Showing papers in "Mrs Bulletin in 1997"

Progress in Electroluminescent Devices Using Molecular Thin Films

Abstract: Organic electroluminescence (EL) is moving from a simple curiosity in laboratories to the reality of commercial use. The day will soon arrive when high-quality green EL displays find practical usage. Charge-injection-type EL involves the combination of positive and negative charge carriers injected from electrodes in contact with an organic thin film. The occurrence of EL through bipolar charge injection into organic solids was clarified for single crystals of anthracene and related compounds in the 1960s. Few new developments in physics exist today for organic EL. However worldwide enthusiasm for charge-injection-type EL, which started in the mid-1980s, has been increasing rapidly. The motivation for this renewed interest is straightforward. High-efficiency surface emission across the whole visible spectral range can be obtained easily, and prospects now exist for full-color flat-panel-display technology.Since the demonstration of high-performance EL devices made of multilayers of vacuum-sublimed dye films by Tang and VanSlyke, much progress has occurred in the research and development of EL devices made from molecular materials. A variety of molecular materials such as vacuum-sublimed dye films, fully π-conjugated polymers, polymers with chromophores orr skeletal chains, or side chains, and polymer-dispersed dye films can be used for EL devices.Among a variety of EL devices, multilayer-structure versions made of vacuum-sublimed dye films exhibit the best performance. Application-oriented research in the development of high-quality flat-panel displays has been performed during the past 10 years, mainly in Japan's private-sector laboratories.

Physical Properties of SiC

Abstract: While silicon carbide has been an industrial product for over a century, it is only now emerging as the semiconductor of choice for high-power, high-temperature, and high-radiation environments. From electrical switching and sensors for oil drilling technology to all-electric airplanes, SiC is finding a place which is difficult to fill with presently available Si or GaAs technology. In 1824 Jons Jakob Berzelius published a paper which suggested there might be a chemical bond between the elements carbon and silicon. It is a quirk of history that he was born in 1779 in Linkoping, Sweden where he received his early education, and now, 172 years later, Linkoping University is the center of a national program in Sweden to study the properties of SiC as a semiconductor.

Particle-Packing Phenomena and Their Application in Materials Processing

Abstract: Particle packing is directly controlled by the particle-size distribution of a material being processed. For this reason, particle packing is important to all particulate/fluid systems. After the solids fraction of a body is defined, interparticle chemistry controls how the body will pack and flow. A system of powders can never pack better than the maximum possible level defined by the particle-size distribution alone. Proper control of interparticle chemistry however can help achieve maximum packing, can be used to open the structure, and/or can be used to modify rheological or other process properties.The main goals of particle-packing research have been to determine how systems of particles pack, to develop algorithms for calculating packing densities and porosities for any distribution of particles (spherical or nonspherical, rough or smooth, wet or dry), and to determine how packing and its properties affect the variety of industrial operations that utilize particulate/fluid systems.

Low-Dielectric-Constant Materials for ULSI Interlayer-Dielectric Applications

Abstract: Continuing improvement of microprocessor performance historically involves a decrease in the device size. This allows greater device speed, an increase in device packing density, and an increase in the number of functions that can reside on a single chip. However higher packing density requires a much larger increase in the number of interconnects. This has led to an increase in the number of wiring levels and a reduction in the wiring pitch (sum of the metal line width and the spacing between the metal lines) to increase the wiring density. The problem with this approach is that—as device dimensions shrink to less than 0.25 μm (transistor gate length)—propagation delay, crosstalk noise, and power dissipation due to resistance-capacitance (RC) coupling become significant due to increased wiring capacitance, especially interline capacitance between the metal lines on the same metal level. The smaller line dimensions increase the resistivity (R) of the metal lines, and the narrower interline spacing increases the capacitance (C) between the lines. Thus although the speed of the device will increase as the feature size decreases, the interconnect delay becomes the major fraction of the total delay and limits improvement in device performance. To address these problems, new materials for use as metal lines and interlayer dielectrics (ILD) as well as alternative architectures have been proposed to replace the current Al(Cu) and SiO2 interconnect technology.

Blue-Green Light-Emitting Diodes and Violet Laser Diodes

Abstract: Short-wavelength-emitting devices, such as blue laser diodes (LDs) and light-emitting diodes (LEDs), are currently sought for a number of applications, including full-color electroluminescent displays, laser printers, read-write laser sources for high-density information storage on magnetic and optical media, and sources for undersea optical communications. For these purposes, II–VI materials such as ZnSe and SiC, and III–V-nitride semiconductors such as GaN have been investigated intensively for a long time. However it was impossible to obtain high-brightness (over 1 cd) blue LEDs and reliable LDs. Much progress has been achieved recently on green LEDs and LDs using II–VI-based materials. The short lifetimes prevent II–VI-based devices from commercialization at present. The short lifetime of these II-VI-based devices may be caused by the crystal defects at a density of 103/cm2 because one crystal defect would cause the propagation of other defects leading to failure of the devices. Another wide-bandgap material for blue LEDs is SiC. The brightness of SiC blue LEDs is only between 10 mcd and 20 mcd because of the indirect bandgap of this material.On green LEDs, the external quantum efficiency of conventional, green GaP LEDs is only 0.1% due to the indirect bandgap of this material. The peak wavelength is 555 nm (yellowish green). As another material for green emission devices, AlInGaP has been used. The present performance of green AlInGaP LEDs is an emission wavelength of 570 nm (yellowish green) and maximum external quantum efficiency of 1%.

GaN/AIGaN Heterostructure Devices: Photodetectors and Field-Effect Transistors

Abstract: In this article, we review recent progress in GaN-based photodetectors and field-effect transistors (FETs), including optoelectronic FETs, and discuss materials parameters and fabrication technologies that determine the device characteristics of these two device families. Many types of visible-blind photodetectors and nearly all types of FETs have been demonstrated in GaN-based materials systems. However many challenges remain, both in improving the existing devices—the performance of which is still quite far from reaching its full potential—and in developing entirely new devices, which use unique properties of this wide-bandgap materials system.

Molecular Mechanisms in Smart Materials

Abstract: The following is an edited version of the David Turnbull Lectureship address, given by recipient Robert E. Newnham at the 1996 MRS Fall Meeting. Newnham received the lectureship for “pioneering the field of ceramic composites for electronic and optical applications, and in recognition of a distinguished career of guiding students, lecturing, and writing.” Newnham is the Alcoa Professor of Solid State Science at The Pennsylvania State University.

Nanoporous Silica as an Ultralow-k Dielectric

Abstract: As feature sizes in integrated circuits approach 0.18 μm, problems with interconnect resistance-capacitance (RC) delay, power consumption, and crosstalk become more urgent. Integration of low-dielectric-constant (k) materials will partially mitigate these problems, but each candidate with k significantly lower than that of dense silica (k ∼ 4) suffers disadvantages. Current low-k commercialization emphasizes spin-on glasses (SOGs) and fluorinated SiO2 with k > 3, and a number of polymers are under development with k in the range of 2–3. These suffer from potential problems including thermal stability, mechanical properties, low thermal conductivity, and reliability. For some low-k materials, a protective liner covering the conductor is necessary. Although the material k is often cited, the value of practical concern is the effective k, which may be quite different because of this protective liner. As feature sizes shrink, the presence of the liner becomes more problematic and necessitates even lower k materials. Another approach employs nanoporous silica with k of ∼1–4. Porous silica has been classified as an aerogel (dried supercritically) or as a xerogel (dried by solvent evaporation). We use the term nanoporous silica since it captures the key material properties that may be independent of how the films are processed. The ultralow dielectric constant results from porosity incorporation. For a porous material, the dielectric constant is a combination of that of air (∼1) and of the solid phase. The variation of k with porosity (volume fraction of pores) appears in Figure 1.

Defects and Interfaces in GaN Epitaxy

Abstract: 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.

Silicon Carbide Electronic Materials and Devices

Abstract: The development of SiC for electronic applications has been a subject of intense research for nearly 40 years. Much of this research is motivated by the extraordinary combination of physical properties possessed by SiC, especially in the development of SiC-based devices for specific high-temperature, high-power, or high-frequency applications that are not suitable for Si- or GaAs-based devices. During the early years of SiC research and development, a significant amount of good, fundamental research was performed, but the development of commercially available SiC-based devices was retarded by low-quality bulk materials and inadequate epitaxial processes. In the late 1980s, research at academic institutions, such as North Carolina State University, and industrial laboratories, such as Westinghouse (now Northrup-Grumman), Advanced Technology Materials, Inc. (ATMI), and Cree Research, Inc., coupled with the commercial offering of highquality SiC wafers from Cree, created an opportunity for further advancement. Improvements in epitaxial processes and device processing strategies were also realized during this time. Together these factors have enabled the fabrication of high-quality device structures and have generated increased research and funding activities in SiC electronic devices.

Polymer Electroluminescent Devices

Abstract: Electroluminescence (EL) is the emission of light generated from the radiative recombination of electrons and holes electrically injected into a luminescent semiconductor. Conventional EL devices are made of inorganic direct-bandgap semiconductors, such as GaAs and InGaAs. Recently EL devices based on conjugated organic small molecules and polymers have attracted increasing attention due to easy fabrication of large areas, unlimited choice of colors, and mechanical flexibility. Potential applications of these organic/polymeric EL devices include backlights for displays, alphanumeric displays, and high-density information displays.Electroluminescence from an organic material was first demonstrated in the 1960s on anthracene crystals by Pope et al. at New York University. Subsequently several other groups also observed this phenomenon in organic crystals and thin films. These organic EL devices had high operating voltages and low quantum efficiency. Consequently they did not attract much attention. In 1987 a breakthrough was made by Tang and VanSlyke at Eastman Kodak who found that by using multilayers of sublimated organic molecules, the operating voltage of the organic EL devices was dramatically reduced and the quantum efficiency was significantly enhanced. This discovery touched off a flurry of research activity, especially in Japan. The Japanese researchers, as welt as the group at Kodak, have since improved the device efficiency and lifetime to meet commercial requirements. This progress is reviewed by Tsutsui in this issue.

Electrically Conducting Polymers: Science and Technology.

Abstract: For the past 50 years, conventional insulating-polymer systems have increasingly been used as substitutes for structural materials such as wood, ceramics, and metals because of their high strength, light weight, ease of chemical modification/customization, and processability at low temperatures. In 1977 the first intrinsic electrically conducting organic polymer—doped polyacetylene—was reported, spurring interest in “conducting polymers.” Intrinsically conducting polymers are completely different from conducting polymers that are merely a physical mixture of a nonconductive polymer with a conducting material such as metal or carbon powder. Although initially these intrinsically conducting polymers were neither processable nor air-stable, new generations of these materials now are processable into powders, films, and fibers from a wide variety of solvents, and also are airstable. Some forms of these intrinsically conducting polymers can be blended into traditional polymers to form electrically conductive blends. The electrical conductivities of the intrinsically conductingpolymer systems now range from those typical of insulators (<10−10 S/cm (10−10 Ω−1 cm1)) to those typical of semiconductors such as silicon (~10 5 S/cm) to those greater than 10+4 S/cm (nearly that of a good metal such as copper, 5 × 105 S/cm). Applications of these polymers, especially polyanilines, have begun to emerge. These include coatings and blends for electrostatic dissipation and electromagnetic-interference (EMI) shielding, electromagnetic-radiation absorbers for welding (joining) of plastics, conductive layers for light-emitting polymer devices, and anticorrosion coatings for iron and steel.The common electronic feature of pris tine (undoped) conducting polymers is the π-conjugated system, which is formed by the overlap of carbon pz orbitals and alternating carbon-carbon bond lengths.

GaN and Related Materials for Device Applications

Abstract: The addition of GaN, A1N, InN, and related alloys to the family of device-quality semiconductors has opened up new opportunities in short-wavelength (visible and ultraviolet [uv]) photonic devices for display and data-storage applications, solar-blind uv detectors, and high-temperature/high-power electronics. Silicon will of course continue to dominate in microelectronics applications, and InP and GaAs and their related alloys will be the mainstays of long-wavelength lightwave communication systems and red, orange, and yellow light-emitting-diode (LED) technology, respectively. There are however many existing and emerging uses for wide-bandgap semiconductors with good electrical and optical characteristics. The purpose of this issue of MRS Bulletin is to furnish a background and summary on the exciting new developments involving GaN and related materials.Strong efforts on the synthesis and device aspects of GaN took place in the 1960s and 1970s because of the potential for realization of blue lasers and LEDs that would extend the existing wavelength range of photonic devices. Progress was hampered because of several severe materials problems. First there was no bulk crystal growth technology for producing substrates, and epitaxial material was grown on highly lattice-mismatched substrates such as sapphire. This heteroepitaxial material was invariably highly conducting because of residual shallow donor defects or impurities. These high n-type backgrounds, combined with the relatively deep ionization levels of all of the common p-type dopant impurities, prevented the achievement of p-type doping and therefore of bipolar or injection devices.

Hydrogen Storage in Quasicrystals

Abstract: Quasicrystals may have important applications as new technological materials. In particular, work in our laboratory has shown that some quasicrystals may be useful as hydrogen-storage materials.Some transition metals have a capacity to store hydrogen to a density exceeding that of liquid hydrogen. Such systems allow for basic investigations of solid-state phenomena such as phase transitions, atomic diffusion, and electronic structure. They may also be critical materials for the future energy economy. The depletion of the world's petroleum reserves and the increased environmental impact of conventional combustion-engine powered automobiles are leading to renewed interest in hydrogen. TiFe hydrides have already been used as storage tanks for stationary nonpolluting hydrogen internal-combustion engines. Nickel metal-hydride batteries are commonly used in a wide range of applications, most notably as power sources for portable electronic devices—particularly computers. The light weight and low cost of titanium-transition-metal alloys offer significant advantages for such applications. Unfortunately they tend to form stable hydrides, which prevents the ready desorption of the stored hydrogen for the intended use.Some titanium/zirconium quasicrystals have a larger capacity for reversible hydrogen storage than do competing crystalline materials. Hydrogen can be loaded from the gas phase at temperatures as low as room temperature and from an electrolytic solution. The hydrogen goes into solution in the quasicrystal structure, often avoiding completely the formation of undesirable crystalline hydride phases. The proven ability to reversibly store variable quantities of hydrogen in a quasicrystal not only points to important areas of application but also opens the door to previously inaccessible information about the structure and dynamics of this novel phase. Selected results illustrating these points appear briefly here.

Organic and Inorganic Spin-On Polymers for Low-Dielectric-Constant Applications

Abstract: Low-dielectric-constant materials (k 3.0) have served as planarizing dielectrics during the last 15 years. The newer spin-on polymers have greatly enhanced mechanical, thermal, and chemical properties, exhibiting lower dielectric constants than the traditional materials.

Hard Carbon Coatings: The Way Forward

Abstract: Since the first reports by Aisenberg and Chabot in 1971 indicating the possibility of producing hard amorphous (so-called diamondlike) carbon (DLC) films, many experimental and theoretical research results have been published outlining the properties and film growth mechanisms of these films deposited by a variety of techniques. Polycrystalline and even quasimonocrystalline diamond thin films have also been produced, thus providing a wide range of wear and corrosion properties. The difference between these materials and graphite led to a prediction of rapid market growth for hard carbon. However this has not materialized. A large number of carbonbased films have been produced with differences in hardness, elasticity, friction coefficient, optical and electronic bandgap, electrical and thermal conductivity, and thermal stability. In addition these materials can show very different practical adhesion properties, which depend also on the substrate material and composition. Cost, deposition rate and temperature, geometry, and size of the coating chamber are additional variables. As a result, many of these materials can only be used for a limited range of applications. It is now possible to better understand the suitability of various coatings and the causes of the early failures that occurred through unsuitable material choices. This improved understanding should allow improvements in the performance and reliability of hard carbon films and perhaps trigger the kind of market growth that was originally foreseen but failed to materialize.

Quasicrystals: Perspectives and Potential Applications

Abstract: For decades scientists have accepted the premise that solid matter can only order in two ways: amorphous (or glassy) like window glass or crystalline with atoms arranged according to translational symmetry. The science of crystallography, now two centuries old, was able to relate in a simple and efficient way all atomic positions within a crystal to a frame of reference in which a single unit cell was defined. Positions within the crystal could all be deduced from the restricted number of positions in the unit cell by translations along vectors formed by a combination of integer numbers of unit vectors of the reference frame. Of course disorder, which is always present in solids, could be understood as some form of disturbance with respect to this rule of construction. Also amorphous solids were naturally referred to as a full breakdown of translational symmetry yet preserving most of the short-range order around atoms. Incommensurate structures, or more simply modulated crystals, could be understood as the overlap of various ordering potentials not necessarily with commensurate periodicities.For so many years, no exception to the canonical rule of crystallography was discovered. Any crystal could be completely described using one unit cell and its set of three basis vectors. In 1848 the French crystallographer Bravais demonstrated that only 14 different ways of arranging atoms exist in three-dimensional space according to translational symmetry. This led to the well-known cubic, hexagonal, tetragonal, and associated structures. Furthermore the dihedral angle between pairs of faces of the unit cell cannot assume just any number since an integer number of unit cells must completely fill space around an edge.

Radiation Effects in Graphite and Carbon-Based Materials

Abstract: Displacement damage in graphite and carbon-based materials can occur when energetic particles, such as neutrons, ions, or electrons impinge on the crystal lattice. The displacement of carbon atoms from their equilibrium positions results in lattice strain, bulk dimensional change, and profound changes in physical properties. This article will discuss the effects of displacement damage in graphites and carbon-based materials. The materials considered here are those whose bonding is sp2—that is, graphites, pyrolytic carbons and graphites, carbon fibers, and carbon-carbon (C/C) composites. Radiation damage in sp3 (diamond) carbon forms is not discussed.Carbon-based materials and graphites are widely used in nuclear applications. For example, polygranular (manufactured) graphites have been employed as a moderator in nuclear reactors since the 1940s. More recently, pyrolytic graphites, artificial graphites, and C/C composites have been adopted as plasma-facing components in fusion devices. Engineering applications, such as those just cited, have necessitated a full understanding of the basic mechanisms of radiation damage, as well as the effects of radiation damage on the physical properties of carbon-based materials.

Ceramic Powder Compaction

Abstract: Powder pressing, either uniaxially or isostatically, is the most common method used for high-volume production of ceramic components. The object of a pressing process is to form a net-shaped, homogeneously dense powder compact that is nominally free of defects. A typical pressing operation has three basic steps: (1) filling the mold or die with powder, (2) compacting the powder to a specific size and shape, and (3) ejecting the compact from the die. To optimize a pressing operation, experienced press operators generally understand and control parameters such as die-fill density, die-wall friction, packing density, and expansion on ejection.Die filling/uniformity influences compaction density, which ultimately determines the size, shape, microstructure, and properties of the final sintered product. To optimize die filling and packing uniformity, free-flowing granulated powders are generally used. Spherical granules (i.e., agglomerates or clusters of finer particles) range in size from ~44 to 400 μm with the average size being ~100–200 μm. They are typically produced from 0.5 to 10-μm median particle-size powders by spray drying a ceramic powder slurry. To produce processable powders, various organic additives are typically added to the slurry prior to spray drying. These include binder(s) for strength, plasticizers that produce deformable granules, and lubricants that mitigate frictional effects. Consistent batching and dispersion of the granulated feed are critical for reproducible and uniform die filling. Granule densities that are 45–55% of the theoretical density (TD), and bulk-powder and die-fill densities of 25–35% TD are typical for ceramic powders.

Thermal Conductivity of Quasicrystals and Associated Processes

Abstract: Quasicrystals (QCs) exhibit unusual physical properties that are significantly different from those of crystalline materials and are not expected for alloys consisting of normal metallic elements. Opposite to conventional metallic alloys, their thermal conductivity and diffusivity are unusually low (with a positive temperature coefficient)—practically that of an insulator—which is atypical for materials containing about 70-at.% aluminum. Moreover the thermal conductivity decreases when the structural perfection is improved. One observes a low, if any, electronic contribution to the heat capacity and thus a vanishing density of electronic states at the Fermi level.The origin of this unexpected behavior was first attributed to the existence of a deep pseudogap at the Fermi level with a localization tendency of electrons near the Fermi level.However experimental evidence led to an alternative approach related to the structure of quasicrystals. In QCs, well-defined atomic clusters form self-similar subsets of the structure over which electronic and vibrational states are expected to extend. According to the inflation symmetry of the icosahedral structure, the so-called recurrent localization effects may then explain the conduction behavior and other striking features of quasicrystals (e.g., brittleductile transition at high temperature, corrosion resistance, low friction, high hardness).In the following, we first present the main thermal properties of quasicrystalline alloys compared to those of conventional materials with an emphasis on the variation of the thermal conductivity with temperature. The combination of such peculiar conduction, mechanical, and tribological properties gives the quasicrystalline alloys a technological interest for applications where superficial thermal and mechanical conditions are of prime importance. This is illustrated with two examples involving a QC coating on a base Al substrate: (1) thermal insulation for which a low conductivity is needed and (2) quenching heat-transfer modification due to a low-effusivity superficial effect. These processes are then explained in the third part of this article in terms of the cluster-modes delocalization mechanism responsible for the low conductivity of the quasicrystals.

Wetting on Grafted Polymer Films

Abstract: Studying the properties of endanchored polymer layers has been a fashionable occupation for numerous physicists, chemists, and material scientists for more than 10 years. Theoreticians have realized that grafted macromolecules are nice statistical objects wriggling around under thermal motion, which give rise to nontrivial long-range entropic effects. These can be described by elegant scaling laws and analogies with quantum or classical mechanics. For experimenters the area turned out to be a marvelous playground in which both very simple and sophisticated techniques such as x-ray or neutron scattering and reflectivity, nuclear magnetic resonance (NMR), Rutherford backscattering, and optical and atomic force microscopy (AFM) have been used to discover interesting and subtle phenomena. All this effort was also motivated by the importance of grafted layers in applications such as paints, adhesives, lubricants, colloidal stabilizers, and composite materials. By anchoring a thin, soft polymer layer to a solid surface, one can tune the surface properties. In this short article, we will discuss how the wetting and spreading of liquids and polymer melts can be profoundly altered by the presence of such protective layers.

Implantation and Dry Etching of Group-III-Nitride Semiconductors

Abstract: The recent advances in the material quality of the group-III-nitride semiconductors (GaN, A1N, and InN) have led to the demonstration of high-brightness light-emitting diodes, blue laser diodes, and high-frequency transistors, much of which is documented in this issue of MRS Bulletin . While further improvements in the material properties can be expected to enhance device operation, further device advances will also require improved processing technology. In this article, we review developments in two critical processing technologies for photonic and electronic devices: ion implantation and plasma etching. Ion implantation is a technology whereby impurity atoms are introduced into the semiconductor with precise control of concentration and profile. It is widely used in mature semiconductor materials systems such as silicon or gallium arsenide for selective area doping or isolation. Plasma etching is employed to define device features in the semiconductor material with controlled profiles and etch depths. Plasma etching is particularly necessary in the III-nitride materials systems due to the lack of suitable wet-etch chemistries, as will be discussed later. Figure 1 shows a laser-diode structure (after Nakamura) where plasma etching is required to form the laser facets that ideally should be vertical with smooth surfaces. The first III-nitride-based laser diode was fabricated using reactive ion etching (RIE) to form the laser facets but suffered from rough mirror facet surfaces that contributed to scattering loss and a high lasing threshold. This is a prime example of how improved material quality alone will not yield optimum device performance.

Deposition and Applications of Quasicrystalline Coatings

Abstract: The key to engineering a material lies in exploiting its beneficial characteristics while minimizing its inherent weaknesses. Whether the weakness is, for example, poor corrosion resistance or low hardness, applying a relatively thin coating of another material that mitigates the shortcoming of the underlying material is a practical solution allowing the composite pieces to be used in demanding environments. This method has been utilized in a wide variety of cases ranging from paint on wooden fences and ceramic thermal barriers on single-crystal superalloy turbine blades to tungsten carbide hard-facing layers on drilling equipment. Some materials may suffer from high cost and therefore are used as a thin layer to impart their desired properties. For instance, gold leaf is applied to buildings for appearance while diamond films are deposited onto normal cutting tools to improve their performance. The specific application typically dictates both the material and the deposition method for the coating. The gold leaf does not need to offer much resistance to abrasion or mechanical stress in order to maintain its beautiful shine far into the future. In contrast the diamond film must be strongly adhered to the underlying cutting tool surface if it is to survive the punishing wear and thermal stresses of machining operations.

Mechanical Behavior of Quasicrystals

Abstract: Scientists have studied the mechanical properties of quasicrystalline materials for quite some time. However the difficulty in obtaining material of reasonable quality hampered systematic investigations. The progress in materials preparation in recent years has triggered new activity in this field. Furthermore the new ternary and multicomponent alloys have demonstrated great promise for use as coatings with good wear resistance and low coefficients of sliding friction. However the physical reasons for these properties and their correlation with the particular structure of quasicrystals are still not understood. As in conventional alloys, experiments under well-defined conditions are required that can serve as a basis for understanding the intrinsic mechanical properties of quasicrystals. Such studies are now increasingly possible after the development of techniques to grow larger single quasicrystals up to a few centimeters in size directly from the melt.Since the mechanical behavior of quasicrystalline alloys is to a great extent determined by a brittle-to-ductile transition at about 70% of the absolute melting temperature, it is useful to discuss the mechanical properties with reference to appropriately defined low-temperature and high-temperature regions.

Surface Properties of Quasicrystals

Abstract: There is currently great interest in the surface reactivity of quasicrystalline materials, generated largely by a model, proposed by Janot, for the bulk atomic and electronic structure. This {open_quotes}hierarchical cluster{close_quotes} model predicts that quasicrystal surfaces should be intrinsically inert and rough, and is being used to rationalize their practical properties such as low friction coefficients and oxidation resistances Surface structure and surface preparation may play a role in the applicability of this model. In this talk, we examine these factors and present experimental measurements of the surface reactivity of Al-based icosahedral alloys. We make some comparisons with surface reactivity of pure, crystalline aluminum, and with that of crystalline alloys which are similar in composition to the quasicrystal.

Modeling of Powder Compaction: A Review

Abstract: The compaction process involves stress transmission via rigid or flexible (die) walls and the propagation of stresses within a powder mass. The particles that comprise the powder distribute the stress by a variety of kinematic processes that involve sliding, rotation, particle deformation, and rupture. In practice the “particles” are often agglomerates of finer particles that have a range of properties. All of these factors must be considered in developing a comprehensive predictive model for compaction.The modeling of powder-compaction processes has a significant history that has been greatly advanced by the relatively recent general availability of powerful computers and their peripherals as well as by appropriate softwares. Compaction modeling may attempt to provide a basis for machine-loading specifications, or it may provide guidelines to help minimize “capping” defects where failure cracks form at the top of the green compact. It may also provide “green-body heterogeneity” through predicted stress and density distributions within a compact. Likewise compaction models may be combined with binder burnout and sintering models to predict internal microstructural features such as grain size and porosity, and the external shape of the sintered product. This article will deal only with the modeling of the compaction process; important elements such as powder flow for die filling and subsequent processing steps such as sintering and net shape predictions are not directly addressed.

Fluorinated Amorphous Carbon as a Low-Dielectric-Constant Interlayer Dielectric

Abstract: Low-k organic polymers such as polytetrafluoroethylene (PTFE) are promising materials for use as interlayer dielectrics (ILD) because their dielectric constants are generally lower than those of inorganic materials. However poor adhesion with Si substrates, poor thermal stability, and production difficulties have hindered their use in microelectronics. On the other hand, plasma-enhanced chemical vapor deposition (PECVD) of polymer films (plasma polymerization) has many advantages that help to overcome these problems. Plasma-enhanced chemical vapor deposition uses a glow discharge to create activated species such as radicals and ions from the original monomer, and the polymer films are deposited through various gas-phase and surface reactions of these active species, including ablation of the deposited film. No water is generated during plasma polymerization, and the influence of a solvent can be ignored. Also a layered structure that promotes adhesion can be easily fabricated by changing the source compounds. Recently the use of fluorinated amorphous carbon thin films (a-C:F) as new low-dielectric-constant interlayer dielectrics has been proposed. These thin films have an amorphous C–C cross-linked structure (including sp 3 and sp 2 bonded carbon) and have the same C–F bonds found in PTFE. The strong C–F bonds decrease the dielectric constant, and the C–C crosslinked structure maintains the film's thermal stability. The a-C:F film can be deposited from fluorocarbon source materials using PECVD. Typically fluorocarbons such as CF4, C2F6, C4F8, and their hydrogen mixtures are used as source materials. First the a-C:F films for low-k ILD, with a dielectric constant of 2.1, were deposited from CH4 + CF4 mixtures by using parallel-plate PECVD.

Fundamentals of SiC-Based Device Processing

Abstract: Since the commercial availability of SiC substrates in 1990, SiC processing technology has advanced rapidly. There have been demonstrations of monolithic digital and analogue integrated circuits, complementary metal-oxide-semiconductor (CMOS) analog integrated circuits, nonvolatile random-access memories, self-aligned polysilicon-gate metal-oxide-semiconductor field-effect transistors (MOSFETs), and buried-channel polysilicon-gate charge-coupled devices (CCDs). In this article, we review processing technologies for SiC.OxidationA beneficial feature of SiC processing technology is that SiC can be thermally oxidized to form SiO2. When a thermal oxide of thickness x is grown, 0.5x of the SiC surface is consumed, and the excess carbon leaves the sample as CO. Shown in Figure 1 are the oxide thicknesses as a function of time for the Si-face and the C-face of 6H-SiC, and for Si. The oxidation rates are considerably lower for SiC than for Si. The oxidation rate of the C-face of 6H-SiC is considerably greater than that of the Si-face. Hornetz et al. have shown that the reason for the slower oxidation rate of the Si-face is due to a 1-nm Si4C4−xO2 (x < 2) layer that forms between the SiC and the SiO2 during oxidation of the Si-face. When oxidizing the Si-face, the Si atoms oxidize first, which inhibits the oxidation of the underlying C atoms that are 0.063 nm below the Si atoms. When oxidizing the C-face, the C atoms readily oxidize first to form CO, with no formation of the Si4C4−xO2 layer for temperatures above 1000°C.