation from a bending magnet or from a specially designed “insertion device.” This new field of x-ray magnetic circular dichroism (XMCD) shows considerable promise in spectroscopy and microscopy of magnetic materials. In this article, we describe the nature of XMCD, offer a few examples of recent progress, and review the prospects for future development using the newly constructed Advanced Light Source (ALS).
Simulation of Microstructure and Surface Profiles of Thin Films for VLSI Metallization
Authors: Smy T. , Dew S.K. , Brett M.J.
Published: August 2016

A crucial step in the manufacture of very large-scale integration (VLSI) integrated circuits is the fabrication of reliable, low-resistance metal interconnects between semiconductor devices. The fabrication of these interconnects is generally performed by depositing a blanket metal film and then patterning it by lithographic and etching techniques. The primary means of depositing thin metal films for VLSI interconnects are sputtering and chemical vapor deposition (CVD).

The creation of reliable interconnects is, however, complicated by a number of issues. In order to obtain low contact resistance, to inhibit reactions with the silicon, and to provide good adhesion to both Si and SiO2, contact, barrier, and adhesion layers are generally deposited prior to the deposition of the low-resistance metal film that forms the bulk of the interconnect. If these layers are to provide an effective barrier to diffusion of the interconnection metal to the silicon, they must be deposited in a uniform, homogeneous form. It is also necessary that the primary interconnect material have as high step coverage as is possible in order to reduce current crowding, local heating effects, and electromigration. Unfortunately, as VLSI circuit densities have increased, the fabrication of interconnects requires high aspect-ratio contact cuts, and relatively severe local topographies can result. These factors make it difficult to deposit films with good step and bottom coverage.

In addition to these concerns with the film surface profile, another factor is becoming increasingly significant. Both sputtering and CVD produce thin films with characteristic microstructures. This microstructure consists of columns or grains separated by grain boundaries and voids.
Quantitative Auger and XPS Analysis of Thin Films
Authors: Slaughter J.M. , Weber W. , Güntherodt Gernot , Falco Charles M.
Published: August 2016

In 1925, P. Auger first observed the so-called Auger electrons in a Wilson cloud chamber. He explained this occurrence as being due to a radiationless transition in atoms excited by a primary x-ray photon source. In 1953, Lander first pointed out that Auger electrons arising from solid samples can be detected in the energy distribution curve of secondary electrons from surfaces subjected to electron bombardment. Moreover, low-energy Auger electrons (∼1 keV kinetic energy) can escape from only the first several atomic layers of a surface since they are strongly absorbed by even a monolayer of atoms. Thus Auger electron spectroscopy (AES) possesses high surface sensitivity. This is one characteristic that makes AES very useful for the study of thin films. For such applications, an important development in AES occurred when Harris showed that the sensitivity of the detection of Auger electrons can be improved by differentiating the electron energy distribution curve with respect to the energy. Furthermore, Weber and Johnson demonstrated that, provided the Auger line profile does not change, the peak-to-peak height in the differentiated energy distribution curves is proportional to the Auger current in the peak. Therefore, in addition to its surface sensitivity, AES also can be used for quantitative studies of thin films.

Like AES, x-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that uses the energy distribution of electrons ejected from a thin film for quantitative analysis. However, in many ways the information provided by AES and XPS is complementary.
Copper-Based Metallization in ULSI Structures: Part II: Is Cu Ahead of Its Time as an On-Chip Interconnect Material?
Authors: Li Jian , Seidel Tom E. , Mayer Jim W.
Published: August 2016

The demand for manufacturing integrated circuit (IC) devices such as dynamic random access memory (DRAM), static random access memory (SRAM), electrically erasable and programmable read only memory (EEPROM) and logic devices with high circuit speed, high packing density and low power dissipation requires the downward scaling of feature sizes in ultralarge-scale integration (ULSI) structures. When chip size becomes smaller, the propagation delay time in a device is reduced. However, the importance of on-chip interconnect RC (resistance capacitance) delay to chip performance, reliability, and processing cost is increasing dramatically. When interconnect feature size decreases and clock frequencies increase, RC time delays become the major limitation in achieving high circuit speeds. The miniaturization of interconnect feature size also severely penalizes the overall performance of the interconnect, such as increasing interconnect resistance and interconnect current densities, which lead to reliability concerns due to electromigration. Lower resistance metal and lower dielectric materials are being considered to replace current Al and SiO2 interconnect materials. Innovative efforts in circuit design, process development, and the implementation of new materials can provide solutions. This issue of the MRS Bulletin focuses on the industrial viewpoint of copper interconnects. (A previous issue of the MRS Bulletin, June 1993, addressed university research approaches to copper metallization.) Articles in this issue, from six major semiconductor companies—IBM, Motorola, AT&T Bell Laboratories, SEMATECH/National Semiconductor, NTT, and Fujitsu—provide a real-world viewpoint of the challenges faced when replacing aluminum with copper. The articles published in both issues also contain a comprehensive list of references (more than 300) to articles, patents, and device applications related to copper metallization for ULSI applications.
Block-by-Block Deposition of Complex Oxide Films
Authors: Locquet Jean-Pierre , Mächler Erich
Published: August 2016

Since the discovery of superconductivity in layered cuprates there has been a strong desire to grow thin films of these materials in a so-called layer-by-layer manner, where each layer represents one atomic monolayer. There has been worldwide interest in this problem and numerous groups have invested time and equipment in an attempt, first, to grow the known materials using a sequential deposition technique so that the atomic monolayers are stacked artificially on top of one another in a sequence defined by the structure of the given unit cell, and second, to search for new stacking sequences of such atomic monolayers, thus making metastable compounds.

Understanding the growth of thin films using any deposition technique requires information about two fundamental processes, the nucleation of a crystal and its growth. Both are strongly affected by the three thermodynamic parameters, temperature, pressure, and chemical potential (composition), at the growth front. The third parameter, the surface composition from which the desired compound can nucleate and propagate, probably provides the greatest degree of freedom in the growth process. The compositional phase diagrams of most cuprates show that several phases can coexist with the superconducting phase at a certain temperature and pressure. This is also the main reason why a widely changing surface composition, such as that present during layer-by-layer or block-by-block deposition, still produces high-quality thin films.
Atomic Layer-by-Layer Engineering of High Tc Materials and Heterostructure Devices
Authors: Eckstein J.N. , Bozovic I. , Virshup G.F.
Published: August 2016

Oxides exhibit most of the interesting phenomena known to occur in solid-state systems. As a class of materials they may be richer in phenomenology than any other comparable class. Oxides can be insulators, semiconductors, or metals. The copperoxide-based compounds we have studied are superconductors with the highest critical temperatures. In some oxides, electrons manifest simple single-particle transport properties, with a high mobility; in others, they show strongly correlated behavior resulting in a Mott-Hubbard transition, localization, and charge- or spin-density waves. In some oxides, electron-phonon coupling leads to polaronic transport. Others show collective states such as magnetism; in some there are large local magnetic moments that can couple to form ferromagnetic or antiferromagnetic phases that exist up to high temperatures. Yet others have large nonlinear dielectric and optical properties. In fact, it would seem there is very little that some such oxide couldn't do for or to the experimenter.
High-Temperature Oxidation of Stainless Steels
Authors: Colson J.C. , Larpin J.P.
Published: August 2016

The first stainless steels, mainly low carbon chromium-iron alloys, have been known since the beginning of this century. These steels show good resistance against wet corrosion and high-temperature corrosion. This article focuses on high-temperature corrosion, with emphasis on gaseous sulfidizing and oxidizing environments. The discussion is limited to these two gases since corrosion involving halogen-and/or carbon-containing gases involves other specific processes. The behavior of binary and ternary alloys will be successively examined, then the role of minor elements will be considered.

Fundamental Mechanisms of High-Temperature Corrosion of Stainless Steel

Usually, a dry corrosion process results in the formation of corrosion products, giving a simple or complex oxide or sulfide scale on a metallic substrate, separating it from the aggressive gaseous environment and, consequently, acting as a protective barrier. Scale growth is controlled by the conductivity of the reaction products which are solid electrolytes. Generally, the mechanism of scale growth is governed by outward cation or inward anion diffusion processes. This is the basis of the model originally put forward by Wagner for a single metal and subsequently developed for alloys, and particularly, for stainless steels. This one-way point-defect diffusion process is responsible for the observed parabolic scaling kinetics characterized by a parabolic rate constantkp. This model is well described in the literature.

In the case of stainless steels, formation of a protective scale is required; this is possible if the oxide or sulfide products have a low diffusivity to cations or anions due to a low density of point defects in the crystal lattice. The protective characteristics of the corrosion products may be experimentally determined by measurement of their electrical conductivity, although the scales should also be effective against short-circuit transport of ions, atoms, or molecules. The best barriers consist of oxides, such as Al2O3, SiO2, and Cr2O3.
Bentonite as a Backfill Material in a High-Level Waste Repository
Authors: Grauer Rolf
Published: August 2016

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by90Sr and137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.

The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:

• Low hydraulic conductivity compared to the host rock;

• Good swelling properties to seal construction-caused joints and rock fractures;

• Plasticity to absorb rock movements and to distribute pressure homogeneously;

• Good retention of radionuclides; and

• Stability over a period of at least 106 years.

Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.
Surfaces, Interfaces, and Changing Shapes in Multilayered Films
Authors: Josell Daniel , Spaepen Frans
Published: August 2016

It is generally recognized that the capillary forces associated with internal and external interfaces affect both the shapes of liquid-vapor surfaces and wetting of a solid by a liquid. It is less commonly understood that the same phenomenology often applies equally well to solid-solid or solid-vapor interfaces.

The fundamental quantity governing capillary phenomena is the excess free energy associated with a unit area of interface. The microscopic origin of this excess free energy is often intuitively simple to understand: the atoms at a free surface have “missing bonds”; a grain boundary contains “holes” and hence does not have the optimal electronic density; an incoherent interface contains dislocations that cost strain energy; and the ordering of a liquid near a solid-liquid interface causes a lowering of the entropy and hence an increase in the free energy. In what follows we shall show how this fundamental quantity determines the shape of increasingly complex bodies: spheres, wires, thin films, and multilayers composed of liquids or solids. Crystal anisotropy is not considered here; all interfaces and surfaces are assumed isotropic.

Consideration of the equilibrium of a spherical drop of radius R with surface free energy γ shows that pressure inside the droplet is higher than outside. The difference is given by the well-known Laplace equation:

This result can be obtained by equating work done against internal and external pressure during an infinitesimal change of radius with the work of creating a new surface.
In Situ EXAFS Study of the Photoexcited State and Defects in Chalcogenide Glasses
Authors: Kolobov Alexander V. , Oyanagi Hiroyuki , Tanaka Kazunobu
Published: August 2016

Amorphous chalcogenides, of which selenium is the simplest representative, exhibit a number of unique properties such as the ability to undergo various transformations under the action of the bandgap light. On illumination, the absorption edge shifts to lower energies, and subsequent annealing near the glass-transition temperature leads to a recovery of the initial parameters as demonstrated in Figure 1. Such a photo-induced change could not be observed either in amorphous group IV semiconductors or a-As or in crystalline chalcogenides. A good review of the initial stage of these studies was made by de Neufville.

Reversible photodarkening can also be observed in pure chalcogens, but this process can be achieved only at lower temperatures, which is understandable if one takes into account that the glass-transition temperature of selenium is just above room temperature. This result indicates that there is a correlation between the temperature at which photodarkening is annealed out and the flexibility of the glassy network.

Reversible changes in the optical absorption are accompanied by (reversible) changes in electrical and photoelectric properties, volume, microhardness, glass-transition temperature, and dissolution rate in various solvents, to name a few. The totality of these changes has led investigators to the conclusion that the photoinduced changes in optical absorption are caused by changes in structure.