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.

While polymers and organic materials have been known and utilized broadly for many decades, finding such materials with the intrinsic properties of semiconductors and metals is a relatively recent phenomenon. The report in 1977 about doping polyacetylene to achieve relatively high conductivity opened up important new vistas for chemistry and physics, and for technology in general. This report recognized that a key feature of the electronic polymers and organic materials was a backbone consisting of alternating single and double bonds resulting in a “π-conjugated network.” This in turn led to a relatively small energy gap, enabling the appearance of both semiconducting and metallic properties.

Initially these polymers were unstable in air and not readily processed. Over the past decade, major advances have occurred in the synthesis of new forms of conducting and semiconducting polymers that enable processing under a broad range of conditions including organic solvents, inorganic solvents, and aqueous media. There are even meltprocessable versions of some of the electronic polymers.

A prime focus of the field has been the determination of the mechanisms for charge conduction and the intrinsic conductivity of these fascinating materials, especially doped polymers. In the past decade, interest has increased in the semiconducting (generally undoped) forms of these polymers and organic materials, including their photophysics and their use in a wide variety of devices. The reports of light-emitting devices fabricated from molecular and oligomeric constituents in the mid-1980s and from polymeric constituents in 1990 stimulated interest in this area of research.

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 Jöns 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 Linköping, 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.

One of the dominant trends in current research in materials science and related fields is the fabrication, characterization, and application of materials and device structures whose characteristic feature sizes are at or near the nanometer scale. Achieving an understanding of—and ultimately control over—the properties and behavior of a wide range of materials at the nanometer scale has therefore become a major theme in materials research. As our ability to synthesize materials and fabricate structures in this size regime improves, effective characterization of materials at the nanometer scale will continue to increase in importance.

Central to this activity are the development and application of effective experimental techniques for performing characterization of structural, electronic, magnetic, optical, and other properties of materials with nanometer-scale spatial resolution. Two classes of experimental methods have proven to be particularly effective: scanning-probe techniques and electron microscopy. In this issue of MRS Bulletin, we have included eight articles that illustrate the elucidation of various aspects of nanometer-scale material properties using advanced scanningprobe or electron-microscopy techniques. Because the range of both experimental techniques and applications is extremely broad—and rapidly increasing—our intent is to provide several examples rather than a comprehensive treatment of this extremely active and rapidly growing field of research.

To take advantage of the superior properties of SiC-based devices, the industry must be able to produce high-quality SiC homoepitaxial films on single-crystal SiC wafers. Silicon- and GaAs-based semiconductor devices have taken advantage of the industry's ability to produce largediameter, high-quality substrates to develop large-volume, low-cost devices. For the same reasons, the development of low-cost, high-quality single-crystal SiC substrates has been pursued since the first Lely (nonseeded growth process) platelets were used to make devices. Currently several groups (ATMI, USA; Northrop Grumman Research Center, USA; Okmetic Ltd., Finland; Nippon Steel Corporation, Japan; and our company, Cree Research, Inc., USA) are working toward SiC substrate fabrication. University researchers also play an important role in the search for solutions to fundamental outstanding issues. These efforts are important because the future success of the SiC industry is closely related to the successful development of the SiC bulk growth process.

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.

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.

As characteristic dimensions in semiconductor devices continue to shrink and as advanced heterostructure devices increase in prominence, the ability to characterize structure and electronic properties in semiconductor materials and device structures at the atomic to nanometer scales has come to be of outstanding and immediate importance. Phenomena such as atomic-scale roughness of heterojunction interfaces, compositional ordering in semiconductor alloys, discreteness and spatial distribution of dopant atoms, and formation of self-assembled nanoscale structures can exert a profound influence on material properties and device behavior. The relationships between atomic-scale structure, epitaxial growth or processing conditions, and ultimately material properties and device behavior must be understood for realization and effective optimization of a wide range of semiconductor heterostructure and nanoscale devices.

Cross-sectional scanning tunneling microscopy (STM) has emerged as a unique and powerful tool in the study of atomic-scale properties in III-V compound semiconductor heterostructures and of nanometer-scale structure and electronic properties in Si micro-electronic devices, offering unique capabilities for characterization that in conjunction with a variety of other, complementary experimental methods are providing new and important insights into material and device properties at the atomic to nanometer scale. In this article, we describe the basic experimental techniques involved in cross-sectional STM and give a few representative applications from our work that illustrate the ability, using cross-sectional STM in conjunction with other experimental techniques, to probe atomic-scale features in the structure of semiconductor heterojunctions and to correlate these features with epitaxial-growth conditions and device behavior.

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.

The science of materials as a specific discipline is a relatively modern development. Within this rather modern field, the study of semiconductor materials is an even more recent development. Modern textbooks on materials science focus primarily on the properties of metals and second on the properties of ceramics, reflecting the commercial importance of these materials 50 years ago when the transistor was demonstrated using germanium. Today the commercial importance of semiconductor materials is comparable to that of metals and ceramics, and the advances in semiconductormaterials technology are driving rapidly growing business sectors in communications, software, and biotechnology.