National Academies Press: OpenBook

Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology (1999)

Chapter: 1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology

« Previous: Overview
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 31

1
Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology

Important and unexpected discoveries have been made in all areas of condensed-matter and materials physics in the decade since the Brinkman report.1 Although these scientific discoveries are impressive, perhaps equally impressive are technological advances during the same decade, advances made possible by our ever-increasing understanding of the basic physics of materials along with our increasing ability to tailor cost-effectively the composition and structure of materials. Today's technological revolution would be impossible without the continuing increase in our scientific understanding of materials, phenomena, and the processing and synthesis required for high-volume, low-cost manufacturing. The technological impact of such advances is perhaps best illustrated in the areas of condensed-matter and materials physics discussed in this chapter, which will examine selected examples of electronic, magnetic, and optical materials and phenomena that are key to the convergence of computing, communication, and consumer electronics.

Technology based on electronic, optical, and magnetic materials is driving the information age through revolutions in computing and communications. With the miniaturization made possible by the invention of the transistor and the integrated circuit, enormous computing and communication capabilities are becoming readily available worldwide. These technological capabilities, which enabled the information age, are fundamentally changing how we live, interact, and transact business. Semiconductors provide an excellent demonstration of the strong

1National Research Council [W.F. Brinkman, study chair], Physics Through the 1990s, National Academy Press, Washington, D.C. (1986).

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 32

interplay between and interdependence of science and technology. Perhaps in no other area are advances in technology more closely linked to advances in understanding.

This chapter is not intended to be comprehensive; rather, it seeks to illustrate the pivotal role of condensed-matter and materials research in providing the understanding required to develop enabling technologies. At the same time, the development of these new technologies has greatly expanded the tools and capabilities available to scientists and engineers in all areas of research and development, ranging from basic research in physics and materials to other areas of physics and to such diverse fields as medicine and biotechnology. The examples discussed also make evident the importance of long-term, sustained research in realizing the benefits to society of improved scientific understanding of materials (see Figure 1.1).

image

Figure 1.1
Incorporation of major scientific and technological advances into new 
products can take decades and often follows unpredictable paths. 
Illustrated here are some selected technologies supported by the 
foundations of electronic, photonic, and magnetic phenomena and 
materials. These technologies have enabled breakthroughs in 
virtually every sector of the economy. The two-way interplay 
between foundations and technology is a major driving force in 
this field. The most recent fundamental advances and technological 
discoveries have yet to realize their potential.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 33

Although technological advances today are most often associated with the information age or communications and the computing revolution, impressive advances continue to be made across a broad spectrum of technologies and scientific disciplines (see Box 1.1). For example, progress in condensed-matter and materials physics has led to advances in biology, medicine, and biotechnology. New tissue diagnostics based on diffusing light probes use understanding borrowed directly from the physics of carrier transport in mesoscopic random materials. The development of new optical microscopies, such as two-photon confocal, optical coherence, and near-field optical microscopy, together with the widespread use of optical tweezers, have started a revolution in the observation and manipulation of submicrometer-sized objects in cell biology, in new forms of spectroscopic endoscopy, and in gene sequencing techniques. The emergence of high-power solid-state lasers and solid-state detectors and the widespread use of fiber optics make new optical approaches for diagnostics, dentistry, and surgery increasingly easy. A new form of magnetic resonance imaging enabled by semiconductor laser pumping of spin-polarized xenon gas has allowed the three-dimensional mapping of lung function. The generation of femtosecond pulses of light by the use of new solid-state lasers has begun another revolution in our understanding of the subpicosecond dynamics of biological molecules on the important frontiers of molecular signal processing and protein folding. Although not covered in detail here, such advances in the use of optics in medicine and biology are discussed in detail in another National Research Council report.2 In addition, semiconductor and other solid-state lasers or enhanced solid-state detector arrays, offshoots from condensed-matter physics, are enabling major advances in the fields of atomic and molecular physics, physical chemistry, high-energy physics, and astrophysics. New optical materials and phenomena are also responsible for a number of advances in the technologies associated with printing, copying, video and data display, and lighting.

In the realm of magnetic materials, the loss of cobalt in the 1980s because of political unrest in Zaire prompted an intense research effort to find cobalt-free bulk magnetic materials. This led to major advances in creating magnetic structures from neodymium and iron, which had superior properties and lower cost compared with cobalt alloys for electric motors and similar applications requiring magnets with high permanent magnetization. These new magnets, which are achieved through complex alloys and even more complex processing sequences, are vastly expanding the industrial use of bulk magnetic materials.

Advances in magnetic materials and their applications are not limited to bulk materials with high permanent magnetization and magnetic materials used in information storage. Improvements in soft bulk magnetic materials play an important role in transformers used in the electric power distribution industry. In-

2National Research Council [C.V. Shank, study chair], Harnessing Light: Optical Science and Engineering for the 21st Century, National Academy Press, Washington, D.C. (1998).

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 34

BOX 1.1 The Science of Information-Age Technology

The predominant semiconductor technology today is the silicon-based integrated circuit. The silicon integrated circuit is the engine that drives the information revolution. For the past 30 years, this technology has been dominated by Moore's Law: that the density of transistors on a silicon integrated circuit doubles about every 18 months.* Moore's Law articulates the increased functionality-per-unit cost that is the origin of the information revolution. Today's computing and communications capability would not be possible without the phenomenal 25 to 30 percent per year exponential growth in capability per unit cost since the introduction of the integrated circuit in about 1960. That sustained rate of progress has resulted in high-density memories with 64 million bits on a chip and complex, high-performance logic chips with more than 9 million transistors on a chip. This trend is projected to continue for the next several years (see Figures 1.1.1 and 1.1.2).

If the silicon integrated circuit is the engine that powers the computing and communications revolution, optical fibers are the highways for the information age. Although fiber optics is a relatively recent entrant into the high-technology arena, its impact is enormous and growing. Fiber is now the preferred technology for transmission of information over long distances. There are already approximately 30 million km of fiber installed in the United States and an estimated 100 million km worldwide. In part because of the faster than exponential growth of connections to the Internet, optical fiber is being installed worldwide at an accelerated rate of

image

 

Figure 1.1.1 Computing power versus time in microprocessors. (Courtesy of Intel.)

*Moore's Law, first articulated by Gordon Moore of Intel Corporation, is not a statement of physics. It is a statement that the industry will perform the R&D necessary and supply capital investment at the rate required to achieve this doubling rate.

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 35

(Box continued from previous page)

image

 

Figure 1.1.2 History of semiconductor technology.

more than 20 million km per year—more than 2,000 km per hour, or around Mach 2. In addition, the rate of information transmission down a single fiber is increasing exponentially by a factor of 100 every decade. Transmission at 2.5 terabits per second has been demonstrated in the research laboratory, and the time lag between laboratory demonstration and commercial system deployment is about 5 years. The analog of Moore's Law for fiber transmission capacity, which serves as a technology roadmap for lightwave systems, is shown in Figure 1.1.3. Figure 1.1.4 summarizes the history of optical communications technology.

Compound semiconductor diode lasers provide the laser photons that transport information along the optical information highways. Semiconductor diode lasers are also at the heart of optical storage and compact disc technology.

In addition to their use in very high-performance microelectronics applications, compound semiconductors have proven to be an extremely fertile field for advancing our understanding of fundamental physical phenomena. Exploiting decades of basic research, we are now beginning to be able to understand and control all aspects of compound semiconductor structures, from mechanical through electronic to optical, and to grow devices and structures with atomic layer control, in a few specific materials systems. This capability allows the manufacture of high-performance, high-reliability, compound semiconductor diode lasers that can be modulated at gigahertz frequencies to send information over the fiber-optic networks. High-speed semiconductor-based detectors receive and decode this information. These same materials provide the billions of light-emitting diodes sold annually for displays, free-space or short-range high-speed communication, and numerous other applications. In addition, very high-speed, low-power compound semiconductor electronics play a major role in wireless communication, especially for portable units and satellite systems.

Another key enabler of the information revolution is low-cost, low-power, high-density information storage that keeps pace with the exponential growth of corn-

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 36

BOX 1.1 Continued

image

 

Figure 1.1.3 Exponential growth in data transmission rate in fibers. (Courtesy of Bell Laboratories, Lucent Technologies.)

puting and communication capability. Both magnetic and optical storage are in wide use. Recently, the highest performance magnetic storage/readout devices have begun to rely on giant magnetoresistance (GMR), a phenomenon that was discovered by building on more than a century of research in magnetic materials. Although Lord Kelvin discovered magnetoresistance in 1856, it was not until the early 1990s that commercial products using this technology were introduced (see Figure 1.1.5). In the past decade, our understanding of condensed matter and materials converged with advances in our ability to deposit materials with atomic-level control to produce the GMR heads that were introduced in workstations in late 1997. It is hoped that with additional research and development, spin valve and colossal magnetoresistance (CMR) technology may be understood and applied to workstations of the future. This increased understanding, provided in part

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 37

(Box continued from previous page)

image

 

Figure 1.1.4 History of optical communications technology.

image

 

Figure 1.1.5 History of magnetoresistance. (Courtesy of IBM Research.)

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 38

BOX 1.1 Continued

by our increased computational ability arising from the increasing power of silicon integrated circuits, coupled with atomic-level control of materials, led to exponential growth in the storage density of magnetic materials analogous to Moore's Law for transistor density in silicon integrated circuits (see Figure 1.1.6).

image

 

Figure 1.1.6 ''Moore's Law'' for magnetic storage: logarithm of storage density versus time. (Courtesy of IBM Research.)

creases in the magnetic permeability and decreases in the losses by incremental improvements in amorphous metglass leads to decreases in the losses suffered in transmission and distribution. Magnetoelectronics, an emerging area based on advances in the understanding of the properties and processing of magnetic materials, shows promise for future applications.

Despite the numerous recent discoveries and technological advances in the understanding and use of magnetic materials, our fundamental understanding of magnetism remains remarkably incomplete. Some of the basic questions and important challenges in magnetism facing the scientific community are discussed in this chapter.

Electronic Materials And Phenomena

Materials and Physics That Drive Today's Technology
Silicon Technology

As noted in the introduction, semiconductor technology is the key enabler of the information age. The science of materials as a specific discipline is a relatively

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 39

modern development. The physics and materials science of semiconductors is an even more recent development. Metals and ceramics were commercially important materials when the transistor was demonstrated about 50 years ago. Despite the fact that the science of semiconductors is relatively new compared to that of metals and ceramics, the commercial importance of semiconductors is now comparable to that of metals and ceramics. Advances in semiconductor technology are driving the rapid growth of business sectors involved with computing, communications, consumer electronics, and software, and are enabling emerging fields such as biotechnology. Today's transistor performance and the incredible advances of integrated circuits in silicon technology are the result of more than 50 years of dedicated research in electronic materials. The understanding achieved from this focused research has enabled high-volume manufacturing of circuits with ever-increasing complexity and performance.

In addition to driving computing and communications, the steady decrease in cost-per-function has created literally hundreds of applications for silicon integrated circuits. Semiconductors are ubiquitous. Microprocessors are used almost everywhere today—from household appliances, to banking and smart cards, to automotive and aircraft control systems, automatic fuel injection engines, and cockpit instrumentation—and will be in even more applications tomorrow. The same sustained rate of progress that permitted the widespread application of semiconductors has created a global semiconductor industry with 1997 revenues of about $150 billion, supported by a materials and equipment infrastructure of about $60 billion. Semiconductor technology is also the heart of the $1 trillion global electronics industry and vital in many other areas of the approximately $33 trillion global economy.

The increasing functionality of integrated circuits, which comes as a by-product of scaling to smaller feature sizes, has been achieved by comparable increases in their complexity and that of the attendant manufacturing process. Today's leading-edge microprocessors are manufactured with minimum feature sizes of 250 nm and require six levels of metallization to connect the transistors and circuit components. A beneficial by-product of the steady decrease in feature size is higher speed devices and circuits. Based on technology projections that form the basis of the National Technology Roadmap for Semiconductors,3 the semiconductor industry expects to manufacture integrated circuits with feature sizes of 180 nm in 1999 and 150 nm by 2001. If the scaling trend continues as indicated by Moore's Law, which the industry has followed since its inception, integrated circuits with minimum feature sizes of 50 nm will be manufactured in high volumes within 15 years (see Box 1.2). Continuing to advance this technology requires that the industry invest in expensive new manufacturing facilities and an ever-increasing scientific understanding and control of semiconductor

3Semiconductor Industry Association, National Technology Roadmap for Semiconductors, SEMATECH, Austin, Tex. (1997).

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 40

BOX 1.2 A Brief History of Ion Implantation

In the early 1940s, the basic machines that were later adapted for ion implantation in the semiconductor business were used at Oak Ridge National Laboratory for uranium isotope separation. This was a critical part of the Manhattan Project. Ion beams were first used as part of semiconductor-device processing at Bell Laboratories in 1952. Bell filed a comprehensive patent in 1954 covering the use of ion implantation for doping semiconductors, but it was not until 1966 that implantation was actually used to manufacture commercial semiconductor devices.

Hughes Research Laboratory used the technique to form junctions in the manufacturing of diodes. In 1970 Texas Instruments began using ion implantation in integrated circuit manufacturing to set threshold voltages. Concurrent with these developments in processing, several companies attempted to enter the implant-tool manufacturing business with only moderate success, most successful among them being Accelerators Incorporated. In 1971, however, a new company, Extrion, was formed to build commercial implanters specifically designed for integrated circuit manufacturing. Extrion soon became the primary supplier of implant tools. This led to the development of a whole new industry in America.

Today, ion implantation is used in several steps of the integrated circuit manufacturing process to control the concentration and depth distribution of dopants. Ion implantation tool manufacturing, an almost exclusively U.S. industry, has grown to a more than $1 billion per year business. Three U.S. companies (Applied Materials, Eaton, and Varian) supply virtually all the commercial ion-implant systems worldwide.

materials and manufacturing processes. Conversely, our rate of understanding has been greatly enhanced by the technology created by the rapid advances in semiconductor-related technologies.

Many daunting scientific and engineering problems must be overcome in order to continue at the Moore's Law rate of progress for the next 15 or even 10 years. For instance, the number of wires needed to connect the transistors grows as a power of the number transistors. As transistor dimensions are shrunk, computer chip manufacturers pack an ever-increasing number of them into their devices. The complexity of wiring the transistors in these devices may eventually reach the limits of known materials. Moreover, the cost of manufacturing increasingly layered and complex wiring structures may limit the performance of these systems. Even if solutions to the interconnect problem can be identified, continued scaling of silicon technology will ultimately encounter fundamental limits. For example, metal-oxide semiconductor transistors can be built today with gate lengths of 30 nm (only about 150 atoms long) that display high-quality device characteristics. Manufacturing complex circuits that rely on devices with these feature sizes will require several hundred processing steps with atomic-level control. However, the performance of complex integrated circuits with tens of millions of transistors may be degraded because of nonuniform operating

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 41

characteristics. In time, continued decreases in device dimensions may result in the information being carried by an ever-decreasing number of charge carriers; ultimately, simple statistical fluctuations will limit the uniformity of device characteristics as the number of charges used to convey information decreases.

To delay this limitation as long as possible, research is under way on new materials with a high dielectric constant for memory applications or to limit leakage from tunneling currents. As understanding of synthesis and processing increase, ferroelectric materials are being introduced for nonvolatile memory applications. Even with these advances, as feature sizes continue to decrease, integrated circuits based on field-effect transistors will eventually encounter fundamental limits, such as interconnect delays caused by the ever-increasing number of interconnects, heat generation, or quantum limits of transistors that are too small to confine the electrons in the channels. Today's approach to the design and manufacture of integrated circuits will no longer be extendible to smaller feature sizes and higher densities. The fundamental limits of the current technology and our addiction to exponentially increasing computational power offer exciting scientific and engineering challenges in the search for the materials and device structures of the future.

Compound Semiconductor Technology

Compound semiconductors, which consist of more than one element, offer intrinsically higher speed and lower noise compared with silicon. These advantages have been exploited to develop very high frequency electronic devices and circuits for microwave and wireless communication applications.

The worldwide electronics market for compound semiconductors is estimated to be growing at about 40 percent per year and is expected to be about $1 billion in 2000. In addition to the high-speed microwave applications for which they have long been the materials of choice, discrete components are widely used in the low-noise receivers of telephone handsets. Compound semiconductors such as GaAs, AlGaAs, InGaAs, SiGe, GaN, and GaAlN are key to the development of next-generation wireless telephones, which will use higher frequency microwaves to transmit more information and allow more channels. GaN transistors, for example, have a high breakdown voltage and great robustness, although extensive research and development is required before the material can be understood and fabricated in a well-controlled fashion. Advancing the limits of semiconductor materials technology is essential for increasing the speed of transistors and advancing our ability to modulate light-emitting diodes and semiconductor lasers for high-speed optical information transmission. Because compound semiconductors are composed of more than one element, they offer a vastly increased range of materials from which to create structures with desired electronic properties.

The technology of modern compound semiconductor device fabrication is predicated on the ability to produce extended planar layers of uniform composi-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 42

tion and thickness. Band-structure engineering through complex heterostructures formed from combinations of compound semiconductors greatly increases the performance, potential applications, and research opportunities. Heterostructure devices such as heterojunction bipolar transistors, field-effect transistors, semiconductor lasers, or light-emitting diodes require the presence of epitaxial layers with different compositions and well-controlled thicknesses in the same device.

This promise can be realized with manufacturing techniques such as molecular beam epitaxy or metal-organic chemical vapor deposition, developed in the 1960s by materials physicists. These techniques allow atomic layer control in the growth of one material on another in single atomic layers to produce materials not found in nature. The use of novel forms of microscopy for fabrication and testing, and the development of comprehensive modeling techniques that take into account all of the materials physics and carrier dynamics of the structures, will determine our ability to design and build such structures on the atomic scale with feature sizes comparable to the quantum de Broglie wavelength of the electrons.

The ability to span the broad spectral region from ultraviolet to long-wave-length infrared was enhanced by the invention of strained-layer systems. Very high crystal quality layers can be grown for systems with different equilibrium atomic spacing, provided the thickness of the layer is less than the critical layer thickness at which detrimental dislocations nucleate and grow. Superlattices of such systems are called "strained-layer superlattices," whereas single layers sandwiched between two layers with identical lattice constant are called either "strained-quantum well" or "pseudomorphic structures." The recognition that alloys with different equilibrium lattice constants, such as GaInl-xAsx on GaAs, could be grown epitaxially to create strained layers opened up major opportunities for ''band-gap engineering." These so-called pseudomorphic layers removed the constraint that epitaxial materials must have equilibrium atomic spacings that match the atomic lattice spacings of the substrate. Recently, dislocation-free growth of a different lattice constant material has been obtained on a thin layer of compliant substrate that has been wafer bonded at an angle with respect to the bulk substrate lattice. Such strained thin layers offer an additional degree of control over the electronic band structure of the resulting artificially structured material.

The ability to create nearly ideal two-dimensional electron gases (2DEGs) through the growth of artificially structured materials with charge carriers confined to potential wells, and modulation doping, in which the dopant impurities that donate charge carriers are located far away from the potential wells that confine the charge carriers, allowed the development of very high performance, high electron-mobility transistors (HEMTs). Today's highest performance transistors, in terms of speed-power product and noise, are pseudomorphic HEMTs.

The ability to fabricate complex structures with atomic-level control permits research into fundamentally new structures for technology applications. One

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 43

example of a concept that is beginning to receive attention is that of quantum-state logic, in which a device can be switched between multiple states, in contrast to a field-effect transistor that is either "on" or "off."

The creation of heterostructures and quantum wells also led to the development of new tools to study the fundamental physical phenomena associated with these artificially structured materials, greatly increasing our understanding of their electronic band structure and optical properties. One of the recent examples is the use of ballistic electron energy microscopy to measure the energy band offsets in buried heterostructures (see Box 1.3).

The Science Underlying Semiconductor Microelectronics Technology

As noted earlier, for the past 30 years silicon technology has been dominated by Moore's Law: the density of transistors on a silicon integrated circuit doubles about every 18 months. This increased functionality-per-unit cost is the basis of the information revolution. The same technology that allows us to shrink the size of devices has allowed us to learn new physics. The synergy of technological development and new physics has been remarkably successful in the past few years, and it is difficult to anticipate the many new directions that synergy will take in the next decade or two.

The technology that fuels Moore's Law rests on the ability to make high-quality silicon field-effect transistors. In these transistors a metal gate confines electrons near the interface between silicon and a SiO2 gate oxide. As feature sizes decrease, quantum mechanical effects become observable. Combining the small feature size available with electron-beam lithography with very high quality artificially structured materials that can be grown in compound semiconductors led to the discovery of entirely new physical effects, such as the fractional quantum Hall effect (FQHE) discovered in two-dimensional electron systems. The elegant physics underlying the FQHE is discussed in Chapter 3.

To continue the increasing levels of integration beyond the limits mentioned above, new approaches and architectures are required. One of the alternative approaches that has received recent attention is quantum-dot (see Box 1.4) cellular automata (QCA). QCA is an approach that takes advantage of the quantum tunneling between dots to perform the operations that transistors perform. In today's digital integrated circuit architectures, transistors serve as current switches to charge and discharge capacitors to the required logic voltage levels. In QCA, logic states are encoded by the positions of individual electrons rather than by voltages. Such structures are scalable to molecular levels, and the performance of the device improves as the size decreases; artificially structured QCA cells studied to date operate only at low temperatures, but molecular-sized QCA cells would function at room temperature.

Electron-beam lithography was developed to make very small semiconductor devices. The short wavelength of electrons allows one to pattern structures with

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 44

BOX 1.3 Ballistic Electron Emission Microscopy

Ballistic electron emission microscopy (BEEM) is a variant of scanning-tunneling microscopy (STM) which probes, with nanometer lateral resolution, the subsurface electronic properties of materials. The technique was first developed in 1988 to investigate lateral variations of Schottky barriers formed at metal/semiconductor interfaces and has since been applied to a wide variety of materials systems. The study of semiconductor heterostructures and quantum structures buried beneath the Schottky barrier has shown particular promise because BEEM is able to access length scales smaller than those available to more traditional techniques.

In BEEM, an STM tip locally injects hot carriers through a thin metal layer and over the Schottky barrier without making direct contact. Carriers that are further transmitted through the heterostructure are collected by a third terminal usually located at the semiconductor substrate. As the tip is scanned or its bias is changed, changes in the collected BEEM current give spectroscopic information about the heterostructure, such as heights of band offsets or positions of quantized electronic states.

Figure 1.3.1 shows schematically a BEEM measurement of ~50 nm-diameter GaSb self-assembled quantum dots located ~7.5 nm below an Au/GaAs interface. Because the dots are close to the surface, their profiles can be seen in the STM topography, so that the tip can be located on and off the dot for comparison. Fig-

image

 

Figure 1.3.1 BEEM measurement of self-assembled quantum dots. [Reprinted from L.D. Bell and V. Narayanamurti, Current Opinion in Solid State and Materials Science 3, 38 (1998).]

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 45

(Box continued from previous page)

ure 1.3.2 shows concurrently scanned STM and BEEM images of a single dot. The BEEM image clearly shows a reduction in BEEM current through the dot, which shows the presence of a localized conduction band offset at the dot.

image

 

Figure 1.3.2 STM and BEEM images of a single dot. [Reprinted with permission from M.E. Rubin, H.R. Blank, M.A. Chin, H. Kroemer, and V. Narayanamurti, "Local conduction band offset of GaSb self-assembled quantum dots on GaAs," Applied Physics Letters 70, 1590 (1997). Copyright © 1997 American Institute of Physics.]

dimensions that are smaller than 100 nm. Such structures have been used as gates on submicron GaAs/AlGaAs devices (see Box 1.5), eliminating the 2DEG under them. In this way, confinement both in the plane and perpendicular to the plane of the 2DEG can be achieved. The simplest structure of this kind is a narrow constriction in the 2DEG that exhibits a resistance quantized in units of h/e2.

Electron-beam lithography can be used to make nanometer-sized metal wires and rings. This opened the field of mesoscopic physics: the study of systems that are larger than atoms but small enough that they are not bulk materials. In such mesoscopic systems, the wavelength of the carriers is comparable to the device dimensions and to the mean free path for phase breaking, and statistical averaging does not eliminate quantum mechanical phenomena. One dramatic phenomenon of this kind is universal conductance fluctuations.

Most mesoscopic effects for systems in one- or two-dimensional confinement are subtle. However, when electrons are confined in all three dimensions the results can be dramatic. Structures in which electrons confined to metals and semiconductors with tunnel junctions connecting the confined regions to the leads (essentially "artificial atoms") enhance the electron-electron correlations,

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 46

BOX 1.4 Transport in Quantum Dots

Quantum dots are semiconducting or metallic regions so small that the electrons are confined in all three dimensions. Like an atom, a quantum dot contains a finite number of charges and has discrete energy levels. The study of electron transport through these minuscule conducting regions has revealed a variety of fascinating phenomena including observable effects caused by individual electrons. Current versus voltage measurements for a quantum dot show discrete staircases where each successive plateau represents the addition of one electron to the quantum dot (see Figure 1.4.1).

image

 

Figure 1.4.1 Current versus voltage measurements for a quantum dot illustrating the discrete electronic states. Each successive plateau represents the addition of one electron to the quantum dot. (Courtesy of Massachusetts Institute of Technology.)

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 47

(Box continued from previous page)

Transmission through the dot can be measured by application of a bias voltage and measurement of the current (Figure 1.4.1, top). If, in addition, one steadily changes the potential of the dot by the application of a bias voltage to a gate electrode, a succession of resonance peaks are observed (Figure 1.4.1, bottom). The peaks occur when the energy levels in the quantum dot coincide with the Fermi level of the electrons in the leads.

Although the transmission probability (which has a shape independent of the number of electrons initially on the dot) is understood, these measurements did not provide information about the coherence of the tunneling electrons. Recent measurements of the phase of the electrons demonstrated that the tunneling was coherent. In the experiments, a semiconducting path around an insulating region penetrated by magnetic flux lines was constructed. This approach exploited the Aharonov-Bohm effect, in which an electron acquires a phase shift as it goes around a magnetic flux line. Interference was observed between the parts of the electron wave function that traveled on opposite sides of the flux line. Using a four-terminal configuration, the change in phase versus potential is measured as an electron enters the quantum dot and interacts with a quasi-bound state and leaves. The phase shift on both transmission and reflection were measured, yielding the unexpected result that the phase shifts are independent of the number of electrons initially on the quantum dot, a result that is not yet understood.

resulting in the quantization of both charge and energy. The energy levels of these artificial atoms can be measured by the voltage required to add an extra electron. These artificial atoms are large enough to display behavior that is not observed in natural atoms; for example, the superconducting energy gap in mesoscopic Al structures is quantized.

From the perspective of potential applications, single-electron transistors (SETs) (see Box 1.6) can be realized in systems with three-dimensional confinement provided by structures with two tunnel junctions and a gate. SETs turn on and off again every time an electron is added. This device not only functions as a transistor, but also provides insight into the physics of mesoscopic structures. Using the sharp peaks associated with the addition of an electron, the equilibrium ground-state energy of the droplet of electrons, as well as some low-lying excited states of the droplet, can be measured. Furthermore, application of a magnetic field reveals phase transitions between different states of the system. The magnetic field alters the balance between the confining potential, which favors a high electron density, and the Coulomb interaction, which favors a low electron density.

Based on recent successes in nanostructures, we can speculate about the kinds of nanostructures likely to yield new physics and technology. Three different physical effects in nanostructures that can be exploited for nanoelectronics are illustrated in Figure 1.2. In resonant tunneling, the probability for charge carriers to tunnel through barriers is greatly enhanced when the energy levels on

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 48

BOX 1.5 Double Electron Layer Tunneling Transistor (DELTT)

The double electron layer tunneling transistor (DELTT) is a recently developed quantum transistor that, unlike previous quantum transistors, does not require tight control over lateral dimensions and is thus easy to reproducibly fabricate in large numbers. It is based on the gate control of two-dimensional-two-dimensional (2D-2D) tunneling between the two electron layers in a double quantum well (QW) heterostructure, first investigated in 1990. Because the DELTT exhibits negative differential resistance, it is multifunctional, allowing the same circuit functions to be performed by fewer devices. It is also expected to be exceptionally fast, although such high speeds have yet to be demonstrated.

The source and drain terminals of the DELTT are formed by electrically contacting both QWs, and then locally depleting electrons (by gating or etching) from the QW one does not wish to contact (see Figure 1.5.1). A third gate terminal controls the tunneling between the two QWs. Close-proximity backgates—necessary for both good I-V peak-to-valley ratios and for small size and high speed—are achieved by an epoxy-bonding flip-chip technique that yields a total device thickness of less than one micron.

image

 

Figure 1.5.1 Schematic illustration of the DELTT. The energy band diagram of the double quantum well heterostructure is shown at left. [Reprinted with permission from J.A. Simmons, M.A. Blount, J.S. Moon, S.K. Lyo, W.E. Baca, J.R. Wendt, J.L. Reno, and M.J. Hafich, ''Planar quantum transistor based on 2D-2D tunneling in double quantum well heterostructures," Journal of Applied Physics 84, 5626 (Nov. 15, 1998). Copyright © 1998 American Institute of Physics.]

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 49

(Box continued from previous page)

Current in the DELTT flows only if both energy and momentum can be conserved in a tunneling event. Because both layers are two-dimensional, this is equivalent to the QW subbands being aligned. This can be achieved by applying a source-drain bias, a control gate bias, or both. Figure 1.5.2 shows source-drain I-V characteristics at several control-gate voltages for an AlGaAs/GaAs DELTT. Both the height and position of the resonant current peak are clearly controlled by the gate. Similarly good behavior has been obtained at 77 K, and bistable memories and digital logic gates have been demonstrated. Although obstacles remain, the DELTT shows excellent promise as a practical, room-temperature quantum transistor.

image

 

Figure 1.5.2 Source-drain current versus source-drain voltage for several values of top control gate voltage (VTC). Both the height and position of the resonant tunneling current peak is controllable by the gate. [Reprinted with permission from J.A. Simmons, M.A. Blount, J.S. Moon, S.K. Lyo, W.E. Baca, J.R. Wendt, J.L. Reno, and M.J. Hafich, "Planar quantum transistor based on 2D-2D tunneling in double quantum well heterostructures," Journal of Applied Physics 84, 5626 (Nov. 15, 1998). Copyright © 1998 American Institute of Physics.]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 50

BOX 1.6 Kondo Effect in an Artificial Atom

The analogy of a quantum dot to an artificial atom has been extended with the demonstration that a quantum dot interacts with nearby metallic leads in much the same way that a single magnetic impurity interacts with a surrounding metal—in the phenomenon known as the Kondo effect. Kondo behavior was found recently in a single-electron transistor, which consists of a semiconductor quantum dot sandwiched between two metallic leads. This miniature device turns on and off as individual electrons controlled by a nearby gate flow on and off the dot.

The theory of the Kondo effect was developed in the early 1960s to explain a long-standing puzzle about the resistance of some metals: Why does the resistance start to increase as the metal is cooled below a certain temperature? According to the picture that has emerged, the increased resistance comes from magnetic impurities whose local magnetic moments couple antiferromagnetically to those of the conduction electrons. The coupling becomes stronger and increasingly impedes the flow of current as the temperature is decreased.

The concept of the Kondo effect is intriguing because it involves the pairing of a localized electron with an electron in an extended state in the metal. Its manifestation in a quantum dot is no less compelling. Although interactions between electrons in quantum dots are known to be important, the Kondo phenomenon is a true many-body effect requiring a coherent state resulting from the coupling of the localized electrons in the dot and a continuum of electron states outside the dot.

Experimenters have tried to see a manifestation of the Kondo effect in quantum dots ever since its presence was predicted in the late 1980s, but succeeded only recently. Kondo behavior for a single spin had been observed in resonant tunneling through a charge trap created unintentionally in a point contact. A collaborative experiment involving the Massachusetts Institute of Technology (MIT) and the Weizmann Institute in Israel has attracted additional interest because it shows the Kondo effect in a way that will allow one to explore the phenomenon in a system with many tunable parameters.

Kondo-like effects in quantum dots are observable only under a very narrow set of conditions. To see the effects of coupling between the dot and the leads, one needs to make the rate for tunneling of electrons between the dot and the leads as high as possible. The higher this rate, the higher the temperature at which the Kondo effect survives. However, if one makes the rate too high, the electrons on the dot become completely delocalized. With a smaller dot, the electrons are more localized to begin with, and a higher rate is possible.

To make a semiconductor quantum dot, one starts with a two-dimensional electron gas of electrons confined in a plane at the boundary between two semiconducting materials. Additional semiconductor layers go on top of this boundary region. At the top of the structure, one lays down electrical gates; the electrical potentials created by these gates confine the electrons in the plane below the gates to a very small region. Typically the quantum dots lie 100 nm below the surface. The MIT-Weizmann team made a much smaller artificial atom by forming the two-dimensional electron gas closer to the surface.

The conductance of a single electron transistor displays a peak when the sum of the voltage (Vg), on one of the gates and of the voltage (Vds) between the two leads on either side of the dot, each multiplied by the appropriate capacitance, is large enough to add an electron to the dot. A gray-scale plot of the conductance

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 51

(Box continued from previous page)

(see Figure 1.6.1) therefore consists of a series of bright diagonal bands, marking the positions of the peaks, whose slopes are determined by the relative capacitances. The highest peaks occur where the bands intersect on the Vds = 0 axis. These maxima cluster in pairs along the Vds = 0 axis, with the intra-pair peak separation smaller than the inter-pair separation. (One pair is shown in the figure.) The two peaks correspond to the addition of a pair of electrons to the same spatial state; one electron enters the state with spin up and the other with spin down. The next electron must go into the next spatial state. Thus, in the region between the paired peaks the artificial atom has an odd number of electrons.

The peak structure described so far is that expected for any artificial atom. One tip-off in the data to the presence of the Kondo effect is the non-zero conductance between the paired peaks, the bright, narrow vertical line along the Vds = 0 axis. In this region the quantum dot has an unpaired electron, which is free to form a singlet with the electrons in the leads. This singlet state couples electrons from the

image

 

Figure 1.6.1 Evidence for the Kondo effect in a single electron transistor. (Courtesy of Massachusetts Institute of Technology and the Weizmann Institute.)

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 52

BOX 1.6 Continued

left lead to the unpaired electron on the dot and thence to the right lead, giving conductance in a region where none is ordinarily expected. As predicted by theory, this interpeak conductance increases as the temperature is decreased.

If the enhanced conductance that appeared between the two peaks were due to the Kondo effect, it would require a symmetric interaction of the unpaired electron on the quantum dot with electrons in both leads. But if one applies a voltage Vds across those two leads, separating the Fermi energy levels of the two reservoirs, that interaction is no longer symmetric, and the conductance must fall. Another signature of the Kondo effect is the disappearance of the enhanced conductance as the voltage between the leads is increased, leading to the narrowness of the vertical line in the figure.

Finally, a magnetic field splits the unpaired electrons, causing the conductance peaks to split as well, by an amount equal to 2 BB. This signature is also observed.

the two sides of the barrier are identical. Large changes in the tunneling current are realized with small changes in the bias voltage across such structures. In structures that confine electrons to regions with dimensions comparable to the electron wavelength, quantum interference effects can be used to switch electronic currents. A conceptual approach to a transistor based on quantum interference is shown in Figure 1.3a.

Quantum confinement structures can be created that serve as electron waveguides, conceptually similar to the waveguides encountered in optical structures. Nanostructure switches based on guided-wave coupling can be created in quantum confinement structures, illustrated in Figure 1.2b. In these switches, illustrated in Figure 1.3b, electrons in input channel 1 (IN1) can either exit through output channel 1 (OUT1) or be switched to output channel 2 (OUT2) depending on the gate bias voltage.

image

Figure 1.2
Illustration of physical effects realizable in nanostructures: (a) resonant tunneling; 
(b) quantum confinement; and (c) Coulomb blockade. (Courtesy of Stanford University.)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 53

image

Figure 1.3
Schematic illustrations of nanoswitching concepts based on the 
physical effects illustrated in Figure 1.2. (Courtesy of Stanford University.)

In the Coulomb blockade structure illustrated in Figure 1.2c, adding an electron to a quantum dot creates a Coulomb field that repels the addition of another electron. A SET can be realized by placing the quantum dot between the input and output channels as illustrated schematically in Figure 1.3c. The transistor is switched between on and off by changing the voltage on the gate.

A number of novel circuits based on capacitively coupled arrays of artificial atoms have been proposed based on SETs; however, because the original SETs operated only at very low temperatures, the effort in this area was initially limited. The operating temperature scales with the energy required to add an electron to the artificial atom, which increases with decreasing size of the mesoscopic structure. Recently, nanometer-size SETs and single-electron memories (SEMs) have been demonstrated that have quantum dots sufficiently small to operate near room temperature (see Box 1.7), stimulating increased interest in these mesoscopic structures.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 54

BOX 1.7 Single-Electron Transistors and Memories

Single-electron transistors (SETs) rely on quantum dots of small semiconducting or metallic regions in which electrons are confined in all three dimensions. In a conventional field effect transistor, electrons (or holes) travel through a semiconducting region, known as a channel. The channel connects two electrically conducting contacts, known as the source and drain. A gate electrode located above the channel is used to control the flow of charge in the channel. In complementary logic, the channel is normally in a nonconducting state. Application of a voltage to the gate electrode increases the conductivity of the channel, allowing charge to flow across it; the transistor turns from off to on.

SETs can be made by replacing the channel between the source and drain in conventional devices with quantum dots surrounded by an insulator. The key is that the quantum dots are so small that, when an electron is on the island, Coulomb repulsion prevents other electrons from crowding on. Changing the gate voltage on the quantum dot aligns the energy level of the dot and the electrode, allowing the electron to tunnel to the electrode. Because the separation of the energy levels in the quantum dot increases with decreasing size of the dot, very small structures, analogous to artificial atoms with dimensions on the order on 1 to 4 nm, are required for room-temperature operation of a SET.

Single-electron memories (SEMs) are made using a variation on the SET. The crucial difference is that, in a SEM, the quantum dots sit between the channel and the gate electrode rather than replacing the channel as they did in the SET. A SEM structure based on silicon technology is shown in Figure 1.7.1. The source and drain are sufficiently separated from the quantum dots to prevent spontaneous electron tunneling. In the uncharged, or 0 state, there are no electrons on the quantum dot and a given geometry-dependent gate voltage allows current to flow between the source and drain. In the 1 state, an electron is injected into the quantum dot. When an electron is on the quantum dot, a different voltage is required to turn on the transistor because of the Coulomb field caused by the presence of the electron. Using quantum dots formed from polycrystalline silicon and standard integrated-circuit processing technology, researchers at Hitachi have recently fabricated 128-Mbit single-electron dynamic random access memories (DRAM) that operate at room temperature.

image

 

Figure 1.7.1 Single-electron memory structure based using a silicon quantum dot. (Courtesy of the University of Wisconsin.)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 55

Challenges, Priorities, and Frontiers of Electronic Materials and Phenomena

Based on the progress in scanning microscopies, we can speculate that the successor to electron-beam patterning for fabrication of nanostructures could be atom-by-atom assembly. In recent years, scanning-tunneling microscopes have been used to arrange atoms on surfaces and to measure changes in the energies of the surface electrons by tunneling spectroscopies. Scanning probes have been used to construct single-atom switches in which the movement of an atom from one position to another opens or closes a circuit created by assembling rows of atoms on a surface.

The inexorable pressure to reduce the dimensions of semiconductor devices has introduced to condensed-matter physicists and materials scientists the concept of self-organizing structures in the nanometer-size regime. Self-assembly promises a broader frontier of nanostructures. A self-assembled array of quantum dots can be grown either by strained-layer nucleation of islands between quantum wells during growth or by assembly of nanometer-sized dots grown by solution chemistry into a macrocrystal. Both approaches are discussed in detail in Chapter 4. Such nanocrystals are a different approach to forming artificial atoms analogous to the artificial atoms discussed above. A major research area will be learning how to control self-assembled materials to create ordered one-, two-, and three-dimensional structures. These self-organized structures will be technologically useful to the degree that their size and nucleation density can be controlled.

The committee concentrates, in this section, on the electronic properties of quantum structures, but photonic lattices are beginning to emerge and are discussed in more detail in the following section. In photonic lattices, photons are confined in arrays of structures with dimensions comparable to the wavelength of light in the medium. Combining such lattices of quantum wells with feature sizes comparable to the wavelength of electrons will lead to coupled electron-photon systems with new and interesting electronic and optical properties.

Numerous outstanding scientific and technological problems have been identified in the research in electronic materials. Beginning with silicon integrated-circuit technology, major materials-related technical questions include, What interconnect technology will be used beyond copper and low k (i.e., beyond normal metals and dielectrics) for silicon integrated circuits? and How do we manufacture SETs and SEMs at reasonable operating temperature and cost?

Recognizing that one cannot continue to scale silicon integrated circuits to smaller feature sizes indefinitely raises the question, What is beyond silicon? Additional questions that need to be investigated to address this question are the following: How can self-assembled materials be controlled to create the desired one-, two-, and three-dimensional structures? How does one create hybrid structures that exploit the best properties of, e.g., organics or plastics and semiconduc-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 56

tors, magnetic materials and semiconductors, or superconductors and semiconductors? How do we understand and exploit quantum state logic?

To continue to advance our fundamental knowledge, and to use this knowledge to continue to advance technology based on electronic materials, the committee offers the following recommendations:

• Perform the long-range research required to allow the semiconductor industry to follow Moore's Law and maintain its historical rate of productivity improvement.

• Accelerate research into materials, structures, and technologies that will go ''beyond silicon," i.e., to discover what technology will be used after today's silicon integrated-circuit technology reaches its limits.

• Expand the research efforts in self-assembled materials to create structures that promise needed technologies.

• Continue to develop the processing and characterization tools required to create and evaluate ever more-complex, ever-smaller, artificially structured materials.

Optical Materials And Phenomena

Materials and Physics That Drive Today's Technology
Optical Communications

As mentioned in the introduction, fueled by the explosion of Internet use and the globalization of voice and data communications, lightwave communication systems capacity and installation are growing exponentially with a growth rate of about a factor of 10 every 6 years. The current global market for lightwave systems is about $8 billion per year and is expected to grow to about $15 billion per year by 2000. Optical telecommunication was introduced into the market in 1980; today, not only is optical fiber the medium of choice for long-distance voice and data communications, but it is also rapidly growing to be a leading player in the local area network (LAN) market. Optical fiber is predicted to have revenues of about $20 billion in the year 2000 and dominate the analog cable television and fixed wireless loop markets.

The first undersea optical cable was installed in 1988, with a capacity of about 8,000 voice circuits per cable, at a cost of about $400 per circuit per year. More than 300,000 km of undersea lightwave cable had been installed by the end of 1996. Cable installed in 1996 cost less than $30 per year per voice channel and had a capacity of 120,000 voice channels per cable (5 Gb/s per fiber).

The first major terrestrial lightwave system installed in the United States linked Washington and New York with a capacity of 90 Mb/s per fiber in 1983. A similar system linked New York and Boston in 1984. More than 100,000 km of fiber had been installed in terrestrial systems, one-third of it in the United

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 57

States, by the end of 1996. The latest systems incorporate wavelength division multiplexing (WDM) that uses many separate wavelength channels per fiber, dispersion-shifted fiber, and optical amplifiers. Currently in deployment are 120-Gb/s-per-fiber systems using 48 channels with 2.5 Gb/s per channel. In the next 2 years 400-Gb/s, 80-channel systems will be introduced into the market.

These advances in technology were made possible by advances in our understanding of materials and growth techniques that reduced the transmission losses of silica-germania optical fiber from 400 dB/km in 1965 to about 0.15 dB/km by the early 1990s. Such losses allow a signal to travel 800 km (about the distance between Washington and Boston) before the signal intensity decreases to about 1/100 of its original value. These advances were accompanied by recent major advances in InP-based electrically modulated single-wavelength semiconductor diode lasers operating in the 1.3-µm and 1.5-µm wavelength regions, where the lowest loss in silica fiber occurs; in fast avalanche photodiode detectors; in erbium-doped fiber amplifiers and other fiber devices (see Box 1.8); and in high-power semiconductor diode lasers used to pump the fiber devices.

Current digital optical telecommunications networks typically use the NRZ (non-return-to-zero) format to transmit data in the linear amplitude regime. Future systems could use nonlinear effects in fiber with high-power lasers to exploit the properties of soliton transmission.

Solitons are wave packets that propagate without changing shape. They are solutions to the electromagnetic wave propagation equation in fiber waveguides that arise from the nonlinear effects of self-phase modulation and dispersion in the group velocity. Solitons are dispersion-free and exhibit a pulse shape that retains its waveform over long distances because the two nonlinear effects are exactly counterbalanced. They were first proposed as a means of data transmission in optical fiber in the 1970s and observed in the research laboratory in 1980. They offer extremely high bit-rate transmission (>100 GHz) at a single wavelength. Extensive research on the use of other nonlinear effects in both fibers and semiconductors and in artificially poled piezoelectric materials is under way to enable future ultrahigh-speed all-optical processing devices.

Local area networks (LANs), optical data links, and optical signal processing are emerging growth areas enabled by new technologies such as vertical-cavity surface-emitting lasers, smart pixels and microelectromechanical systems. All of these technologies were implemented as devices within the past decade. The emergence of low-loss graded index multimode plastic optical fibers in the past 5 years could lead to a low-cost medium to deliver high bandwidth communications over short links from a single-mode glass fiber backbone to the desktop. The advantage of extremely low-cost connectors and low-cost transceivers could outweigh the high cost of fluorinated polymer materials compared with the cost of glass fiber based LANs. The predicted annual market for optical data links is $1 billion by the year 2000 and $3.3 billion by 2005, with approximately half in computers and half in LANs.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 58

BOX 1.8 Fiber Devices

The revolution in optical communications over the last decade began with the invention of the erbium (Er)-doped optical-fiber amplifier in the late 1980s. With the invention and implementation of a number of key optical-fiber devices, evolution of an all-optical network architecture has begun. Fiber gratings were first made in 1975. The technological revolution in fiber devices was enabled by the discovery in 1993 that when exposed to ultraviolet (UV) light, an index of refraction change as large as 0.01 occurs in the cores of silica fiber doped with hydrogen-loaded germania. This UV-induced irreversible chemical change permits stable fiber Bragg gratings to be easily written into the cores of standard optical fiber. These Bragg gratings serve as key building blocks for a large range of both active and passive fiber devices such as filters, amplifiers, fiber lasers, dispersion compensators, pump laser reflectors, demultiplexers, and equalizers.

The optical power in communications systems increased sharply with the introduction of the Er-doped fiber amplifier. This amplifier is basically a single-pass laser consisting of several meters of a spliced silica fiber doped with 1,000 ppm Er3+ in the core with an input coupler for the pump light. Optical amplification can be achieved through stimulated emission from the excited states of Er atoms in the glass if a population inversion is created with pump light from a semiconductor diode laser.

The optical properties of rare-earth impurities in a glass matrix were first studied in the 1960s. Rare-earths are ideally suited for use as an amplification and lasing medium; they have strong optical transitions in the infrared and their properties are nearly independent of the host material. Er, pumped with 1488-nm or 980-nm light, is ideal for an amplifier in the 1.55-µm communications window.

Optical amplification has many advantages over electronic regeneration: amplification occurs over a relatively wide (80-nm) gain curve, ideal for dense wavelength division multiplexing systems; amplification is transparent—i.e., independent of modulation format and bit rate—and in principle the gain is bidirectional. It also allows watts of optical power, which is important for higher data rates of wavelength division multiplexing, increased passive split architectures where one source is split into many channels, and extended repeater spacings. Systems with optical amplification are far simpler to upgrade to higher bit-rate systems after initial installation because all optical repeaters are independent of bit rate. Er-fiber amplifiers were first deployed in undersea communications systems in the mid-1990s.

Because silica fiber has more wavelength dispersion in the 1.55-µm region than at its minimum in dispersion at 1.31 µm, additional dispersion compensating fibers were installed at intervals in the system. New fiber designs have shifted the dispersion minimum to 1.55 µm. New installations use this fiber to minimize the effects of dispersion at high data rates. In the 1.31-µm optical communications window used by most of the installed terrestrial base, amplifiers using praseodymium (Pr) ions in fluoride glass hosts (because Pr in silica does not emit at this wavelength) and Raman-shifted silica fiber amplifiers have been recently demonstrated in the research lab.

Fiber lasers using rare-earth ion dopants and fiber Bragg gratings as cavity mirrors were demonstrated in the early 1990s in both silica and fluoride fiber hosts. A cascaded Raman-shifted laser was demonstrated in 1990 in standard silicagermania fiber. Raman-shifted lasers eliminate the need for specialty fiber doped

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 59

(Box continued from previous page)

with rare-earths. Figure 1.8.1 shows a block diagram of a Raman-shifted laser operating at 1.3 µm. The laser consists of five stages of amplification of successively Raman-shifted light. Each stage is a cavity defined by its own tuned set of fiber Bragg grating mirrors. The Raman shift of the light in each stage is the inelastic scattering of the high-intensity light in the laser cavity from the phonons of the silica core. Higher conversion efficiencies and output power were recently obtained by sending the pump light into an optimally designed cladding that couples efficiently to the single mode waveguide core.

image

 

Figure 1.8.1 Block diagram of a Raman amplifier at 1.3 µm. (Courtesy of Bell Laboratories, Lucent Technologies.)

Future demand for broadband communications is expected to drive deployment of fiber-to-the-home communications access systems that will bring GHz bandwidths from the central office to each home. Over the next 20 years the predicted expenditure on deployment of wideband access in the United States is $150 billion. Predicted expenditures on access infrastructure globally is a factor of 3 to 5 more. The limiter for both the technology and deployment of access is low-cost, reliable components, which poses major challenges for materials and device physics. For example, lasers that can operate at elevated temperatures without active cooling are necessary, as are passive optical distribution systems and low-cost upstream communication devices such as optical modulators.

Multiple wavelength optical transmission systems for long-distance networks (DWDM, or dense wavelength division multiplexing), which use sophisticated integrated optoelectronic devices and waveguide circuits such as multiwavelength lasers, optical routers, and all optical cross-connects will be rapidly deployed. These technologies are currently in their infancy compared to their silicon microelectronics counterparts.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 60

Optical Data Storage

The data storage market, fueled by the insatiable appetite of consumers for generating, collecting, and storing information, is growing exponentially. The total number of stored bits is expected to exceed 1020 by the year 2000 with the total data storage market about $100 billion. Of that, the market for optical storage is projected to be about $12 billion. In addition, the enormous consumer entertainment electronics market of digital audio and video storage will add another $20 billion in compact disk (CD) and digital video disk (DVD) technology sales and $40 billion in media sales.

The essential idea of optical storage is to store bits by forming pits or spots on a reflective surface. A spot is read by shining a focused laser beam onto the disk and measuring the intensity of the reflected light. Optical disk technologies can be classified by the number of times each portion of the surface can be written. Read-only disks, such as the ubiquitous audio CDs and computer compact-disk read-only memories (CD-ROMs) introduced in the early 1980s, are manufactured with data already recorded as pits stamped from a master onto the polycarbonate disk surface, which is subsequently aluminized and covered with a protective transparent layer. Write-once disks (also called WORM, for write once, read many) were introduced in the late 1980s. They are manufactured in an erased condition, and each sector can be written once by burning pits into the surface, such as with a laser in the disk drive. Read-write disks, also called rewritable disks, were introduced in the mid-1990s. They record data via reversible effects such as light absorption inducing a phase change between amorphous and polycrystalline states (phase-change, PC) or by flipping the direction of a macroscopic magnetic polarization in an optical medium in conjunction with an applied magnetic field (magnetooptic, MO). In PC drives data are read by monitoring the reflectivity change between an amorphous and crystalline spot, while in MO drives data are read by detecting the polarization rotation of a laser beam as it traverses the thickness of the medium.

A key enabler for today's optical recording technology was the invention of the compound semiconductor laser diode in the mid-1960s. Also necessary was the subsequent development of these tiny diode lasers into low-cost, robust, reliable, relatively high-power devices. Today's diode lasers are about the size of a grain of salt, compared to table-top-size gas-ion lasers that require expensive parts, water cooling, and high-voltage, high-current operation. Today's optical drives use near-infrared GaAlAs diodes that operate at 780 nm and 830 nm. Next-generation DVD technology will migrate to AlInGaP lasers operating near 530 nm (yellow) around 1999. Other enablers were PC media based on binary and ternary chalcogenides and MO media based on thin films of magnetic FeTb and CoPt alloys. These alloys, invented by materials physicists in the 1960s and 1970s, are still undergoing intense materials development.

Currently a typical optical drive has a slower positioning time, by a factor of

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 61

3, than a magnetic disk drive, a data transfer rate 4 to 10 times slower, and a cost per gigabyte about 5 times higher. However, the platter of an optical drive is typically removable and costs about a factor of 10 less than a typical magnetic disk of equivalent capacity. Optical drives occupy niche markets of data back-up and distribution rather than the far larger market of main drives in computers. This latter market is dominated by magnetic drives (discussed below). Future improvements in optical storage technology could potentially allow higher density optical storage with access speeds similar to those of magnetic drives, and thus become a displacement technology for magnetic storage. Development is under way in four areas for discontinuous improvements in the speed or data density of optical drives.

One development to improve storage density involves focusing a laser beam into the desired storage layer of a three-dimensional sandwich produced by stacking semitransparent layers. Data storage density is thereby increased by as much as a factor of 10.

Also under development are shorter wavelength lasers, permitting smaller spot sizes. Extensive materials research is required to make low-cost high-reliability blue-green lasers commercially available (see Box 1.9).

The third emerging development is near-field optical recording. Flying a special design optical head very close to the storage medium allows write and read spots smaller than the wavelength of laser light. This direct outgrowth of condensed-matter and materials physics and optical physics was initiated in 1928 when British scientist E.H. Synge proposed the physics of near-field optics for microwaves. The first near-field visible optical microscope was not constructed until the late 1980s, after the invention of the scanning-tunneling microscope (STM) in 1981. The STM created the technology required for scanning a small tip in a controlled manner a specified distance away from a surface with atomic-scale resolution. Near-field technology has the potential for data storage density one to two orders of magnitude higher than conventional optical and magnetic storage projections to the year 2000. Another potential advantage of this technology is the ability to use very low mass optical heads mounted directly onto sliders that have been developed for magnetic storage to reduce seek times.

The fourth development on the horizon is holographic data storage. In holographic storage a page of binary data is stored as pixels of a monochrome image. It is possible to record thousands of holograms in a spot of a storage medium with resolution on the order of the wavelength of light. Because of the three-dimensional capability, holographic storage promises a projected density two to three orders of magnitude larger than conventional optical storage. In addition, it has extremely high data rates because an entire image is transferred simultaneously. Development of low-cost, reliable, blue-green lasers and solid-state spatial light modulators, along with low-cost, robust, and reliable storage media for three-dimensional holograms, are needed to enable commercialization of the technology.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 62

BOX 1.9 GaN Lasers

A major breakthrough in condensed-matter and materials physics in the last decade was the development of blue-green semiconducting laser diodes in the direct wide band gap GaN/AlN/InN quantum well materials system. Serious materials problems hampered research efforts concerned with the growth of quantum wells of these materials in the 1960s and 1970s, however: there was no bulk substrate crystal for lattice matching; high impurity concentrations during growth permitted growth of only very high concentration n-type material; and the relatively deep levels of p-type donors made it difficult to achieve p-type material required for current injection devices. By the mid-1970s most major U.S. industrial research efforts in this area were terminated because of the apparent intractability of these problems. Meanwhile, research in the much less robust II-VI (Zn,Cd,Mn/S,Se) materials revealed major difficulties caused by the migration of dislocation-induced dark-defect regions into the active laser area causing catastrophic damage.

Two groups in Japan, lead by Akasaki and Nakamura, continued to try to improve materials properties in the far more robust GaN system. A major breakthrough was reported in 1989 by the Akasaki group, who found p-type material after exposing as-grown GaN to an electron beam in an electron microscope. Soon thereafter Nakamura discovered that GaN grown by standard metalloorganic chemical vapor deposition was passivated by a high density of H impurities. With these breakthroughs in understanding, Nakamura was able to produce n-type material. By 1994 Nichia Chemical announced blue, and soon thereafter green, light-emitting diodes (LEDs) with extended color range. These early diodes had external quantum efficiencies higher than 5 percent, five times higher than the competing yellow-green lasers in the AlInGaP materials system. The high efficiencies of commercial LEDs in the AlGaAs and InGaN materials have opened up new markets in vehicle and brake running lights, and in highway status and traffic control signs. This market is now in excess of $1 billion per year. The addition of blue and green LEDs rounds out the visible spectrum and opens up new markets in long-life traffic lights and high efficiency, high brightness, white lighting systems. In 1996 these LEDs were already incorporated into commercial full-color displays. Figure 1.9.1 shows an outdoor full-color display incorporating blue and green InGaN LEDs and red GaAlAs LEDs by Arami Electric Co., Ltd. Several groups have subsequently demonstrated a blue-green laser in the GaN/AlInGaAs system, and a worldwide race is now under way to achieve a reliable continuous wave laser at room temperature.

(Box continued on next page)

Display, Printing, and Copying Technologies

Exciting recent developments in synthesis of semiconducting organic materials have enabled researchers to demonstrate a variety of optoelectronic devices based on electronically active organics. These include light-emitting diodes, thin-film transistors, photovoltaics, and nonlinear optical elements. The great potential for these devices resides primarily in the ability to process organics using cost-effective methods such as spin casting and screen printing, not in performance considerations. The potential to produce large-area devices and

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 63

(Box continued from previous page)

image

 

Figure 1.9.1 A black-and-white depiction of a full-color outdoor display incorporating blue and green InGaN LEDs and red GaAlAs LEDs. [MRS Bulletin 22, 30 (February 1997).]

patterns easily compatible with plastic substrates is the vision driving scientific investigation of semiconducting organics.

Application of organic materials in electrophotographic photoreceptors is already commercially successful. In that case, large-area molecularly doped polymer films with high-charge photogeneration yield have demonstrated extremely high contrast between photoactivated and dark conductivity. This property, combined with the ability to rigorously exclude deep-charge transport traps, has made organic materials superior for photocopying applications.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 64

The new classes of semiconducting organic materials fall into two categories: discrete evaporable molecular systems and conjugated polymers. The former tend to be compositionally pure and easily suitable for layering; the latter are more thermally stable and are usually amenable to solution processing. The most mature application of these materials is light-emitting diodes; and commercial low-resolution pixelated monochrome display products based on the evaporable organics are currently available (see Figure 1.4). It is likely that analogous products based on conjugated polymers will be available in 1999. Material stability, device efficiency, and low operation voltage remain important areas of research and engineering. Further progress in synthesis of systems resistant to oxidation and electrochemical degradation, synthesis of stable electron transport materials for the polymeric devices, preparation of materials where aggregation quenching of luminescence is negligible, and development of contact modification treatments that improve injection efficiency remains a challenge for organic chemists. Meanwhile, understanding injection and transport, which determine current voltage characteristics, is crucial. Identification and passivation of luminescence-quenching sites and study of the effects of high fields present in light-emitting diodes remain important areas for basic research. A number of

image

Figure 1.4
Flexible light-emitting diode display based on 
evaporated organic materials. (Courtesy of University 
of California at Santa Barbara.)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 65

systems issues associated with electrical drive scheme, color fidelity, and patterning must also be resolved if organic emitters are to have wide application in display technology.

The requirements for other promising applications of light-emitting organics such as printing and lighting are substantially the same. A flurry of activity directed at making electrically pumped organic lasers, however, has raised a number of additional issues. The current injection requirements are significantly larger than what is presently achievable, and improved transport will be required. Also, design of resonator geometries suitable for retaining these large injection currents, while avoiding absorption loss associated with metallic contacts, will be required. A significant body of clever work involving distributed feedback, photonic gap, and microdroplet resonators has begun to address this problem. Hot carrier effects in the organics may also become quite important at the applied electric fields necessary to drive lasers, and ''interband" emission analogous to that in inorganic semiconductors has been reported recently under conditions near dielectric breakdown. The high density of excited states required for stimulated emission gain has also spurred interesting new research on exciton-exciton interactions and cooperative emission phenomena. These rely on optical excitation of the organic films. It may turn out that the best way to make suitable solid-state organic lasers is a hybrid design using passive organic gain media that are optically pumped by electrically pumped inorganic semiconductor lasers such as those based on InGaN. For these applications, traditional laser dyes doped into inert polymers may be satisfactory.

Many of the scientific issues noted above are common to the application of organic semiconductors to transistor applications and photovoltaics. These transistor applications are likely to be further in the future because performance remains somewhat inferior to alternative technologies. Organic thin-film transistors that perform within an order of magnitude of amorphous silicon have been fabricated and may be promising as pixel-switching transistors in active-matrix liquid-crystal displays. Charge injection and transport are critical to these applications. Interface chemistry and physics at contacts remain poorly understood; control of these is essential to stable low-voltage operation. Exciting and promising results using dipole layers to reduce injection barriers have been reported. Dipole layers function much as graded gap contacts do in traditional semiconductors. Charge transport has been studied extensively, and deep traps are commonly observed that both reduce mobility for transistor applications and raise injection voltages resulting from space charge limitations on the current. Research to identify and reduce trap sites is also important to make these applications viable.

A final general class of promising applications involving active organics in electronics (see Box 1.10) relies on the conductivity of doped conjugated polymers. Doped trans-polyacetylene, although unstable in ambient conditions, has exhibited conductivities higher than metals. More stable systems, such as doped

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 66

polypyrrole, that are more easily processed and are more compatible with plastic than traditional systems like indium tin oxide, show promise for application as transparent contacts for displays.

We anticipate a variety of scientific opportunities, for which the ultimate technological implications are as yet unclear, to emerge from studying electronic polymers in the next decade. Some of these opportunities may come as a result of photochemically modifying single domains or arrays of domains with a near-field optical microscope, synthesizing materials with giant optical nonlinearities, linking organic materials to inorganic quantum dot structures, examining the phase diagrams of mixed polymeric systems, and designing functional polymers that interface to biological systems. Most likely, even more exciting but unanticipated phenomena will arise from this young and vital area of research.

Although the vision for future consumer electronics as described in Box 1.10 is somewhat hyperbolic, the past decade has marked the synthesis of new active organic materials and advances in the science underlying their electronic, optical, and mechanical properties. Technologies based on these materials have the promise that comes with the processability of organic materials—namely, the potential to use inexpensive manufacturing methods like spray painting, dip coat-

BOX 1.10 A Future Vision for Organic Consumer Electronics

As you walk into the train station, you notice the large multicolor advertisement for the evening paper glowing on the electroluminescent schedule board. The large-area light-emitting diodes are made from organic conducting electrodes and luminescent polymers that have been spray painted onto the board. You want your usual sections of the local paper and then a few from different papers, so you slide your plastic profile card into the newspaper machine. The card is a few thousand transistors made from organic materials that have been printed by an ink jet printer modified to deposit organic charge transporters and electrodes. It contains your medical records, frequent flier numbers, custom newspaper preferences, and a host of other information. It also serves as a cash card. The machine asks you to put in your display. You unroll your pocket display and insert it into the machine. Several organic lasers write your customized newspaper into a thin patch of organic material about the diameter of a dime in the upper right corner of your portable display that functions as an erasable compact disk. On the train, you plug your display into the seat back. The reader is a moving organic laser whose reflection from the writing on the disk is recorded by a photovoltaic cell, which is also made from organic materials, and the display is a high-resolution luminescent color display made from organic light-emitting diodes. The active switching matrix for the display pixels is made from hybrid materials, inorganic charge transporters that have been solubilized to be processable using organic chemistry. The display was printed on a flexible polymer in a reel-to-reel manufacturing process. A tiny dot of silicon circuitry containing a microprocessor to interpret the information read from the disk and containing display drive circuitry was attached. You marvel that the display cost less than your monthly rail pass.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 67

ing, and reel-to-reel processing. The organic materials can be designed at a molecular level to incorporate different functional entities that bring the desired optical, electrical, and mechanical properties and can then be modified to make them suitable for economical processing schemes. Active work to develop hybrid organic/inorganic structures, copolymers with regular blocks of several different types, and layered materials will likely spur development of new devices and new ways of integrating them into systems. Further research and development promises to allow us to make materials that self-organize into complex supramolecular arrays. Three-dimensional structures made in this way may be useful in fabricating photonic gap structures for telecommunications, color reflective displays, artificial photosynthetic cells, synthetic membranes, and improved electrophotographic materials.

Applying the physical science that underlyies these prospects will offer many challenges. The practical limits to charge photogeneration, injection, and transport are still poorly understood, especially at a microscopic level that would be prescriptive for synthetic chemists. The basic physics of charge and energy transfer is well established; but predictive understanding, given the complexity of morphologically disordered and compositionally impure systems such as polymers, is both an experimental and theoretical challenge. This is especially true at interfaces between materials, which are becoming increasingly important as device dimensions shrink. Overcoming challenges associated with making efficient electrical contact to organic materials is central to developing efficient light-emitting diodes, transistors, and electrically pumped lasers. Similarly, the basic science of material stability is critical in making relatively delicate organic materials commercially viable. An enormous amount of empirical progress has been made toward this end, and we expect commercial products based on organic electroluminescence to be widely available in 2000. It is likely that a deeper understanding of the chemistry of degradation will lead to even more robust and widely applicable materials.

The Physics of Optical Nanostructures and Artificially Structured Materials

As electronic devices are made smaller and faster, it will become increasingly difficult to transmit electrical signals over wire interconnects at low power consumption for chip-to-chip or, at very high speeds, on-chip communications. One possible development will be the use of miniature optical interconnects to solve the timing, power, and switching-speed limitations of electrical interconnects. This will require the development of low-power optical nanodevices that are compatible with silicon technology. Other possible applications for arrays of tiny light-emitting diodes are displays and optical correlators for producing parallel computation of images. In the last decade major advances in optical microcavity lasers, light-emitting diodes, and detectors resulted from increased under-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 68

standing of the physics of quantum microcavities and advances in III-V and II-VI compound semiconductor growth and processing techniques. Simultaneously, these new materials fabrication and processing techniques have led to beautiful insights into the science of quantum optical structures, which will in turn enable more advances in technology.

By changing the dimensionality of a material by growth or processing, one can greatly alter the density of electronic states, as shown in Figure 1.5. A bulk three-dimensional distribution of electrons in a metal or semiconductor has a density of available electronic states that rises as the square root of the energy. The nature and the energies of these electronic states are greatly affected by ''quantum confinement." Enclosing a thin layer (with thickness comparable to the electron's de Broglie wavelength, about 20 nm in GaAs) of lower band-gap material between two slabs of higher band-gap material, yields a two-dimensional quantum well with sharp steps in the electronic density of states (Figure 1.6a). Confining the electrons into a one-dimensional quantum wire produces a series of sharp peaks at the onset of each new quantum mode in the wire (Figure 1.6b). Confining the electrons into a zero-dimensional "quantum dot," a box comparable in size to the wavelength of the electron in all directions, produces a series of even sharper spikes that correspond to a series of confined quantum levels for the electrons in the box (Figure 1.6c). Therefore lowering the dimensionality of a structure on the atomic scale of a few nanometers will cause very large changes in the physics of electronic transport. The advances in simulations of electronic and optical processes in semiconductors, along with programmable molecular-beam epitaxy (MBE) techniques for fabrication of multiple quantum wells with atomic-scale accuracy—("quantum engineering" of structures), have led to the invention of the quantum cascade laser, which operates between confined levels of electrons in a series of tailored quantum wells (see Box 1.11), giant "pseudomolecules" with large optical nonlinearities, and resonant tunneling devices.

The optical properties of a semiconductor are greatly affected by reducing the dimensionality of its structure. For example, an exciton, an optical excitation of the system near the band edge, comprises a bound state between a conduction band electron and a valence band hole with wave functions similar to those of Rydberg atoms. Either an electron or a hole, or both, can be bound in a lower dimensional material by being confined in one, two, or three dimensions. This can also greatly affect the excitonic levels and thus the optical properties near the band gap. The dimensionality of the system is reduced if the size of a dimension (for example, the thickness of a slab) is comparable to the diameter of the exciton, which in GaAs is about 5 nm. In materials with strong coupling to light, the optical and electronic plasma modes of the material interact to form coupled exciton-polariton modes that are also greatly affected by the dimensionality of the system. Theoretically, manipulating the density of states of the optical excitations of a system can produce lasers with zero current threshold, in contrast to

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 69

image

Figure 1.5
Illustration of the effect of quantum confinement on the density of 
electronic states. [Reprinted from Scientific American 8(1), 26 (1997) with 
permission from John Deecken (artist).]

image

Figure 1.6
Two-, one-, and zero-dimensional small optical devices. [Reprinted 
from Scientific American 8(1), 27-29 (1997) with permission from 
(a) S.N.G. Chu, Lucent Technologies; (b) Lucent Technologies; 
(c) James S. Harris, Stanford University.]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 70

BOX 1.11 Quantum Cascade Lasers

In conventional semiconductor lasers used for applications such as fiber-optic communications and compact disk players, light is generated by the annihilation of a negative and a positive charge (an electron in the semiconductor's conduction band and a hole in the valence band) in the active region. As such, the laser wavelength is determined by the energy difference between the conduction and valence bands (the so-called energy band-gap) and its differential efficiency, in the limit of zero optical losses, cannot exceed unity, corresponding to one laser photon created per electron-hole pair annihilated. These constraints are overcome in a radically new semiconductor laser, called a quantum cascade (QC) laser, invented at Bell Laboratories in 1994. In a QC laser, light is not generated by electrons and holes recombining across the band-gap but by electrons alone as they make a transition between two excited states of ultrathin quantum wells. The QC laser is the first semiconductor unipolar laser. These energy levels arise from size quantization when their thickness becomes comparable to or less than the electron de Broglie wavelength. The wavelength is therefore not fixed by a material property (the band-gap), but can be tailored over a very wide range by changing the thickness of the layers using the same combination of materials. QC lasers are an excellent example of materials by design. The energy levels and wave functions, the optical matrix elements and the electron-phonon scattering times are designed to achieve population inversion and the desired wavelength along with other laser characteristics. The active regions of a QC laser are alternated with electron injectors from which electrons tunnel into the upper excited state of the laser transition (Figure 1.11.1 and see Figure 1.6). Electrons tumble down an energy staircase, emitting a photon in each active region. Thus in an N-stage device (where N is typically 25) N photons are created by a single electron traversing the structure. This leads to much higher optical powers than in a conventional diode laser operating at the same wavelength. The Bell Laboratories group has been able to demonstrate QC lasers based on quantum wells made of AlInAs/GaInAs material grown by molecular beam epitaxy with wavelengths spanning a large portion (3 to 13 µm) of the mid-infrared spectrum. This spectral range includes the two atmospheric windows (3 to 5 µm and 8 to 13 µm). In operating temperature and optical power these devices outperform all other semiconductor lasers emitting at these wavelengths and are the first to operate at room temperature and with powers of several hundred milliwatts in pulsed and continuous wave operation. Single-mode operation with wide wavelength tuning has also been demonstrated. QC lasers are important in the detection by absorption spectroscopy of trace gases for pollution monitoring applications. Other potential commercial uses include industrial process control, combustion diagnostics, and medical applications such as breath analyzers for the early detection of ulcers, diabetes, and various forms of cancers. Military applications include countermeasures and battlefield detection of toxic gases and biological toxins via point sensors and lidar techniques.

(Box continued on next page)

(Box continued from previous page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 71

image

 

Figure 1.11.1 The active regions of a QC laser are alternated with electron injectors from which electrons tunnel into the upper excited state of the laser transition. (Courtesy of Bell Laboratories, Lucent Technologies.)

conventional lasers in which gain is only achieved after sufficient excitation such that the excitons have overlapped in space and have ionized into an electron-hole plasma. Because excitons are bound states of two fermions, they should therefore obey Bose statistics and can themselves exhibit Bose condensation into a macroscopic quantum ground state. The phase diagram of the excitonic matter in semiconductors and its interaction with photons, the observation of lasing between sharp excitonic levels or lasing from Bose-condensed excitons is still controversial, however, and is a field of much current interest.

Over the last decade four approaches for forming small optical devices have been used:

1. MBE has been used for growing programmed series of two-dimensional quantum wells (Figure 1.6a).

2. Cleaved-edge overgrowth with MBE has also been used to make one-dimensional quantum wires (Figure 1.6b). Quantum wells are grown by MBE, the sample is cleaved in vacuum perpendicular to the original growth direction, and new quantum wells are grown to make T-intersections of two quantum wells, forming wires of lower binding energy. Alternatively, growth in the intersection of wells forms one-dimensional wires.

3. Two successive cleaved-edge overgrowths were used to make a series of

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 72

double-T zero-dimensional quantum dots (Figure 1.6c); or, another approach, the clever use of strain in pseudomorphic overgrowth was used to produce strained dot arrays (Figure 1.3.1).

4. Solution chemistry, described in Chapter 5, was used to produce a mono-disperse colloidal suspension of semiconductor quantum dots.

Selective area overgrowth has also been successfully used to tailor the quantum well thickness and composition laterally to make integrated semiconductor optical devices; examples include electro-optical modulators that consist of a ridge waveguide semiconductor laser and an optical modulator adjacent to each other on a single chip. The fabrication of novel optical nanodevices is in its infancy; many advances in design and manufacturing (such as self-assembly) are required to allow mass applications.

Enclosing a material between highly reflecting mirrors produces an optical microcavity, which can be tailored to control the angular distribution of the output light from the structure as well as the spectrum and spontaneous emission from any emitters inside. This is the principle behind vertical-cavity surface-emitting lasers, or VCSELS, which are made by enclosing an active gain medium between two highly reflective dielectric stacks of mirrors grown vertically on a substrate. Microcavities with very sharp resonances (very high Q) have been achieved by making whispering gallery mode resonators out of semiconductors or droplets of dye solution or polymer in which the index change between the material and air produces a high-Q waveguide around the outside diameter of the structure (Figure 1.7). In fact, optical nanocavities have been produced that have Q as high as several thousand. Because of the enormous field enhancement in the cavity with such Q, nonlinear effects should be observable with only a few incident photons. The intense emission of light observed, and yet to be conclusively understood, from porous silicon is to some extent both a confinement and microcavity effect. Another exciting new field involves tailoring optical materials to achieve periodic opposite polarities of the ferroelectric polarization on a length scale tailored to maximize the intensity of optical nonlinearities by efficient phase matching of coherent four-wave mixing.

Extending the concept of optical microcavities into three dimensions leads to the prediction of photonic band-gap materials, structures with periodic variations of dielectric constant on a length scale comparable to the wavelength of light. The idea is to design materials such that they can affect the properties of photons in a manner similar to the way semiconducting crystals affect electrons. In a semiconductor, the atomic lattice presents a periodic potential to an electron propagating through the electronic crystal. The geometry of the lattice and the strength of the potential are such that, owing to Bragg-like diffraction from the atoms, a gap in energy for which an electron is forbidden to propagate in any direction appears. In a photonic crystal, the periodic potential is caused by a lattice of periodic dielectric media instead of atoms. If the dielectric contrast is

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 73

image

Figure 1.7
Very high-Q microcavity. (Courtesy of Bell Laboratories, Lucent Technologies.)

sufficient, Bragg scattering of light off the lattice can also produce a forbidden band that extends over a certain energy range in which light cannot propagate in any direction. However, a defect in the periodicity will introduce localized states in the photonic band-gap in much the same way that localized states exist for electrons within the semiconducting gap. The nature and shape of the localized states will depend on the dimensionality of the defect: a two-dimensional slab or a one-dimensional line will define mirrors and waveguides in the dielectric array, and a zero-dimensional defect will define a microcavity. The design and manipulation of these defects in the photonic band-gap material promises far more control of photons.

As the technology for fabricating photonic lattices in the near infrared (IR) and visible spectral regions advances, they will offer a radically different means for controlling light. For example, Figure 1.8 shows a theoretical model of how light propagates in a periodic square lattice of dielectric rods with a waveguide produced by the intersection of two missing rows of rods. Remarkably, propagation is predicted to occur with no losses even though the bend in the waveguide is on a length scale comparable to the wavelength of light! In ordinary dielectric waveguides today, the bending losses caused by leakage from evanescent fields requires very smooth bends with bending radii of 10 cm—thus the waveguides are large, making manufacture and packaging of integrated optical structures

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 74

image

Figure 1.8
How light propagates in a periodic square lattice. 
(Courtesy of Massachusetts Institute of Technology.)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 75

difficult. Low-cost fabrication of photonic band-gap waveguides for communications wavelengths of 1.3 and 1.5 µm, or wavelengths in the near IR and visible regions important for display, copying, and data-storage technologies, would revolutionize the field of integrated optical devices in much the same way as the integrated circuit has revolutionized electronics.

Challenges, Priorities, and Frontiers of Optical Materials and Phenomena

The frontiers in the field of optical materials and phenomena are novel manufactured materials with tailored optical properties made by self-assembly, poling, lithography, MBE and other techniques not yet invented, organics and other complex materials, nanostructures and micromachined components, biology, and ultrafast phenomena.

Some of the outstanding scientific questions about optical materials involve understanding the physics of lasing and the coupling of exciton-polariton-phonons in nanostructures and quantum cavities, fundamental understanding of the physics of electro-optical processes in organics, and understanding the physics of ultrafast nonequilibrium processes in semiconductors, metals, and biological molecules and tissues.

Some outstanding technology needs are low-cost all-optical communications network and consumer access components such as new fiber materials and devices; all-optical buffer memory, add-drop filters, amplifiers, semiconductor blue lasers, fast light switches, fast spatial light modulators; materials with tailorable optical properties such as better nonlinear optical materials, resists, and photonic-band gap materials in the near IR communications wavelength region; and low-cost assembly and manufacturing techniques for optical components such as self-assembly, stamping, and printing.

Science and Technology of Magnetism

Beginning with the Ancient Mariner's compass and continuing with such applications as automobile starter motors, refrigerator magnets, and computer hard disk drives, the importance of magnetic materials (Figure 1.9) in a wide range of technological uses steadily grows. Such materials display a host of fascinating properties of scientific interest. Many of these properties have proved to be useful in technological applications. The interplay of science and applications has made magnetism an extremely exciting segment of condensed-matter and materials physics. The last decade has seen an acceleration in the advances of both the technology and science of magnetism—advances that have set the stage for even more profound discovery and technological developments in the near future.

The field of magnetism enjoys an unusually strong technology pull, second in condensed-matter and materials physics only to that of semiconductors, par-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 76

ticularly silicon. Various forms of magnetic storage technology dominate this pull. Bulk materials have long had applications in technologies such as motors, generators, and transformers. In addition, the emerging field of magnetoelectronics, with its potential impact on microelectronics technology, is stimulating a fresh wave of enthusiasm along with a broadened research agenda.

This section begins with a review of a few exciting recent developments and future directions of selected important technologies based on magnetic materials and phenomena. Against this backdrop, selected scientific accomplishments of the past decade and opportunities for the future will be presented. The section concludes by summarizing some of the major outstanding scientific questions and suggesting priority directions for future research.

Technology Pull

The industry of magnetic recording, in all its forms, constitutes an enterprise

image

Figure 1.9
Magnetic response of a ferromagnetic particle. The initially 
unmagnetized magnetic particle exhibits a hysteresis loop in 
its magnetization (M) as an external magnetic field (H) is 
increased from zero to a maximum value, reversed through 
zero to a minimum value, and subsequently returned to zero. 
Characteristic features of hysteresis curves include the 
saturation magnetization, Ms (the maximum magnetization 
of the sample); the remnant magnetization, Mr (the 
magnetization that persists in the sample when the 
external field is zero); and the coercive field, Hc (the 
external field necessary to return the magnetization to 
zero). ''Soft" magnetic materials have low Hc, high 
permeability (~dM/dH), and a small area enclosed 
by the hysteresis loop. "Hard" magnetic materials have 
high Hc, high Mr, and a large area enclosed by the 
hysteresis loop.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 77

with annual revenues in excess of $100 billion. This consists primarily of magnetic disk, magnetic tapes, and optical disks for digital data storage together with various forms of magnetic recording for audio and video. This industry is experiencing an overall compound annual growth rate in revenue of about 10 percent per year. This growth rate is expected to continue for at least another decade, with magnetic storage playing the dominant role. The United States "owns" about 40 percent of the magnetic storage business, the largest single component of which is hard disk drives (see Box 1.12). This component alone is a $30 billion a year business.

The accelerating interplay of the science and applications of magnetism is well illustrated by the phenomenon of magnetoresistance. Lord Kelvin first observed this effect in 1856. Beginning in the early 1980s, a decade of research and development (at IBM) with this basic laboratory phenomenon perfected a product of major commercial importance. The first magnetoresistive sensor used in the recording head of a hard disk drive has an intricate structure in which data is sensed by a 20 nm thick permalloy (a NiFe alloy) layer. The useful change in resistance of this film as it passes in close proximity to a small magnetized region of a magnetic disk is about 2 percent. The time from discovery of the phenomena to a high-volume product was 135 years.

Recent research activities led to the discovery of a superior form of magneto-resistance, called giant magnetoresistance (GMR). GMR requires the interaction between at least two very thin ferromagnetic films and can register a resistance change at room temperature of about 10 percent in the same magnetic field range as permalloy. Moreover, as an interfacial phenomenon, its performance in so-called spin-valve recording sensors improves with decreasing film thickness, which also increases the storage density. In contrast to the case of magnetoresistance, only 10 years have passed since the original discovery of GMR before an initial product was produced. In the exponentially growing global hard-drive industry, GMR sensors will be needed to sustain this rate of improvement into the third millennium.

A range of magnetic tape storage systems with applications from audio and video to data storage constitute another third of the magnetic storage business. Storage densities in this arena are experiencing a single-digit compound annual growth rate with continuing cost reduction. Here, too, magnetoresistive heads play an important role in the continued scaling to higher densities. Magnetic particle tapes are still the industry standard, but thin film tapes have been introduced and will undoubtedly dominate some time in the future.

In a completely different arena, bulk magnetic materials constitute a $4 billion global market, with the United States holding approximately 20 percent of the market share. This market is projected to grow to more than $6 billion by the year 2000. "Hard" bulk magnetic materials are essential constituents in a wide variety of electric motor and generator technologies. For such applications, the strength of the permanent magnetism, or so-called "maximum BH-product," is

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 78

BOX 1.12 Magnetic Storage in Hard Disks

The basic configuration of a hard-disk drive is illustrated in Figure 1.12.1. Key aspects of this technology include the read/write head, media, head-disk interface, tracking of the head with respect to the disk, and signal processing to distinguish the ones and zeros as the disk spins beneath the head.

The evolution of the areal storage density in leading-edge products was illustrated in the introduction. This exponential growth in storage density has led to an associated growth in the number of hard-disk drive bytes shipped per year, as shown in Figure 1.12.2.

image

 

Figure 1.12.1 Magnetic recording disk drive assembly with a magnetoresistive head. Three disks and one of the movable arms supporting a read/write head are visible. The arrow zooms in on the magnetic sensor of a recording head. The NiFe film measures 20 nm in thickness by 1 or 2 µm in width. The permanent magnetism of the hard bias induces a reference state of the magnetization inside the NiFe film. The magnetic field emanating from small magnetized regions, representing the stored data, in the rotating disk changes the resistance in the NiFe films thus reading out the data as voltage changes across the contacts, which also supply a constant current through the NiFe film. (Courtesy of IBM Research.)

(Box continued on next page)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 79

(Box continued from previous page)

image

 

Figure 1.12.2 The number of hard disk drive bytes shipped in products per year. The compound annual growth rate of 95 percent is projected to continue for the foreseeable future. (Courtesy of IBM Research.)

Exponential improvement in storage density has been achieved through continued advances and scaling of all aspects of the technology. As an example, consider advances in magnetic head technology. Through the first half of the decade thin film inductive heads dominated the industry and became ever more difficult to scale. The first magnetoresistive heads were introduced in 1992 and played a pivotal role in increasing the compound annual growth rate of the areal storage density from 30 percent to 60 percent (with a 40 percent annual reduction in cost per bit!). Giant magnetoresistive heads introduced at the end of 1997 will help ensure the continuation of this trend.

the defining materials characteristic. Research and development leading to increased BH-products (Figure 1.10) has steadily decreased the costs, sizes, and weights of motors in diverse devices like auto starters, cordless shavers, hand-held drills, vacuum cleaners, washing machines, dryers, machine tools, motorized toys, and disk drives in laptop computers. Motor vehicles alone account for 70 percent of permanent-magnet usage in starters, electric windows, speakers, and cassette and CD players. A luxury car may contain 80 motors.

The most recent permanent-magnet development spurt occurred in the mid-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 80

1980s and dramatically illustrates the unpredictable effects of the interplay between politics, economics, research, and development. Samarium-cobalt was the leading hard magnetic material in the late 1970s in spite of the high cost of samarium. The cost of samarium-cobalt magnets ballooned fivefold when the world's principal source of cobalt disappeared during the 1978 national upheaval in Zaire. Intense focused research established that samarium and cobalt could be replaced by neodymium and iron, respectively. Not known in advance was that the attainment of permanent magnetization in the new compositions hinges on complicated processing sequences including, in one case, creation of amorphous material by melt-spinning followed by crystallization through severe mechanical treatment and subsequent annealing. Equally unexpected was that the product would be stronger, both magnetically and mechanically, and less expensive. The rapidly growing NdFeB segment of the bulk magnetic materials market is projected to reach $4 billion by the year 2005.

''Soft" bulk materials play key technological roles in radio frequency (rf) and power distribution applications. At frequencies higher than 100 kHz, ferrites remain the materials of choice. They are widely used in all manner of rf and microwave elements such as antennas, filters, circulators, and insulators. Historically, advances in ferrite performance (higher permeability, higher frequency

image

Figure 1.10
Chronological trend of (BH)max where the data represent initial 
demonstration in the laboratory. [Reviews of Modern Physics 63, 
819 (1991).]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 81

response, and lower losses) have been driven by niche military applications, with commercialization following rapidly. At lower frequencies, soft magnetic materials are used extensively in transformers for the power-distribution industry. Here, incremental improvements can have an enormous technological and economic impact. Particularly important is magnetic metglass, an amorphous material in ribbon form prepared by ultra-fast quenching. The absence of magneto-crystalline anisotropy has the consequence that the magnetization vector can be easily rotated. Therefore, a metglass has high magnetic permeability and low losses. When used in the core of a power transformer, metglass is more expensive than crystalline materials and increases capital costs; however, this increased capital cost is often rapidly amortized by continuing savings from decreased transformer loss in electric power transmission. Worldwide savings of several billion dollars have been realized by the introduction of magnetic metglass.

Magnetostriction is another area of magnetism of considerable, and growing, interest for both military and commercial applications. Most of the materials development work in this area has been focused on rare-earth transition metal alloys. Strains of up to 1 percent, along with considerable force actuation, have been demonstrated in practical applied fields. Applications range from sonar pulse generation to high-reliability replacement of hydraulic systems in aircraft and even tanks.

A final area is magnetoelectronics (exclusive of magnetic storage), which includes a variety of devices and associated assemblies. The largest component of this industry today is sensors used for commercial, scientific, and military applications. Such sensors range from Hall effect sensors, to superconducting quantum interference devices (SQUIDs), flux gate magnetometers, search-coil magnetometers, magnetoresistance (MR) and GMR sensors, to magnetic force microscopes. Magnetoelectronics is characterized by a number of small but stable niche markets. The worldwide market for SQUID instrumentation, for example, is about $20 million per year; the market for magnetic force microscopes is similar.

The potential sleeping giant in the field of magnetoelectronics is a growing collection of novel devices and circuits that possibly can be integrated onto a high-performance chip to perform some complex function. More realistically, key elements may be integrated with high-performance semiconductor technology to produce new generations of microchips with function, density, and/or performance beyond that achievable with semiconductor technology alone. Non-destructive read out memory chips in which bits are stored in small electrically addressable magnets have been demonstrated at capacities of up to 256 kb (Figure 1.11). The nonvolatile radiation-hard nature of this memory together with potential for scaling to much higher density and performance, especially as new magnetic elements are developed, show considerable promise.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 82

image

Figure 1.11
Micrograph of prototype 256-kb nonvolatile magnetic random access 
memory chip. (Courtesy of Honeywell.)

The Physics of Magnetism

It is evident from the above discussion that there is a strong technology pull for research in magnetism. The last 10 years have seen numerous exciting discoveries, a number of which have already had a direct impact on technology. Others are providing us with a better understanding of the world around us and/or are helping lay the groundwork for future technology. It is amazing that, although much is known about magnetism and while effects drive a $100 billion per year industry, our basic understanding of magnetism, even in a material such as iron, is incomplete. As an example, Figure 1.12 shows the results of a simula-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 83

tion of a two-dimensional array of magnetic moments approximating a 10 nm × 500 nm × 1000 nm sheet of permalloy. One might anticipate that for a system of this size, the moments would all work in unison, switching together under the influence of an applied field; however, as shown in Figure 1.12, such is not the case. The behavior of this rather elementary system exhibits a great deal of structure and complexity. There is neither experimental nor theoretical consensus on the detailed behavior of such a system. Nor is it understood how small such a system must be to ensure true, single-domain behavior in which all of the moments always remain parallel to each other.

The fundamental limit on stability of magnetic domains is an important area of basic investigation in magnetism. The advancing march of magnetic technology makes investigation of these limits inevitable, but probing these limits raises some of the most challenging questions for condensed-matter physics and materials science, such as, What is the smallest size magnetic element stable against external perturbations such as temperature fluctuations? and, Given that quantum mechanics sets bounds on the lifetime of any magnetic state, how do such bounds ultimately establish limits on the size of the smallest possible magnetic entities useful for technological applications?

Molecular magnets such as the ferric wheel shown in Box 1.13 are the subject of a wide variety of physical analyses aimed at shedding light on these and other challenging questions about magnetism in small structures. Traditional

image

Figure 1.12
Micromagnetic simulation of the switching behavior of a 
10 nm × 500 nm × 1000 nm permalloy dot. The arrows 
indicate the direction of the magnetization (m) at the point 
in the hysteresis cycle indicated on the curve in the lower left. 
(Courtesy of IBM Research.)

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 84

BOX 1.13 Nanomagnets

Fundamental questions about nanomagnets could not be addressed were it not for the remarkable advances made by materials scientists and chemists in the synthesis of exquisitely controlled nanometer-scale magnetic structures. Many "traditional" synthesis techniques have been applied successfully in this area: sub-100 nm scale permalloy particles can be made with lithographic techniques, and growth assisted by scanning-tunneling microscopes has proven that it is possible to fabricate pure iron particles with a variety of sizes and aspect ratios. The most exciting new techniques involve various "self-assembly" strategies that are emerging as extensions of chemical synthesis techniques. The relatively low-tech techniques of colloid growth have been adapted to make cobalt particles 8 nm in diameter with close to atomic control. Actual atom-by-atom control in the magnetic domain is now achieved by metallo-organic molecular synthesis; magnetic molecules containing exactly 10 iron atoms (the "ferric wheel,'' see Figure 1.13.1), or exactly 12 manganese ions, for example, are now routinely available.

image

 

Figure 1.13.1 Structure of the Fe10 "ferric wheel" cluster, where the large solid circles represent the iron atoms and the empty circles are, in order of decreasing size, chlorine, oxygen, and carbon. The 10 Fe3+ ions, each with a magnetic moment corresponding to the same angular momentum or spin, are bound together into a perfectly regular ring. High magnetic field experiments have shown that the Fe3+ ions exhibit antiferromagnetic behavior; neighboring spins prefer to be antiparallel. The spin structure of the molecule passes through a rich sequence of phase transitions resembling those in bulk layered antiferromagnets. These experiments open the prospect of precisely controlling the structure, interactions, and dynamics of nanomagnets. [Reprinted with permission from D. Gatteschi, A. Caneschi, L. Pardi, and R. Sessoli, "Large clusters of metal ions: The transition from molecular to bulk magnets," Science 265, 1056 (1994). Copyright © 1994 American Association for the Advancement of Science.]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 85

characterization gives the basic magnetic parameters of the particle: ionic moment (i.e., the local state of spin), exchange interaction (the coupling strength between the spins), and anisotropy (the height of the energy barrier in the double-well potential separating the "up" state from the "down" state). Such characterizations demonstrate that the ferric wheel behaves very much as an atomic-scale analog of a layered antiferromagnet. Less traditional characterizations are required to understand the ultimate stability of such molecular magnets, which is determined by quantum tunneling. One particularly interesting example is the observation of what might be called "quantized hysteresis" in the Mn(12) molecule: at low temperatures, this structure shows a propensity to switch from up to down at a sequence of regularly spaced magnetic fields. Some evidence suggests that these magnetic fields coincide with resonances between quantum levels of the up and down wells, resulting in enhanced tunneling. Although some details of the tunneling mechanism remain to be understood, this is a particularly simple example of the stability (up versus down) of the moment of a molecular magnet being ultimately limited by a purely quantum effect.

Of more profound significance than the observation of quantum tunneling would be the observation of "quantum coherence" in these nanomagnets. The phenomenon is closely analogous to the microscopic quantum coherence (MQC) effect sought in small SQUIDs for years—the creation of a quantum state in a controlled superposition of up and down states. The regular advance of the phase of this superposition would result in the sinusoidal oscillation of the magnetic domain between the up and down states. Observation of such coherence oscillations would foreshadow a significant change in the role that quantum mechanics might play in the dynamics of magnetic domains. Although we might view magnetic tunneling as a nuisance, destroying the stability of a bit stored in the magnetic domain, coherence, if controllable, could be the resource needed to realize the basic element of storage and processing in quantum computing. Signs of magnetic quantum coherence have in fact been observed in a naturally occurring magnetic nanoparticle.

In the past 10 years rapid progress has been made in the characterization and understanding of magnetic multilayers, exchange coupling, and spin-dependent transport through magnetic materials and interfaces. Results from an experiment representative of this exciting, ground-breaking work is shown in Figure 1.13. This experiment measured the oscillatory exchange coupling between iron layers separated by a chromium spacer of varying thickness. The chromium wedge was grown epitaxially on the nearly perfect surface of an iron whisker crystal whose magnetization is split into two opposite domains along the [001] direction. A thin iron film was deposited on top of the chromium, and its magnetization was measured using scanning electron microscopy with polarization analysis (SEMPA). The SEMPA image, drawn on the wedge schematic, clearly shows that the exchange coupling reverses direction with almost every single monolayer change in chromium thickness. The oscillatory coupling period, which arises

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 86

from nesting features in the Fermi surface of chromium, is actually slightly incommensurate with the chromium lattice, producing the phase slips observed at chromium layers 24 and 44.

The applications focus provided by GMR has helped to stimulate and invigorate the search for new magnetic heterostructures and nanostructures and new magnetoresistive materials. GMR materials consist mainly of nanometer thicknesses of interleaved metallic layers that are alternately ferromagnetic and non-ferromagnetic. (Analogous nano-dispersed two-phase composites can also exhibit GMR properties.) Key to strong in-plane GMR are spin-dependent scattering at layer interfaces and an electron mean free path (approximately 10 nm) greater than sublayer thickness. The relative resistance change can be greater for current flow perpendicular to the layer planes. Resistance changes as large as 100 percent relative to the low-resistance state have recently been reported.

Work on magnetic multilayers is also stimulating new thinking concerning novel devices that can be made by integrating magnetic materials with standard semiconductor technology. An example of this is shown in Figure 1.14 where a GMR (Co/Cu) multilayer serves as the base of an n-silicon metal base transistor. Biased in the common base configuration, this device exhibited a 215 percent change in collector current in a magnetic field of 500 G at 77 K with typical GMR characteristics. The in-plane GMR of the multilayer was only 3 percent. Although by no means a practical transistor, this structure allows the study of spin-dependent scattering of hot electrons in magnetic multilayers. More practical spin transistors may be forthcoming, particularly if ways to achieve 100 percent spin-polarized injection can be devised.

image

Figure 1.13
Oscillatory exchange coupling in Fe/Cr/Fe. [Physical Review Letters 67, 140 (1991).]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 87

Spin-polarized tunneling experiments in magnetic thin-film planar junctions are helping to elucidate novel magnetic properties as well as demonstrate features of considerable device potential. The first successes in spin-polarized tunneling between two ferromagnets through an insulating tunnel barrier occurred only very recently, even though spin-polarized tunneling was predicted more than 20 years ago. A simplified structure of this type of junction is shown in Figure 1.15. The magnetization of the ferromagnetic base electrode is pinned in the direction indicated. The magnetization of the ferromagnetic counter electrode is shown as aligned with that of the base electrode but can be reversed by application of a modest magnetic field. A proper magnetic design yields a hysteretic response curve symmetric about H = 0. Resistance changes of greater than 30 percent have been demonstrated. Such junctions are of considerable interest as potential storage elements in one approach to magnetic RAMs. Slightly different configurations give devices with nonhysteretic characteristics but with similar magnetoresistance. These devices are attractive candidates for sensor applications and may provide a follow-on to GMR sensors for hard-disk drives.

The enormous surge in the synthesis and study of high-Tc perovskite materials spawned a concerted effort to explore the magnetic properties of similar materials. Some of these materials exhibit what has been termed colossal magnetoresistance (CMR). The magnetoresistance of doped manganite structures such as La1-xSrxMnO3 changes by a factor of 2 or 3, although not at temperatures and magnetic fields suitable for practical device applications. These systems share much in common with high-Tc cuprate superconductors, from which dozens of new crystal structures have emerged. Replacing copper with manganese, for example, could generate a platform of new crystal chemical systems, some of which will undoubtedly exhibit promising CMR properties.

image

Figure 1.14
Schematic cross section of a prototype spin-valve transistor. 
[Physical Review Letters 74, 5260 (1995).]

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 88

image

Figure 1.15
Magnetic tunnel junction structure. (Courtesy of IBM Research.)

Steady advances in improving the characteristics for technological applications has been realized in the more traditional bulk materials. For example, in the case of high-permeability soft materials, we have learned how to compensate for the deleterious magnetic anisotropy from one magnetic element by introducing a second element having an anisotropy of the opposite sign. Another way to cancel anisotropic effects, and thereby increase permeability, is to rapidly quench a magnetic ribbon, as with magnetic metglass, thus making its structure amorphous. The preferred axes of the atomic moments are now random and therefore the atomic contributions to anisotropy energy tend to cancel, making it easy to remagnetize. In the case of permanent magnets, the magnetic anisotropy, and therefore the coercivity of a ferromagnet, decrease steeply as its temperature approaches the Curie critical point, where these parameters necessarily vanish. We have learned how to increase the Curie temperature, and thereby the coercivity, of permanent magnet materials of the RexFey type by introducing interstitial N or C or, to lesser degrees, Ti, V, W, Mo, or Si. The complexity as well as the importance of processing details in the synthesis of high-performance magnetic materials is well demonstrated in the case of NdFeB discussed previously.

Measurement techniques are vital in the research of magnetic materials and phenomena. Experimental advances that have contributed to breakthroughs in the last decade include scanning-tunneling microscopy (STM), magnetic force microscopy (MFM), magneto-optic Kerr imaging, and scanning electron microscopy with polarization analysis (SEMPA). STM has been critical to understanding how subtle differences in physical structure can make profound differences in magnetic structure or properties. Characterization techniques based at major facilities are equally important. For CMR, as for high-Tc research, neutron dif-

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 89

fraction is needed to obtain the position of the oxygen atoms within the unit cell as a function of temperature, field, pressure, and doping. Electron microscopy is needed to understand growth inclusions that form two-dimensional stacking faults. Synchrotron sources enable advanced spectroscopies to identify the +3 and +4 valence states of Mn and their ratio. Diffuse x-ray scattering and quasi-elastic neutron scattering are used to investigate the presence and dynamics of polaronic distortions. A number of other techniques based on such effects as spin-polarized photoemission, magnetic circular dichroism, and second harmonic generation are becoming increasingly prevalent, while many others are in the initial stages of demonstration.

Major Outstanding Materials and Physics Questions and Issues in Magnetism

Many outstanding scientific questions remain in the field of magnetism. Answers to a number of these questions will have an important technological impact and are necessary to continue the momentum and growth of the magnetics industry.

With a few notable exceptions, we lack detailed understanding of the magnetic properties of nanostructured magnetic elements and arrays of such elements. Examples include the following:

• The nature of domain structure and its influence on switching behavior;

• The dynamics and switching times in such elements or systems;

• The influence of temperature, both in the context of stability against thermally induced switching and in the context of structural change at elevated temperature;

• The nature of the interaction of spin-polarized currents with such elements, both reversible and irreversible; and

• The role and the impact of quantum coherence and macroscopic quantum tunneling in the smallest of such structures consisting of a cluster of atomic spins.

We need to understand the impact of the issues above and related issues regarding technologies such as magnetic recording and the synthesis of new materials with improved properties such as higher BH-products. Because of the resurgence of the science and applications of magnetism, it is important that we reestablish the teaching of magnetism as a priority in our universities as a whole rather than at only a few institutions that presently teach it.

Much remains to be learned concerning the nature of spin-polarized transport. Questions need to be answered about the role of structure and the relationship of surface and interface structure to magneto-transport, the scattering mechanisms at interfaces in GMR, and the physics of the temporal and spatial decay of nonequilibrium magnetism. Also required is a detailed understanding of the mechanism of spin injection, either directly or through tunneling barriers from

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 90

magnetic metals into metals and semiconductors. Answers to these types of questions are needed to engineer better GMR, CMR, and magnetic tunnel junction (MTJ) materials and devices. In a particularly useful spin transistor with true on and off states separated by many orders of magnitude in conductance will require very nearly 100 percent spin-polarized current injection from one region of the device into another.

Advanced synthesis and processing techniques need to be developed to produce novel material with a high potential for scientific and technological impact. Layered structures, nanostructured three-, two-, and one-dimensional materials, and materials with higher BH-products are all attractive areas for further exploration. As in other areas of condensed-matter and materials physics, a systematic approach—technically and organizationally—is needed to explore the vast phase space of magnetic materials. As applications develop, methods to bridge the gap between fabrication techniques that serve to produce initial demonstrations and more controlled and reliable techniques that can be migrated to development and manufacturing will be needed.

Several difficult challenges remain in the measurement area. We need to understand how to magnetically probe individual electron and nuclear spins directly. Questions to be addressed include, What is the ultimate spatial resolution of magnetic measurement techniques? And, Can we fabricate a spin-polarized STM? Advances in measurement technology related to such questions will have a profound impact on our ability to understand the nature of magnetism in nanostructures.

Finally, in the technology arena, continued focus on scaling the density for magnetic storage is needed. This will involve a strong, systematic, ongoing program in both media and detectors that will draw heavily on several of the condensed-matter and materials physics areas mentioned above. In addition, there is enormous opportunity in the arena of magnetoelectronics for new magnetic effects and devices that may set the stage for magnetism to play a key role in future microelectronic chip technology. A key element will be integration of complex magnetic materials with mainstream semiconductor technology.

Future Directions and Research Priorities

Numerous outstanding scientific and technological research needs have been identified in electronic, photonic, and magnetic materials and phenomena. If those needs are met, it is anticipated that these technology areas will continue to follow their historical exponential growth in capability per unit cost for the next few years. Silicon integrated circuits are expected to continue to follow Moore's Law at least until the limits of optical lithography are reached; transmission bandwidth of optical fibers is expected to grow exponentially with advances in optical technology and the development of soliton propagation; and storage density in magnetic media is expected to continue to grow exponentially with the

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 91

maturation of GMR and development of CMR and MTJs in the not too distant future. Although these changes will have major impact on computing and communications over the next few years, it is clear that extensive research will be required to produce new concepts, as will new approaches to reduce research concepts to practice, if these industries are to maintain their historical growth rate over the long term.

Continued research is needed to advance the fundamental understanding of materials and phenomena in all areas. For example, despite the extensive technological application and impact of magnetic materials and, despite more than a century of research in magnetic materials and phenomena, we lack a first-principles understanding of magnetism. By comparison, the technology underlying optical communication is very young. The past few years has seen enormous scientific and technological advances in optical structures, devices, and systems. New concepts such as photonic lattices, which are expected to have significant technological impact, are emerging. We have every reason to believe that this field will continue to advance rapidly with commensurate impact on communications and computing.

As device and feature sizes continue to shrink in integrated circuits, scaling will encounter fundamental physical limits. The feature sizes at which these limits will be encountered and their implications are not understood. Extensive research is needed to develop interconnect technologies that go beyond normal metal and dielectrics in the relatively near term. Longer term, technologies are needed to replace today's silicon field-effect transistors. One approach that bears investigation is quantum state switching and logic as devices and structures move further into the quantum mechanical regime.

A major future direction is nanostructures and artificially structured materials, which was a general theme in all three areas. In all cases, artificially structured materials with properties not available in nature revealed unexpected new scientific phenomena and led to important technological applications. As sizes continue to decrease, new synthesis and processing technologies will be required. A particularly promising area is that of self-assembled materials. We need to expand the research into self-assembled materials to address such questions as how to control self-assembled materials to create the desired one-, two-, and three-dimensional structures.

As our scientific understanding increases and synthesis and processing technologies of organic materials systems mature, these materials are expected to increase in importance for optoelectronic and, perhaps electronic, applications. Many of the recent technological advances are the result of strong interdisciplinary efforts as research results from complementary fields are harvested at the interface between the fields. This is expected to be the case for organic materials; increased interdisciplinary efforts—for example, between condensed-matter and materials physics, chemistry, and biology—offer the promise of equally impressive advances in biotechnology.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×

Page 92

In conclusion, the committee identifies a few major outstanding scientific and technological questions and research and development priorities.

Major Outstanding Scientific and Technological Questions

• What technology will replace normal metals and dielectrics for interconnect as speed continues to increase?

• What is beyond today's FET-based silicon technology?

• Can we create an all-optical communications/computing network?

• Can we understand magnetism on the meso/nano scales needed to continue to advance technology?

• Can we fabricate devices with 100 percent spin-polarized current injection?

Priorities

• Develop advanced synthesis and processing techniques, including those for nanostructures and self-assembled one-, two-, and three-dimensional structures.

• Pursue quantum state logic.

• Exploit physics and materials science for low-cost manufacturing.

• Pursue the physics and chemistry of organic and other complex materials for optical, electrical, and magnetic applications.

• Develop techniques to magnetically detect individual electron and nuclear spins with atomic-scale resolution.

• Increase partnerships and cross-education/communications between industry, university, and government laboratories.

Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 31
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 32
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 33
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 34
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 35
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 36
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 37
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 38
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 39
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 40
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 41
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 42
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 43
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 44
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 45
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 46
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 47
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 48
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 49
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 50
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 51
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 52
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 53
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 54
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 55
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 56
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 57
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 58
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 59
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 60
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 61
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 62
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 63
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 64
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 65
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 66
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 67
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 68
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 69
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 70
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 71
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 72
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 73
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 74
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 75
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 76
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 77
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 78
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 79
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 80
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 81
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 82
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 83
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 84
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 85
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 86
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 87
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 88
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 89
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 90
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 91
Suggested Citation:"1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
×
Page 92
Next: 2 New Materials and Structures »
Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology Get This Book
×
Buy Paperback | $85.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

This book identifies opportunities, priorities, and challenges for the field of condensed-matter and materials physics. It highlights exciting recent scientific and technological developments and their societal impact and identifies outstanding questions for future research. Topics range from the science of modern technology to new materials and structures, novel quantum phenomena, nonequilibrium physics, soft condensed matter, and new experimental and computational tools.

The book also addresses structural challenges for the field, including nurturing its intellectual vitality, maintaining a healthy mixture of large and small research facilities, improving the field's integration with other disciplines, and developing new ways for scientists in academia, government laboratories, and industry to work together. It will be of interest to scientists, educators, students, and policymakers.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!