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Suggested Citation:"2 New Materials and Structures." 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.
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2
New Materials and Structures

Our ability to make new materials and structures—both in bulk and in reduced dimensions or length scales—is inextricably linked to the advancement of our understanding of fundamental phenomena in condensed-matter and materials physics. This chapter describes some of the past decade' s advances in inorganic materials and structures. Some of the advances and promising new areas in organic materials are discussed in Chapter 5. As described in Box 2.1, an astonishing array of new materials with unexpected properties has come over the horizon. Improvements in synthesis and processing have led to dramatic improvements in the properties of established materials and our ability to exploit these properties. As a result, we can now fabricate new combinations of materials, features of reduced dimensions, and other characteristics that differ in significant ways from previous possibilities. Some of these developments have provided fertile ground for condensed-matter and materials physicists to explore novel fundamental phenomena; others show promise for finding applications quickly; some have the potential to change our lives.

New materials underlie the science and technology described throughout this report. Beyond condensed-matter and materials physics, they enable both science and future technologies. In some cases, entirely new and unexpected phenomena appear in a class of new materials. Layered cuprate high-temperature superconductors are a new class of materials that has kept experimentalists and theorists alike searching to understand the physical basis of high-temperature superconductivity. New materials sometimes allow entirely new device concepts to be realized or lead to a dramatic change in their scale, such as single-molecule wires made of carbon nanotubes; and new forms of already known materials can

Suggested Citation:"2 New Materials and Structures." 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.
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possess different properties. Semiconductor nanoclusters, which emit light whose wavelength depends on cluster size, offer the possibility of tailoring material properties to suit a particular need. Even mature techniques, such as those for bulk crystal growth, demand continuous improvements in process control to produce the size or quality of material required for either technological applications or fundamental studies.

Better understanding of the mechanisms at play in materials that have been known for decades can lead to new approaches that alleviate detrimental properties. An excellent example is the introduction of metallic oxide electrodes in ferroelectric devices, which reduces aging effects dramatically. Better understanding of the details of materials preparation can give rise to improvements in processing. Improved insight into the kinetics of epitaxial growth can dramati-

BOX 2.1 Additions to the Zoo: New Materials and Structures of the Past Fifteen Years

There have been far too many new developments in the past 15 years or so to document them all in detail, but all these developments have been made possible by advances in two intertwined areas: complexity and processing. Many of the new materials and structures are dramatically more complex, compositionally or structurally, than have been studied previously. In general, this trend has required advances in processing to allow control of the increased complexity. In other cases, the final product may not be much more complex than other well-known materials or structures, but the processing itself may need to be altered to achieve more control over the growth process in order to obtain the new material.

Advances giving rise to new materials and structures fall into three categories. Some involve the synthesis of an entirely new compound or material. The advance may have been revolutionary, meaning that the properties of the new material (or in some cases its existence) could not have been predicted. In other cases, advances in processing have allowed fabrication of new or modified materials or structures whose properties were suspected before the material was actually made. This may allow a well-known compound to be remade in a new form with different properties. Third, well-known materials are sometimes found to exhibit new (in some cases unexpected) properties that appear when the ability to process them is improved. The new property may be found in a known material simply by looking at it in a new light, which shines on it as a result of insight gained from another materials system.

The materials advances listed in Table 2.1.1 were driven by different motivations. Many addressed a technological need, such as the need to transfer or store information. Others were driven by scientific curiosity. Although the driver can be clearly identified in each case, the two sets are not mutually exclusive. Many discoveries that result from pure scientific curiosity ultimately find their way into products. For example, low-temperature superconductors are now used in magnets for magnetic resonance imaging. Other discoveries, though originally motivated by a technological need, give rise to very beautiful and fundamental insights.

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Suggested Citation:"2 New Materials and Structures." 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.
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(Text box continued from previous page)

For example, the fractional quantum Hall effect was first observed in high-mobility semiconductor structures now used in high-frequency applications.

TABLE 2.1.1 Some New Inorganic Materials of the Past Fifteen Years

Advance

Driver

Nature of Advance

New compounds/materials

High-temperature superconductors

Science

Revolutionary

Organic superconductors

Science

Revolutionary

Rare-earth optical amplifier

Technology

Evolutionary

Intermetallic materials

Technology

Evolutionary

High-field magnets

Technology

Evolutionary

Organic electronic materials

Technology

Evolutionary

Magnetooptical recording materials

Technology

Evolutionary

Bulk amorphous metals

Technology

Evolutionary

New structures of known materials

Quasicrystals

Science

Revolutionary

Buckyballs and related structures

Science

Revolutionary

Nanoclusters

Science

Evolutionary

Metallic hydrogen

Science

Evolutionary

Bose-Einstein condensates

Science

Evolutionary

Giant magnetoresistance materials

Technology

Revolutionary

Porous silicon

Technology

Evolutionary

Diamond films

Technology

Evolutionary

Quantum dots

Technology

Evolutionary

Foams/gels

Technology

Evolutionary

New properties of known materials

Gallium nitride

Technology

Revolutionary

Silicon-germanium

Technology

Evolutionary

 

cally lower the growth temperature in semiconductor processing. Understanding and exploiting fundamental growth mechanisms can lead to previously unattainable structures as in the use of strain to induce the self-assembly of quantum dots.

Many advances in condensed-matter and materials physics are the direct result of the availability of materials and structures of a quality not previously attainable. These materials and structures in turn exist because of improvements in the technology used to make, study, measure, and see them. The impetus for improvements in the materials is often a technological need, not the search for new knowledge. The new knowledge generated, however, in some cases itself becomes the enabler of revolutionary technology. This interplay is explored in Box 2.2.

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.2 The Science—Technology Circle

Tremendous advances in compound semiconductor devices were enabled by dramatic improvements in the growth of thin films that began in about 1970 with the invention of molecular beam epitaxy (MBE, see Figure 2.2.1). The direct antecedents of MBE were developments in vacuum technology beginning in the 1960s and continuing into the 1970s, driven by accelerator development and space physics. As the attainable vacuum improved, it became possible to keep a surface atomically clean for long enough to study it. Surface probes such as Auger spectroscopy and electron diffraction techniques allowed the clean surfaces to be studied.

MBE enabled the controlled, layer-by-layer growth of compound semiconductors. The composition of the film could be changed abruptly. Extremely high mobility was achieved in GaAs-GaAlAs heterostructures through ''modulation doping.'' Research into these structures was pursued because of their utility in high electron-mobility transistors (HEMTs, see Figure 2.2.2) which are used today in high-speed electronics.

Study of these layers at low temperatures in extremely high magnetic fields led to the discovery of the quantum Hall effect, which takes place in a two-dimensional electron "gas" produced in a transistor-like device. Under these conditions, electron correlations dominate, leading to precise quantization of the Hall conductance. As the quality of the layers was improved further, the mobility also improved, and the fractionally quantized Hall effect (FQHE) was discovered, in which the quantum number describing the system is a fraction rather than an integer (see Figure 2.2.3). The FQHE has subsequently been used for unprecedentedly accurate measurements of the fundamental quantity h/e2 (Planck's constant divided by the square of the charge of the electron).

image

 

Figure 2.2.1
Molecular beam epitaxy was invented at Bell Laboratories in about
1970 as an outgrowth of advances in vacuum technology
and surface science techniques. (Courtesy of Bell Laboratories,
Lucent Technologies.)

(Box continued on next page)

Suggested Citation:"2 New Materials and Structures." 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.
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(Box continued from previous page)

image

 

Figure 2.2.2
A high electron-mobility transistor (HEMT)
such as those used in cellular telephones.
The round bonding pads are 100 µm
in diameter, roughly the size of a human hair.
The gate of the transistor, just 0.05 µm across,
appears as the two narrow lines in the
center of the scanning electron micrograph.
(Courtesy of Sandia National Laboratories.)

image

 

Figure 2.2.3
A pictorial representation of the
many-particle state that underlies
the fractional quantum Hall effect.
The height of the landscape represents
the amplitude of the quantum wave
of one electron as it travels among its
companions (shown as balls). The
arrows indicate the vortices induced
by the magnetic field, which attach
themselves to the electrons to
form composite particles. (Courtesy
of Bell Laboratories, Lucent Technologies.)

Suggested Citation:"2 New Materials and Structures." 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.
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The remainder of this chapter examines a few of the past decade's most impressive advances in materials and structures. The selections emphasize a number of themes that have emerged in materials research. Some of the discoveries have been completely unexpected. Others were predicted, although the experimentalists did not always know of these predictions when they did their work. Our thinking about new materials has changed fundamentally; we now consider dramatically more complex possibilities in our search for new materials than we did a decade ago. In some classes of materials that have been studied for many decades, we have achieved a much deeper understanding of physical and chemical mechanisms that govern their properties. This understanding in turn has led to improvements in the properties of the materials, either through elimination of problems inherent in existing materials by improved processing or by the introduction of new materials. Even in a material as thoroughly studied as carbon, a myriad of new forms has been discovered, exhibiting a wide range of properties. Shrinking the dimensions of well-known materials such as semiconductors has led to properties dramatically different from those of the bulk. New concepts in thin-film growth have led to improved film properties by changing the growth and processing windows. Finally, there has been a change in the attitude toward strain in heteroepitaxial systems that allows strain to be used to tailor the morphology as well as the electrical properties of the layers. The culmination of this effort is in the use of strain to induce self-assembly of quantum dots.

Complex Oxides

Surely one of the most surprising developments since the publication of the Brinkman report1 has been the discovery of high-temperature superconductivity in complex oxide materials, beginning in 1986 with the observation by Bednorz and Müller of superconductivity near 30 K in La2-xBaxCuO4. This discovery was rewarded with the 1987 Nobel Prize in Physics (see Table O.1). The field exploded with the discovery of superconductivity at temperatures in excess of the boiling point of liquid nitrogen (77 K). The family of known high-temperature superconducting materials now numbers near 100, with the highest superconducting transition temperature (Tc) above 130 K. High-temperature superconductivity has significantly altered the direction of condensed-matter and materials physics in several ways. The excitement generated by this totally unexpected discovery attracted researchers from throughout the field of condensed-matter and materials physics and beyond to the study of these fascinating materials. More recently, the principles that have been successful in the study of these materials have proven valuable in the study of other areas of condensed-matter and materials physics, most notably other sorts of oxides.

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

Suggested Citation:"2 New Materials and Structures." 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.
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High-temperature superconductors are much more complex than many of the materials that have occupied the attention of condensed-matter and materials physicists for many decades (see Figure 2.1). This complexity, however, is a two-edged sword, giving rise to a richness in the possible structures and properties of the materials, but also making the materials extremely challenging to produce, control, and understand.

The crystal structures of these materials are dramatically more complicated and have lower symmetry than those of low-temperature superconductors or semiconductors. The physical properties are similarly anisotropic. This makes the control of crystallographic orientation extremely critical. The unit cell is large. A large unit cell and low symmetry offer many opportunities for the formation of defects during materials preparation, either in individual atomic sites or in long-range crystallographic perfection.

The superconducting coherence length is of the order of the interatomic spacing in some crystallographic directions. This makes the materials exquisitely sensitive to defects—from atomic-scale defects, such as vacancies, interstitials, and substitutional atoms, to grain boundaries and other larger-scale imperfections. Separating the intrinsic properties of such materials from artifacts caused by defects is critical to gaining full understanding of the high-Tc phenomenon. It places extreme emphasis on materials preparation and serves as an example of the true collaboration that must exist between those who seek to understand and control the growth of the materials and those who probe their underlying physics, as illustrated in Box 2.3. Conversely, the carefully controlled introduction of

image

Figure 2.1
Historical development of inorganic superconductors, 
with Nobel prizes indicated by stars. Increasing 
superconducting transition temperature correlates with 
increased chemical complexity and more constituent 
elements, as shown by the numbers 1 to 5. (BCS stands 
for the Bardeen-Cooper-Schrieffer theory of classical 
superconductors.) [MRS Bulletin 19, 26 (1994).]

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.3 Vortex Matter: A Novel Window on Materials Physics

In the past 5 years, a new area of condensed-matter investigation has emerged, based on the remarkable behavior of vortices in superconductors. For decades, type-II superconductors in a magnetic field have been understood as arrays of quantized tubes of magnetic flux, each surrounded by a circulating "vortex" of supercurrent that defines its interaction with its neighbors and the outside world. In traditional superconductors, thermal energy is limited to about 20 K by the superconducting transition temperature, and the vortex tubes form an elastic solid. High-temperature superconductors offer a new possibility: thermal energies up to about 100 K may melt the vortex solid, creating a novel liquid state with dramatically different properties that arise from the relative motion of vortices. As early as 1988, motion of vortices below Tc was found to create undesired dissipation in high-Tc cuprates. The thermodynamic nature of the melting phase transition and of the resulting vortex liquid has been vigorously debated. Theorists soon realized that vortex phases and phase transitions embody many fundamental features of condensed-matter physics, including reduced dimensionality, entanglement of flexible line objects, and the role of disorder on elastic media. Studies of vortex matter provide new insight into these basic materials physics issues in other condensed-matter environments. The diversity of equilibrium vortex phases is illustrated in the phase diagram of Figure 2.3.1.

Although theoretical analysis of vortex liquids and solids abounded, experimentalists were frustrated by the quality of the available high-temperature superconducting materials. Real materials contain defects like impurity clusters, dislocations, twin boundaries, and rough surfaces. In superconductors, these defects generate pinning sites that immobilize vortices and remove them from participation in equilibrium behavior. The experimental observation of vortex phase transitions had to await more perfect materials with dramatically reduced defects and vortex pinning.

In 1992, the first indications of vortex lattice melting were observed in electrical transport experiments. These measurements accurately located the melting line in the H-T plane and gave tantalizing but indirect evidence that the transition was first-order in clean materials. Further transport experiments suggested that first-order melting was destroyed by controlled pinning disorder and suggested the existence of a critical point in the melting line. These and other experimental observations created new interest and activity in the field.

Experimentalists then sought the next level of fundamental information—thermodynamic characterization of the order and entropy changes on vortex melting—with magnetization and specific heat experiments. Such experiments require an even higher level of sample perfection to ensure thermodynamic equilibrium in the solid phase, where pinning effectiveness is significantly enhanced by shear elasticity. Sample size was a second serious problem: the most perfect crystals are also the smallest, making it extremely difficult to resolve the tiny magnetic and thermal signatures of melting from the much larger background. Nevertheless, sample preparation techniques continued to improve with better understanding of the roles of composition, growth rates, and annealing procedures. Improved materials enabled several landmark thermodynamic experiments, which have now settled the question of the thermodynamic order of the transition and raised new questions about critical points, vortex entanglement, and the dimensionality of the liquid.

(Box continued on next page)

Suggested Citation:"2 New Materials and Structures." 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.
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(Box continued from previous page)

image

 

Figure 2.3.1 A suggested phase diagram of vortex matter in the magnetic field-temperature plane. Several vortex liquid and solid phases are illustrated, including a liquid of entangled vortex lines, a perfect hexagonal lattice, a polymer glass of entangled lines, and solid phases disordered by point pinning defects (vortex glass) or by line pinning defects (Bose glass). The melting transition is first-order from a lattice and proposed to be second-order or continuous from a glass. A critical point may occur on the melting line, where the first-order character disappears. The normal and vortex liquid states are separated by a fluctuation dominated by crossover rather than by a true phase transition. (Courtesy of Argonne National Laboratory.)

Vortex matter has emerged as a vital field, with its own developing issues and international community of researchers. It extends traditional studies of atomic matter in several ways. For example, vortex density is linear in the applied magnetic field, so it can easily be changed by an order of magnitude with the twist of a dial. Experimental access to such a large density range is unheard of in atomic matter. The interactions among vortices are well-known Lorentz forces, which can be treated analytically or in simulation with no uncontrolled approximations. Advanced materials development has produced clean crystals with few pinning defects, revealing intrinsic thermodynamic behavior and its evolution under controlled disorder induced by electron or heavy ion irradiation. Finally, vortices can be set in motion by the Lorentz force from an externally applied transport current, enabling studies of driven phases, steady-state motion, and the new area of dynamic phase transitions. This remarkably rich microcosm of condensed-matter physics owes its existence to two materials developments: the landmark discovery of high-temperature superconductors, which introduced large thermal energies into the vortex phase diagram, and dramatic improvements in materials perfection, which enabled experimental studies of the delicate thermodynamics of collective vortex behavior.

Suggested Citation:"2 New Materials and Structures." 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.
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defects of a particular type into the material by, for example, ion irradiation or judicious atomic substitution allows the properties to be adjusted.

The superconducting oxides with Tc above 77 K all contain at least four elements, two of which are copper and oxygen. Oxygen moves readily in these materials, during both sample preparation and subsequent processing. Changing the oxygen content by just a few percent can determine whether a material is a superconductor or an insulator. It can also govern the symmetry and crystal structure of the material, resulting in phase transformations during specimen preparation that, to date, have been unavoidable. Precise control of the stoichiometry of the metal constituents is also required to optimize the superconducting properties, although the consequences of deviations from ideal stoichiometry are not nearly as critical for the metals as for oxygen.

Current interest in the high-temperature superconducting materials centers around two general areas: superconducting electronics and the carrying of large currents. The electronics applications can be further subdivided into logic and high-frequency applications. Electronics applications require thin films, generally in combination with films of other materials. The fabrication of reproducible tunnel junctions with useful properties for logic applications has been very challenging because of the incompatibility of high-temperature superconducting materials with most nonoxide barrier materials and the extremely short coherence length of the superconductor. Quite a few metallic oxides with compatible crystal structures have been identified and studied as a result of considerable research into suitable barrier materials. A promising area of application is in components for communications, particularly in the gigahertz frequency domain. The major issues are the surface resistance of the material and electrical nonlinearities at high frequencies. Though there has been considerable progress in improving surface resistance in the past few years, detailed understanding of the relationships between this and other relevant properties and the structure of the materials is still emerging.

Technological applications demand large-area films that can be deposited fast enough to be economically viable. There has been dramatic progress, with high-quality films of YBa2Cu3O77-x (see Figure 2.2) now available on substrates several hundred square centimeters in area.

Current-carrying applications require bulk material or thick films. Grain boundaries, especially those with significant misorientation between grains, are extremely detrimental to high critical currents because of both the extreme anisotropy of the materials properties and the properties of the grain boundaries themselves. The most successful approach for bulk materials with properties of potential technological interest has been the use of drawn, multifilament wires, especially in the bismuth system. The drawing induces alignment of the grains in the filaments and increases the critical-current density. More recently, biaxial orientation has been achieved in thick YBa2Cu3O77-x films deposited on metal substrates, either coated with an aligned buffer layer fabricated by ion beam-assisted deposition or with strong crystallographic alignment induced in the substrate by rolling.

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.2
The orthorhombic crystal structure of 
superconducting YBa2Cu3O7-x. The superconducting 
transition temperature of this material is above 90 K. 
Note the presence of four elements in the compound 
and the low symmetry of the structure. These 
characteristics make materials synthesis challenging 
and give rise to dramatic anisotropy in the physical 
properties of the material. (Courtesy of Princeton 
University.)

It has proven very fruitful to apply the principles discovered and techniques developed for high-temperature superconductivity to other classes of complex oxides. In some cases, this research has been driven by the need for materials with specific electronic or magnetic properties that are chemically and structurally compatible with high-temperature superconductors. These materials are typically needed as buffer or barrier layers. Compatible materials with other properties could be needed in the future if high-temperature superconducting devices are to be successfully integrated with devices having other functionality, such as memory and optical devices. Perhaps the most impressive demonstration of the application of lessons from high-temperature superconductivity has been the recent interest in colossal magnetoresistance in LaMnO3-derived materials (see Box 2.4).

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.4 Colossal Magnetoresistance: A Rediscovered Property of Old Materials

The discovery of high-temperature superconductivity changed our thinking about complexity in materials composition and structure. It emphasized that truly different properties could be obtained when new dimensions of complexity are considered. A particularly good example of the change in paradigm that high-temperature superconductivity has caused is the observation and study of colossal magnetoresistance (CMR). CMR was first observed in a perovskite material, lanthanum manganate (LaMnO3) in which some of the lanthanum is substituted by an alkaline-earth element: calcium, strontium, or barium (see Figure 2.4.1). The manganates have been known for many decades and had previously been studied for their promising catalytic properties. Presumably, more recent advances in magnetic storage technology sensitized researchers to rediscover the CMR effect and to pursue it with the vigor and determination sparked by the potential applications.

As innumerable materials with perovskite-based crystal structures received new attention in the aftermath of the high-temperature superconductivity discovery, the alkaline-earth-substituted manganates were found to have magnetoresistance effects up to three orders of magnitude larger than the previously known

image

 

Figure 2.4.1 Resistivity (r), magnetoresistance ratio (DR/RH), and magnetization (M) as a function of temperature curves for an epitaxial La-Ca-Mn-O film. [Reprinted with permission from S. Jin, M. McCormack, T.H. Tiefel, R.M. Fleming, J.M. Phillips, and R. Ramish, Applied Physics Letters 64, 3045 (1994). Copyright © 1994 American Institute of Physics.]

(Box continued on next page)

Suggested Citation:"2 New Materials and Structures." 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.
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(Box continued from previous page)

giant magnetoresistance metal multilayers. This observation was not predicted, and an understanding of the phenomenon is even now just being developed. The current consensus is that the relevant magnetic behavior of these compounds results from the motion of electrons between adjacent manganese ions via the intervening oxygen ion. This "double-exchange" interaction favors parallel (ferromagnetic) spins of neighboring Mn3+ and Mn4+ ions produced by the alkaline-earth substitution. This interaction competes with the antiferromagnetic coupling of manganese ions in the absence of the mobile electrons.

Ultimately, the symmetry of the spin ordering on neighboring manganese sites determines the electrical resistivity: parallel spins lead to low resistivity while anti-parallel spins give high resistivity. Thus the variation of resistivity with temperature can be understood in terms of the transition between a semiconducting paramagnetic state above the Curie temperature, Tc, and a metallic ferromagnetic state below Tc. Above Tc, a magnetic field enhances spin alignment and reduces resistance, but it has little effect below Tc. Therefore the largest bulk magneto-resistance effect is observed in the temperature region near Tc.

Magnetoresistive effects, Tc, and the mobility of electrons between manganese ions are thus closely connected. This is one of the key ingredients of manganite physics, and it has been tested recently in experiments in which the manganese-manganese overlap has been systematically varied through controlled bond-angle variations induced by substitutions of ions with various sizes. Because the richness of physical phenomena derives from the interplay among these electronic properties, local lattice strains, and other parameters of comparable magnitude, control and modification of materials properties are of paramount importance to exploration and exploitation of the manganites.

Although the bulk properties of manganites are of great interest in their own right, the recent discovery of spin-dependent electron transport across grain boundaries holds further promises. These phenomena occur in fields low enough for possible sensor applications in magnetic storage. Active research has now started on the development of small-area thin-film devices that rely on the spin-dependent resistance across interfaces, and this work will benefit from the many advances in materials engineering developed in contact with high-temperature superconductors.

Progress in these materials demonstrates once again the complex interplay among the detailed chemistry, materials structure, and electronic and magnetic properties, which provides a fruitful research field for fundamental studies and hope for future technology.

Although ferroelectric materials have been a topic of considerable research for a long time, developments in high-temperature superconductivity within the past decade have aroused new interest and insight, leading to improved electrode materials and better control over the structure and properties of the ferroelectrics themselves. An outgrowth is the current interest in high dielectric constant materials, generally complex oxides, for use in high-density semiconductor memories.

Suggested Citation:"2 New Materials and Structures." 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.
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Research on complex oxides in general and high-temperature superconductors in particular has spawned new growth and processing techniques as a result of the unusual properties of these materials. This research has given rise to materials of ever-improving quality, which allow physicists to probe the fundamental mechanisms at work and technologists to explore more fully their promise for applications. The area is vital and will be a source of exciting physics and materials research for the foreseeable future.

Electroceramics

Electroceramic materials have been studied and used for many decades because of their interesting and in some cases novel properties, such as ferroelectricity, piezoelectricity, pyroelectricity, and electro-optic activity. Current interest in the ferroelectrics centers on their potential in nonvolatile memories and high dielectric constant capacitors. Micromachines, such as accelerometers, displacement transducers, actuators, and so on, require piezoelectric materials. Room-temperature infrared detectors make use of pyroelectric properties. Electro-optic properties enable color filter devices, displays, image storage systems, and the optical switches required in integrated optical systems.

Electroceramics can serve as ''smart'' materials, functioning as both sensors and actuators (see Box 2.5). All smart materials have at least two phase transitions (e.g., crystallographic and electronic), and their synthesis and processing must be carefully controlled to regulate their excursions through phase space. The complexity of these phenomena and the materials that display them has made this an exciting area. There has been dramatic progress in the control of electro-ceramic materials properties, in understanding the relationships between properties of interest and the underlying microstructural mechanisms that control them, and in integrating various materials to give improved properties or even new behavior.

Progress has been especially impressive in the ferroelectric materials. Extensive research has focused on understanding the mechanisms responsible for the degradation of ferroelectric and high-permittivity perovskite thin films with time, temperature, and external field stress. The three most important degradation phenomena are ferroelectric fatigue, ferroelectric aging, and resistance degradation.

Ferroelectric fatigue, the loss of switchable polarization with repeated polarization reversals, is caused by pinning of domain walls, which inhibits switching of the domains. Elimination of fatigue is critical for nonvolatile memory applications. Recent results have shown that charge trapping at internal domain boundaries is the primary fatigue mechanism. Fatigue also induces changes in the oxidation states of isolated impurity point defects, which are much more stable than optically generated ones in unfatigued samples.

Fatigue can be largely eliminated in some ferroelectric systems [e.g., lead

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.5 Electroceramics in Composite Smart Materials and Biomimetic Systems

As the sensitivity and selectivity of a sensor increases, its structure generally becomes more complex. This often involves moving from single-phase materials to composites in which the connectivity, symmetry, or scale of the composite is designed to give a desirable field concentration or composite symmetry. Each constituent material has an associated phase transition. For example, polymeric materials with phase transitions in which the elastic properties change dramatically may be combined with ferroelectric materials in which the dielectric properties have an associated instability. The different types of instability allow fabrication of structures especially good for sensing and actuating. Different connectivity patterns optimize the tensor coefficients that contribute to the figure of merit for the application. Figure 2.5.1 shows the figure of merit for composite piezoelectric materials with different connectivities. The largest figure of merit by far is for the "moonie" structure, which consists of either a piezoelectric ceramic disk or a multilayer stack, sandwiched between two specially designed metal end caps. This design provides a sizable displacement, as well as a large generative force.

The evolution of these sensors and actuators moves the composites progressively closer to the configurations adopted by biological systems that perform the same function. The 1-3 composite hydrophones, for example, mimic the geometry and sensing function of the lateral line of the North Atlantic cod, a series of fibrous sensors spaced along the length of the fish. Similarly, the air space in the moonie transducer is a resonant cavity that corresponds to the fish swim bladder. By vibrating the bladder wall, a fish emits a low-frequency grunt that propagates well through the ocean. The same cavity also makes the moonie and the fish more sensitive. As more complex sensing and actuating functions are designed, inspiration can be gained by studying the analogous biological "composite" systems.

image

 

Figure 2.5.1
Hydrophone figure of merit for composite
piezoelectric materials. The connectivity
is indicated schematically above each bar.
(Courtesy of Pennsylvania State University.)

Suggested Citation:"2 New Materials and Structures." 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.
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zirconate titanate (PZT)] by using electrodes of metallic oxides, such as (La,Sr)CoO3 (LSCO) and (RuO2). The interaction between the ferroelectric material and its electrode plays a critical role in determining fatigue performance, perhaps because of the accumulation of oxygen vacancies near the electrodes during cycling. This suggests that electrodes with a large tolerance for oxygen deficiency, such as some of the metallic oxides, should offer better fatigue characteristics than those that serve as ineffective sinks for oxygen vacancies. The improvement in fatigue characteristics offered by this approach is shown in Figure 2.3.

Layered perovskite materials such as SrBi2Ta2O9 have recently received attention because of their lack of polarization fatigue even with simple metal electrodes. The emerging picture attributes this to less oxygen vacancy accumulation at the electrodes caused by either a smaller vacancy population or reduced

image

Figure 2.3
Developments in new electrode materials for ferroelectric capacitors 
have reduced the fatigue in these devices by more than 6 orders of 
magnitude. The upper image shows a "conventional" capacitor 
structure of the ferroelectric material Pb((Nb,Zr)Ti)O3 (PNZT) with 
unbuffered platinum electrodes, along with the fatigue of the remanent 
polarization. The polarization decays to half its initial value after 105 
cycles. The lower image shows a capacitor structure with the platinum 
electrodes buffered by La0.5Sr0  5CoO3. This capacitor shows no 
fatigue even after 1012 cycles. (Courtesy of the University of Maryland.)

Suggested Citation:"2 New Materials and Structures." 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.
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mobility. Smaller polarization in these materials should also result in weaker pinning and thus higher unpinning rates.

In addition to their ferroelectric and dielectric properties, many of the same materials also exhibit piezoelectric properties. PZT is the most widely used. On cooling from high temperature, the crystal structure of PZT undergoes a displacive phase transformation, and the point symmetry changes from cubic to tetragonal. To make use of piezoelectric ceramics, compositions near a second phase transition are chosen. At the Curie point, PZT converts from a paraelectric state with the ideal cubic perovskite structure to a ferroelectric phase located near a morphotropic phase boundary between the tetragonal and rhombohedral states. Very large piezoelectric coupling between electric and mechanical variables is obtained near this phase boundary. Much of the current research in this field involves looking for other morphotropic phase boundaries to further enhance the electromechanical-coupling factors. Ferroelectric thin films have been successfully used in a variety of microelectromechanical systems applications, including accelerometers, microvalves, pressure sensors, and infrared detectors. Micro-actuators and microsensors are designed to make use of the strong piezoelectric response of ferroelectrics such as PZT and to ease the fabrication and incorporation of on-chip electronics. The development of fabrication methods such as surface micromachining, low-stress silicon nitride deposition, and solution deposition of ferroelectric thin films has been essential.

Control of the growth of ferroelectric films has been a prerequisite for the progress that has been made. Film crystallinity has improved dramatically, and techniques such as ion bombardment have allowed the growth temperature to be lowered, improved the selection of the desired perovskite phase over the pyrochlore phase, improved the degree of preferred alignment in the films, and resulted in denser, smoother films. The use of oxide electrodes and templates for growth has helped eliminate unwanted orientations and dramatically improved the electrical properties. Control of the film microstructure has led to improved leakage current.

Nanoscale force microscopy has begun to allow examination of the switching of individual grains in a polycrystalline matrix with resolution of about 10 nm, as shown in Figure 2.4. The ability to follow localized processes will be important in unraveling the physical phenomena that govern these complex materials.

New Forms Of Carbon

The first documented conjecture of the existence of hollow-cage molecules of carbon appeared in 1966. Four years later, the existence of C60 (buckminster-fullerene) was predicted theoretically, with more rigorous treatments appearing in subsequent years. Nevertheless, the molecule itself was not observed until 1985, when it appeared serendipitously during a series of graphite laser vaporiza-

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.4
Scanning-force microscopy of topographic (a) and piezoresponse 
(b-f) images of a PZT film grown on a LSCO/TiN/Si substrate. 
The central grain was switched completely from the polarization 
direction down (dark) to up (white). The switch back of the 
central grain into the polarization down direction starts mainly 
at the grain boundaries with the surrounding grains. [MRS 
Bulletin 23, 39 (1998).]

Suggested Citation:"2 New Materials and Structures." 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.
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tion experiments designed to simulate the chemistry in a red giant carbon star. Finally, in 1990, a method was developed to produce macroscopic quantities to allow the intense investigation of this and related compounds that have been a focus of research in the present decade. As suggested in Figure 2.5, C60 turns out to be just one of a veritable menagerie of three-dimensional closed carbon molecules: spheres, tubes, particles, and combinations thereof, with one or multiple layers. The discovery of fullerenes by Curl, Kroto, and Smalley was recognized with the 1996 Nobel Prize in chemistry (see Table O.1).

The remarkable geometry of these molecules is enabled by slight deviations from the hexagonal bonding configuration found in graphite resulting from the desire to eliminate energetic dangling bonds at the edges of graphite sheets. The addition of twelve pentagons to the hexagonal array transforms the open graphite structure into any of the observed closed molecules that have only positive curvature. Heptagonal rings give rise to a saddle-shaped surface when buried among hexagons.

Carbon nanotubes, which were originally grown as a by-product in the fullerene-generating chamber, are quasi-one-dimensional structures with a simple and well-understood atomic structure. A chemist might think of a carbon nanotube as a monoelemental polymer. The nanotube is an ideal model for quasi-one-

image

Figure 2.5
Some representative forms of carbon. (Courtesy of Rice University.)

Suggested Citation:"2 New Materials and Structures." 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.
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dimensional structures because its known atomic structure makes computer simulations more reliable. Nanotubes can be as much as several microns long, and tube diameters range from one to a few tens of nanometers. A metal serves as a catalyst for nanotube formation, preventing the growing tubular structure from wrapping around and closing into a smaller fullerene cage. Nanotube growth is believed to take place at the open ends of the tubes. During growth, the open tubule end required to fabricate long, single-wall nanotubes can be maintained by a high electric field, by the entropy opposing orderly cap termination, or by the presence of a metal catalyst. The tube ends tend to close quickly when the growth conditions become inappropriate, for example, when the temperature drops or when the carbon atom flux is too low. As long as the tube end is open, carbon atoms can be deposited on the tube-end peripheries, and they can grow. When pentagons are formed for some reason, the tubes will be capped. If the axial growth rate is dominant over the radial one, the tubule will become a single-shell tube. A comparable growth rate in both the axial and radial directions will form spheroidal particles.

Characterization of carbon nanotubes has been slow compared to fullerene research activity, partly because of the inability to synthesize macroscopic quantities of the tubules and to refine them. Many nanotubes are in the form of a multiple-shell structure of nested cylindrical tubes separated by about 0.34 nm, which is the same as the d0002 lattice spacing of graphite. Cylindrical crystals are often seen in biological protein crystals but rarely in inorganic materials. Recent measurements on single-wall carbon nanotubes have shown that they do indeed act as genuine quantum wires, confirming theoretical predictions, as shown in Figure 2.6.

Electronic and mechanical properties of nanotubes deviate from those of a bulk graphite crystal. Depending on tubule diameter and helicity, both of which affect the band gap, the behavior can range from metallic to semiconducting. Because it makes for a more symmetrical structure, less helicity leads to better conductivity. This leads to true molecules that are also true metals, something chemistry has never had before. Because of the quasi-one-dimensionality of these nanotubes, conduction is quantized.

One unexpected phenomenon in nanotubes is the ability to fill them with a material. Nonhexagonal carbon rings in the hexagonal network are responsible for tubule morphologies and presumably local strain. After deposition of a small amount of lead on tubule surfaces and heating, some of the metal clusters move to heptagon sites. Nanotubes can be opened by mild oxidation at the reactive site at the closed end. On heating, some of the lead is transported into the central hollow in the tubule. The intercalated material is crystalline and not pure lead but lead carbonate or oxide. This finding suggests that the tubule tips react selectively in air at elevated temperature, but the rest of the tubules do not react. Strain induced by including pentagons in the tubule tips may be responsible for the selective reaction. Carbon onions have also been stuffed with metals and metal carbides.

Suggested Citation:"2 New Materials and Structures." 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.
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image

image

Figure 2.6
(a) Atomic-force microscope tapping-mode image of a 
carbon nanotube on top of a Si/SiO2 substrate with two 
15-nm-thick platinum electrodes and a corresponding 
circuit diagram. This single-wall nanotube has a diameter 
of ~1 nm. Its total length is 3 mm, with a section of 140 nm 
between the contacts to which a bias voltage (Vbias) is 
applied. A gate voltage (Vgate) applied to the third 
electrode in the upper left corner of the image is used 
to vary the electrostatic potential of the tube. (b, top) 
Current-voltage curves of the nanotube at a gate 
voltage of 88.2 mV (trace A), 104.1 mV (trace B), and 
120.0 mV (trace c). The inset shows more I-Vbias curves 
with Vgate ranging from 50 mV (bottom curve) to 136 mV 
(top curve), with vertical offsets for clarity. The variation 
with Vgate of the gap around Vbias = 0 implies Coulomb 
charging of the tube. The stepwise increase of the 
current at higher voltages may result from an increasing 
number of excited states entering in the bias window. 
(b, bottom) Current versus gate voltage at Vbias = 30 mV. 
The two traces shown were performed under the same 
conditions. [Reprinted with permission from S.J. Tans, 
A.R.M. Verschueren, and Cees Dekker, "Individual 
single-wall carbon nanotubes as quantum wires," 
Nature 386,474 (1997). Copyright © 1997 Nature.]

Suggested Citation:"2 New Materials and Structures." 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.
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A large family of structures can be generated in the high-temperature regimes of the arc experiments used to produce fullerenes and nanotubes. In addition to nanotubes, the arc yields a large quantity of polyhedral graphitic particles with well-defined faceting and a wide-ranging size distribution (8 to 60 nm). If electrodes incorporating a metallic salt or oxide are used, a small percentage of encapsulated metallic particles within closed graphitic shells is produced. The quasi-spherical onionlike structure (buckyonion) is the exclusive result of the irradiation process. The same effect may be obtained from diverse starting carbon materials (polyhedral graphitic particles, buckytubes, and even disordered forms of carbon). The onions grow by a kind of internal epitaxy as the layers reorganize, progressing from the surface to the central shell. The model structure for the final product is the concentric arrangement of spherical fullerenes, formed by 60n2 carbon shells. Small buckyonions (2 to 4 shells) are very stable under intense electron bombardment, suggesting that they may be the most stable forms of carbon cluster.

By virtue of their unique structures, fullerenes exhibit novel chemical transformations. These molecules are spherical or nearly so. Molecules with high point-group symmetry, which are not bound strongly in the solid state, tend to crystallize into structures with long-range periodicity of the molecular centers of mass, but the molecular orientations are random or even dynamically disordered. Because C60 has high electron affinity, it forms anion salts with alkali and alka-line-earth metals as well as with strong organic molecules. K3C60 and other alkali fulleride salts exhibit superconductivity with Tc above 30 K, as discussed in Box 2.6.

The organic molecule tetrakis dimethylaminoethylene (TDAE) is known to be an effective electron donor. A C60-TDAE salt has been formed that exhibits a ferromagnetic state below 16 K. This material, which has a low-symmetry mono-clinic structure because of the highly nonspherical nature of TDAE, holds the record for the highest Curie temperature of any purely organic molecular solid.

Nanoclusters

Ever since the early 1980s, when scientists began discovering the various potentially advantageous properties of ultrasmall grains of material—nano-clusters—there has been tremendous activity as researchers strive to create and control new types of particles. Much of the recent research has been directed at finding ways to make small clusters of uniform size with common optical, electrical, and mechanical properties. These efforts have already begun to have commercial payoffs, as in the case of ceramics and chemical catalysts that have increased efficiency because of their high surface-to-volume ratio.

In any material, substantial variation of fundamental electrical and optical properties with decreasing size will occur when the electronic energy-level spacing exceeds the temperature. The variation is especially pronounced for semi-

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.6 Superconductivity in Alkali and Alkaline-Earth-Doped C60

Because fullerenes act as electron acceptors, they can form different types of salt. Those of the alkali and alkaline-earths have particularly interesting electronic properties. Photoemission studies probing the occupied electronic states, as well as inverse photoemission measurements probing the unoccupied electronic states, have allowed direct monitoring of the nature of electron doping into the C60 levels. The valence band of solid C60 is derived from a fivefold degenerate hu orbital. The stability of this orbital makes it difficult to remove electrons from C60. On the other hand, C60 is a good acceptor because of the threefold degenerate t1 u and t1 g levels. Exposing C60 to alkali vapor results in electron filling of the t1 u level. Because the Fermi energy is pinned to the top of the filled level, with increased filling, the spectral manifold is shifted to lower energy. The threefold degeneracy of the level means that half-filling corresponds to three electrons. K3C60 is a metal that becomes superconducting at temperatures lower than 19 K. Further filling leads to the compound A6C60 (where A is an alkali element). Because this material has a fully filled t1 u lowest unoccupied molecular orbital, it is insulating. The structures of C60, K3C60, and Cs6C60 are shown on the right in Figure 2.6.1. The structures are cubic but are represented in tetragonal form. The alkali metal atoms sit in the tetragonal and octahedral voids of the fcc-C60 structure. With alkaline-earth metals, the t1 g orbital derived band is also partially filled. Thus Ca5C60, Sr6C60, and Ba6C60 are also superconducting.

image

 

Figure 2.6.1 Normal (PES) and inverse (IPES) photoemission density of states of C60 as a function of exposure to potassium vapor. The gradual filling of the C60 t1 u lowest unoccupied molecular orbital is clearly seen. The spectral manifold shifts to lower energy with increasing exposure because of Fermi level pinning. On the right, body-centered tetragonal representations show the structures of Cs6C60 (x = 6), K3C60 (x = 3), and C60 (x = 0). [Left: Journal of the Physics and Chemistry of Solids 53, 1433 (1992); Right: MRS Bulletin 19, 28 (November 1994).]

Suggested Citation:"2 New Materials and Structures." 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.
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conductors, in which size dependence emerges at relatively large size compared with metals, insulators, or molecular crystals. This arises because the bands of a solid are centered about atomic energy levels, with the width of the band related to the strength of the nearest-neighbor interactions. In the case of van der Waals or molecular crystals, which have weak nearest-neighbor interactions, the bands in the solid are very narrow, giving little size dependence of optical or electrical properties in the nanocrystal regime. As the cluster grows, the center of a band develops first and the edges last. Thus, in metals, for which the Fermi level lies in the center of a band, the relevant energy-level spacing is small, and at temperatures above a few Kelvin, even small clusters (10 to 100 atoms) have electrical and optical properties that resemble those of a continuum. Because the Fermi level lies between two bands in semiconductors, the band edges dominate the low-energy optical and electrical behavior. Optical excitations across the gap thus depend strongly on size even for clusters as large as 10,000 atoms (see Figure 2.7). Electrical transport also depends heavily on size, mainly because of the large variation in energy required to add or remove charges on a nanocrystal. As a consequence, many useful size-dependent phenomena are observed in clusters characterized by an interior that is structurally identical to the corresponding bulk solid and a surface layer that contains a substantial fraction of the total number of atoms in the cluster.

image

Figure 2.7
Quantum confinement causes the optical spectra of CdSe 
nanocrystals to sharpen and move to higher energy as the 
size of the particle shrinks. [MRS Bulletin 20, 23 (1995).]

Suggested Citation:"2 New Materials and Structures." 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.
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The surface of a semiconductor exerts a critical influence on optical and electrical properties. Passivation is critical to the control of these properties, and clusters are no exception. As in the bulk, the ideal termination removes the surface reconstruction, leaves no strain, and simply produces an atomically abrupt jump in the chemical potential for electrons or holes at the interface. A great deal of current research into semiconductor clusters is focused on the properties of quantum dots with the bulk bonding geometry and with surface states eliminated by immersion in a material of larger gap.

There have been two approaches to the fabrication and investigation of quantum dots. The top-down approach involves the gradual reduction of the extent and dimensionality of solid matter. The quantum dots thus produced are between 1 mm and 10 nm in size. They are well passivated and immobilized on a substrate where they may be investigated optically and electrically. The bottom-up approach views quantum dots as extremely large molecules or colloids. Nanocrystals vary in diameter from 1 nm to about 20 nm. Their surfaces are derivatized with organic molecules, which prevent them from aggregating and render them soluble. Whether these organic molecules provide electronic passivation as well is an open question. These samples may be manipulated chemically in a wide variety of ways, yielding entirely new sample configurations (see Figure 2.8).

Much of the work on semiconductor nanocrystals started with the realization that it is possible to precipitate a semiconductor out of an organic liquid. A set of precursors are injected into a very hot liquid. Upon injection the temperature immediately rises above the nucleation limit so that nucleation occurs, and then the temperature quickly drops. The concentration drops quickly because dilution occurs, resulting in crystallites in a fluid. The crystallites become encapsulated in a layer of organic material so that they do not collide and fuse. The crystallites are each single-crystalline, and they have a preponderance of low-energy, low-index facets with very few high-index surfaces. One issue that quickly emerges in the study of nanocrystals is that approximately half the atoms or more are on the surface of the crystal, making control of the surface even more important than in bulk materials or conventional films.

Although the high-pressure behavior of semiconductor nanocrystals is ultimately the same as that of the bulk, the details differ rather dramatically. For example, CdSe nanocrystals 4.2 nm in diameter require almost three times greater pressure than does bulk material to transform from the wurtzite to the rock salt crystal structure. When the nanocrystal does transform, it remains a single crystal, indicating that only a single nucleation event has occurred per crystallite. The smaller the crystallite, the higher the transformation pressure. This difference has been explained by noting that the transformation involving the least atomic motion requires transforming from a wurtzite crystallite with only low-index surfaces to a rock salt structure with numerous high-energy faces. This consideration increases in importance with decreasing crystallite size, as more and more atoms in the crystallite occupy its surface.

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.8
Gallery of quantum dot structures: (a) Positions of cadmium and sulfur atoms 
in the molecular cluster Cd32S55, as determined by single-crystal x-ray 
diffraction. This cluster is a small fragment of the bulk CdS zincblende 
lattice. The organic ligands on the surface are omitted for clarity. (b1) and 
(b2) Transmission electron micrographs of CdSe nanocrystals with hexagonal 
structure, as viewed down different crystallographic axes. These nanocrystals 
were prepared colloidally and exhibit well-defined facets. The surfaces are 
passivated with organic surfactants. (b3) and (b4) Transmission electron 
micrographs of CdS/HgS/CdS quantum dot quantum wells. The faceted 
shapes show that epitaxial growth for passivation is possible in colloidally 
grown nanocrystals. (c) Transmission electron micrograph of a CdSe 
quantum dot superlattice. (dl) Scanning electron micrograph of two coupled 
GaAs quantum dots about 500 nm in diameter. The strength of the coupling 
can be adjusted by adjusting the gate voltage. (d2) Transmission electron 
micrograph of coupled CdSe nanocrystal quantum dots 4 nm in diameter. 
These crystallites are joined by an organic molecule. The coupling can be 
tuned by changing the linker length. (e) Transmission electron micrograph 
of InAs quantum dots in a GaAs matrix, prepared by molecular beam epitaxy. 
[Reprinted with permission from A.P. Alivisatos, ''Semiconductor clusters, 
nanocrystals, and quantum dots,'' Science 271, 934 (1996). Copyright © 1996 
American Association for the Advancement of Science.]

The main reason for the high level of interest in semiconductors of reduced dimensionality results from their large quantum-size effects. The band gap in cadmium selenide can be tuned between 4.5 and 2.5 electron volts (eV) as the size is varied from the molecular regime to the macroscopic crystal, and the radiative lifetime for the lowest allowed optical excitation ranges from tens of picoseconds to several nanoseconds. The energy above the band gap required to add an excess charge decreases by 0.5 eV. The melting temperature increases from 400 to 1600ºC, and the pressure required to induce transformation from a

Suggested Citation:"2 New Materials and Structures." 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.
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four- to a six-coordinate phase decreases from 9 to 2 GPa. There are many questions in the literature about what would happen to an indirect-gap material, such as silicon, as the nanocrystallite size decreases. As silicon crystallites become smaller and smaller, they become more emissive because of the quantum-size effect. However, the fundamental matrix element does not change; it is only a density-of-states effect. Silicon will never become a direct-gap material in the relevant size range. On the other hand, a direct-gap semiconductor like cadmium selenide already has an allowed electronic transition, and as it shrinks, it is more allowed. Direct-gap semiconductors will retain their advantages over indirect-gap materials proportionally. As they are made smaller and smaller, they will continue to radiate more efficiently than indirect-gap semiconductors. One recent development with potentially far-reaching impact is the development of ultra-small, highly efficient semiconductor lasers, known as quantum dot lasers. Because of quantum confinement, controlling the size of the nanocluster leads to control of the color of light emitted (see Figure 2.9). Quantum dots are so small that they tightly confine normally mobile electrons, so the charges spend less energy on their wanderings. Thus more energy is released when the electron and

image

Figure 2.9
Solid lines show optical absorption (ABS) and photoluminescence 
(PL) spectra at 10 K for close-packed solids of CdSe quantum dots 
3.85 nm (curve a) and 6.2 nm (curve b) in diameter. Dotted lines are 
photoluminescence of the same dots but in dilute form dispersed in 
a frozen solution. [Reprinted with permission from C.B. Murray, 
C.R. Kagan, and M.G. Bawendi, "Self-organization of CdSe 
nanocrystallites into three-dimensional quantum-dot superlattices," 
Science 270, 1336 (1995). Copyright © 1995 American Association 
for the Advancement of Science.]

Suggested Citation:"2 New Materials and Structures." 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.
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hole combine, resulting in a shorter wavelength. The smaller the dot, the greater the frequency shift. Making a true quantum dot laser has proven difficult. It is not straightforward to make the dots the same size, and the result has been that the devices emit a range of light frequencies. Very recent work in this area has yielded dots of more uniform size, with characteristics more indicative of true laser activity.

Thin Films, Surfaces, And Interfaces

The revolution in the control of the growth and properties of thin films, surfaces, and interfaces traces its origin to advances in vacuum technology in the 1960s and 1970s and to the development of surface-sensitive probes such as surface spectroscopies and electron diffraction techniques. Although the field is no longer new, there have been impressive gains in the past decade, both in the enabling technologies for thin-film growth and in the insights that growth and surface studies have provided. The result has been more control over and understanding of the growth process, and hence the properties of the resulting film. Defects can be placed as desired or eliminated altogether. More complex materials can be grown with acceptable quality as a result of the increased control.

The base vacuum available in molecular beam epitaxy systems has improved by more than an order of magnitude to ~10-12 torr. Such improvement allows the surface contamination prior to growth to be lowered and also results in less contamination in the growing film. Alternatively, the growth rate can be lowered while preserving low levels of contamination, improving control over the process. Various growth techniques have been developed or refined, as discussed elsewhere in this report. Of particular relevance are pulsed-laser deposition, which has proven particularly useful for the deposition of complex materials such as high-temperature superconductors; ion beam-assisted deposition, which induces crystallographic alignment in a growing film independent of the crystallography (or lack thereof) of the substrate; and various refinements of traditional techniques such as molecular beam epitaxy, chemical vapor deposition, and sputtering.

The ability to image surfaces and films in real space, as discussed in Chapter 6, has revolutionized studies of film growth. Most of the earliest studies using the new scanning probe microscopies corroborated surface structures previously arrived at through tortuous interpretation of surface spectroscopic and diffraction data. It quickly became apparent, however, that the ability to observe surfaces and films in real space with atomic resolution could enable far more understanding of surfaces than could be derived through more indirect techniques. Nucleation sites, evolution of surface morphology, etc., can now be observed directly, as discussed in Box 2.7. Another advance in the area of monitoring that has had a significant impact on thin film studies is the ability to monitor film growth in real time. Perhaps the most impressive demonstration of the power of this technique is the in situ studies of chemical vapor deposition growth performed at

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.7 Early Stages of Film Growth

The study of film growth has been increasingly characterized by the application of surface science methods to understanding growth at the atomic level. Both technology and the desire for fundamental knowledge at the atomic level are driving the search for atomic-level control of the fabrication processes for novel materials and new devices.

Growth of thin films from atoms deposited from the gas phase is intrinsically a nonequilibrium phenomenon, governed by a competition between kinetics and thermodynamics. Precise control of the growth and thus of the properties of thin films becomes possible only through an understanding of this competition. Experiment and theory have both made impressive strides in exploring the kinetic mechanisms of film growth, including adatom diffusion on terraces, along steps, and around island corners; nucleation and dynamics of the stable nucleus; atom attachment to and detachment from terraces and islands; and interlayer mass transport. The synergism between experiment and theory has tremendously improved our understanding of the kinetic aspects of growth.

The diffusion of an adatom on a flat surface or terrace is by far the most important kinetic process in film growth. Despite the vital importance of surface diffusion, accurate determination of the surface diffusion coefficient in a broad range of environments has been a major challenge. Scanning-tunneling microscopy (STM) has improved the situation considerably. STM can image a vastly broader range of surfaces than can field ion microscopy, which has traditionally been used for such studies. Atom-tracking STM has been especially valuable because it allows an atom or cluster to be followed as it migrates. Information from such experiments can then be fed into theories to provide deeper understanding of the mechanisms at play in adatom diffusion.

The availability of new probes of the initial stages of nucleation and growth has meant that even well-studied systems have continued to yield new insights. Much recent attention has focused on the possible pathway for nucleation of a silicon addimer, the stable nucleus for a wide range of growth conditions for homoepitaxy on Si(100) (see Figure 2.7.1). A silicon adatom may have multiple diffusion pathways on the surface before finding a partner, as all calculations have suggested. Recent experiments have focused on determining the relative stability of different dimer orientations and have been able to distinguish slight differences. Studies have also focused on the preferred locations of dimers, where there are still significant differences between experiment and theory. Experiments have revealed some surprisingly large anisotropies in larger islands as they grow.

As islands grow, specific island shapes or morphologies develop. One class is compact, whereas another is fractal-like, with rough island edges or highly anisotropic shapes. Recent studies of two-dimensional island formation in metal-on-metal epitaxy have identified several aspects of atom diffusion along island edges that are important in controlling the formation of fractal islands. Fractal island growth is very dependent on bonding geometry, having been reported only on face-centered cubic (111) or hexagonal close-packed (0001) substrates, both of which have approximately triangular lattice geometry. Growth on face-centered cubic (100) surfaces with square lattice geometry has so far resulted only in compact islands. This observation has required modification of the classic diffusion-limited aggregation model.

(Box continued on next page)
Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.7 Continued

Explorations seeking improved understanding of the relative importance of the various atomistic rate processes important in the initial stages of film growth have led to discoveries of various ways of rate manipulation to improve the quality of films grown by vapor-phase epitaxy. For example, any enhancement of downward diffusion of adatoms below their landing site would improve layer-by-layer or two-dimensional growth. Increasing the ability of an atom to cross a corner of an island at which two edges meet would lead to more compact islands. Larger surface diffusion would lead to earlier achievement of step-flow, hence layer-by-layer growth. Such insights are already leading to greater control over the precise morphology of thin films to achieve desired structures. The rate of progress in this area will surely increase as our understanding continues to grow.

image

 

Figure 2.7.1 Scanning-tunneling microscope images showing the rotational dynamics of a silicon ad-dimer formed on top of a dimer row in Si(001). (a) The bond of the ad-dimer (arrow) is parallel to the substrate dimer rows in the 2 × 1 reconstruction, as schematically shown below the image. (b) The same ad-dimer has rotated by 90 degrees. The two images were taken 40 seconds apart at room temperature. The orientation in (a) is energetically slightly more stable. [Reprinted with permission from Z. Zhang and M.G. Lagally, "Atomistic processes in the early stages of thin-film growth," Science 276, 377 (1997). Copyright © 1997 American Association for the Advancement of Science.]

Suggested Citation:"2 New Materials and Structures." 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.
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synchrotron sources, as described in Chapter 6. The power of this technique has just begun to be tapped.

There have been major strides in understanding the kinetics of epitaxial growth. In silicon molecular beam epitaxy, it has been shown that the notion of a minimum temperature for epitaxial growth is incorrect, even at a fixed growth rate. Instead, for all temperatures of the regime studied (30 to 300ºC), the growing epitaxial film becomes amorphous above a limiting thickness. The epitaxial thickness for the epitaxial-amorphous transition follows an Arrhenius temperature dependence, depending on the deposition rate. Other systems, including gallium arsenide, also exhibit this behavior. Such improved understanding is relevant to the problem of dopant incorporation in silicon epitaxy, in which controlled growth of a highly doped layer of arbitrary thickness can be achieved at a temperature low enough to avoid dopant segregation. This low growth temperature approach allows very sharp doping profiles.

The ability to control the structure of inorganic thin films on an unprecedented scale has recently been demonstrated using a technique called "glancing angle deposition," which maximizes atomic shadowing and minimizes adatom diffusion. By making use of extremely high adatom angles of incidence, coupled with substrate rotation (or other motion) around the substrate normal, slanted, zigzag, helical, or other morphologies have been demonstrated, as shown in Figure 2.10. The helically structured films have been shown to rotate the plane of polarization of light in a manner analogous to other chiral media; pitches between 50 and 2000 nm have been demonstrated.

Major progress has been made in substrate engineering and control, along with understanding of the role of surface properties in film growth, in the last decade. Compliant substrates allow control of the strain induced by the constraint of epitaxy so that the electronic and structural properties of the film can be tailored to fit the needs at hand. If the substrate is sufficiently thin, or if the top layer is sufficiently weakly bonded to the rest of the substrate, the substrate can readily deform elastically when a lattice mismatched material is grown on it. The extent to which this approach can be reduced to practice is as yet unclear. Epitaxy has been shown to be strongly affected by the presence of an adsorbate (surfactant), which changes the surface properties because of its bonding arrangement, usually accompanied by a change in the surface reconstruction (see Figure 2.11). The saturation of the dangling bonds yields a chemical passivation of the surface, a change in electronic structure, and a reduction of the surface free energy, which causes the strong segregation of the surfactant layer. The adlayer floats on the growing film, and only a small fraction is incorporated. These properties can have a profound effect on the growth mode of the film, converting a film that normally grows in an island fashion to layer-by-layer growth. In heteroepitaxy, interfaces prepared by growth with an adsorbate present, either hydrogen or a dopant species, remain sharper at a higher growth temperature than interfaces prepared by growth on a bare surface.

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.10
Films deposited by glancing angle deposition (GLAD): (a) oblique 
evaporated flux at 85 degrees from the substrate normal produces 
a slanted, porous micro-structure; (b) periodically alternating the 
oblique flux from angles of 85 degrees to -85 degrees produces a 
porous film composed of isolated "zigzags"; (c) rotating the 
substrate about an axis normal to the wafer center while maintaining
 obliquely incident (85 degree) flux produces isolated helical 
structures on the substrate. [Reprinted with permission from 
K. Robbie and M.J. Brett, "Sculptured thin films and glancing 
angle deposition: Growth mechanics and applications," Journal 
of Vacuum Science and Technology A 15, 1460 (1997). 
Copyright © 1997 American Vacuum Society.]

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.11
The use of a surfactant dramatically alters the morphology 
of a growing film. In this figure are medium energy ion 
scattering (MEIS) spectra for germanium films on Si(111) 
at 470 ºC. Both random (solid line) and channeling 
(dotted line) data are shown. (a) Ten monolayers of 
germanium deposited with no gallium. Note the island 
morphology. (b) Twenty monolayers of germanium 
deposited with one-third of a mono-layer of gallium 
as a surfactant. Note the columnar morphology. (c) 
Twenty-eight mono-layers of germanium with one 
monolayer of gallium surfactant. Note the smooth 
morphology. [Reprinted with permission from J. Falta, 
M. Capel, F.K. Legroves, and R.M. Tromp, 
Applied Physics Letters 62, 2962 (1993). 
Copyright © 1997 American Institute of Physics.]

Suggested Citation:"2 New Materials and Structures." 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.
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Artificially Structured Materials

Artificially structured materials—materials whose structure or composition differs in some intentional way from materials available in nature—have enabled many of the advances in condensed-matter and materials physics over the past decade that are described in this report. Such materials are frequently dominated by interfaces, a feature that often leads to properties very different from those of bulk materials. Because artificially structured materials are frequently prepared far from thermodynamic equilibrium, they can exhibit phases or properties that are not otherwise achievable. Their multilayer structural length may be on the order of the length-scale characteristic of nonlocal physical phenomena in solids, making such materials ripe for fundamental investigations. The physical limits to their fabrication have been pushed to the greatest extent for semiconducting materials. The coming decade will undoubtedly see the same limits pushed for other classes of materials as well: complex oxides, polymers, biological materials, and composites are a few of the most exciting.

Many if not most artificially structured materials involve heteroepitaxy, the crystallographically oriented growth of one material on a dissimilar one. In nearly all cases, heteroepitaxy involves a lattice mismatch between the different materials, which produces strain in the initial epitaxial layer. The strain is relieved as the epitaxial film thickness increases, by a roughening of the surface of the epitaxial layer or by the introduction of defects such as dislocations into the epitaxial layer or both. Controlling when and how strain relief occurs is a key issue in heteroepitaxy.

The strained films discussed here are grown near the thermodynamic limit. Sputter-deposited films, such as those discussed in Box 2.8 and in the section on magnetic multilayers in Chapter 1, are deposited in the kinetic limit, which is required for alternating layers of extremely disparate materials.

One distinct trend has been toward the use of more highly strained heteroepitaxial combinations, such as InGaAs/GaAs (see Figure 2.12) and SiGe/Si. Such systems must be approached with great care in order to achieve the optimum structural (and consequently electrical or optical) quality. Morphology-related strain relief is not a new phenomenon. Mounding in a film may partially relieve elastic stress of the epitaxial material within each mound. Even though there is additional compression of lattice planes at the grooves between the mounds, the roughened morphology is energetically favorable because the volume of material subjected to additional stress is much less than the volume experiencing partial stress relief. Another very important factor is the surface free energy. Roughening generally increases this energy, so that roughening is suppressed until the free-energy reduction in the system by stress relief is greater than the free-energy increase caused by surface area increase and step formation.

Strain-induced roughening can be problematic in the fabrication of coherently strained device structures, for which it is important to understand the early

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.8 Multilayers for X-Ray and Extreme Ultraviolet Optics

Multilayers are artificially structured materials that are periodic in one dimension in composition or both composition and structure. These layered materials are, if perfect, equivalent to single crystals in one dimension. Thus the multilayer acts as a superlattice, diffracting longer-wavelength radiation in a manner directly analogous to the diffraction of x-rays by crystals. This application of multilayer structures as dispersion elements for soft x-rays and extreme ultraviolet radiation was the impetus for the first attempts to synthesize multilayer materials. Many factors determine the character of the multilayer response to an incident spectrum. The important parameters are the substrate quality (roughness and figure), the uniformity and thicknesses of the component layers, the x-ray optical constants of the component elements, the number of layers in the structure, the interfacial width between layers (i.e., interfacial abruptness in atomic position and composition), and roughness at layer interfaces. Many of these factors depend in turn on the synthesis process and the materials. Therefore, understanding of multilayer performance depends on a knowledge of the relationships among synthesis process, resultant microstructure, and properties for these engineered microstructure materials.

The individual layers of the optics have a specific set of properties related to bulk forms of the materials. Primary issues include the compositions and structures of the layers, the x-ray optical properties of the layers, and uniformity of the areal density of atoms in the layers (see Figure 2.8.1). Specific synthesis questions relate to the film nucleation and growth behavior because deposition of material A onto a substrate or layer B may differ substantially from deposition of material B onto a substrate or layer A. Interfaces within the multilayer must also be controlled to an excruciating degree. They must be compositionally abrupt, smooth, clean, and flat.

Recent work has shown that precise control of sputtering parameters during multilayer deposition allows control of individual layer thicknesses to an accuracy of better than ~0.01 nm, which greatly enhances reflectivity for both nickel-carbon and tungsten-carbon multilayers. Sputter deposition of multilayers typically produces higher quality structures than thermal source techniques. This has been attributed to ion bombardment by the sputter plasma resulting in smoother interfaces and higher reflectivities. Results of ion beam-assisted deposition support this proposal. Thermal-evaporation-source synthesized rhodium-carbon multilayers with and without argon ion bombardment (300 eV) at an incidence angle of 10 degrees show the effect. A gain of more than a factor of two in reflectivity was found for the samples "polished" by the incident ion beam. This increased reflectivity is attributed to smoothing of the interfaces between the carbon and rhodium by a factor of 30 percent by the ion bombardment. Combining these two improvements in control are likely to facilitate fabrication of higher quality multilayer structures, particularly of smaller periods.

Multilayer structures may be optimized and engineered for specific spectral ranges by an analysis for optimum materials on the basis of their x-ray constants and an assessment of their suitability for multilayer microstructure synthesis. As an example, there are difficult spectral regions in which the lowest absorption materials useful as spacer layers are either toxic, such as beryllium, or unstable, such as lithium. Candidate materials such as magnesium are difficult to deposit as

(Box continued on next page)
Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.8 Continued

a uniform thin film because of its low melting point and high vapor pressure. Mg2Si has been identified as a possible new material for this application, and W/Mg2Si multilayers have reflectivities that are the highest reported in the 800 to 1300 eV range.

Multilayer x-ray optics and instrumentation are now mature enough to be both an enabling technology and an area of scientific investigation in their own right. The promise that was held for soft x-ray and extreme ultraviolet multilayer optics is now coming to fruition, and many of the advanced optical systems envisioned in the late 1970s are becoming reality. Such x-ray optics will likely form the critical element of vacuum ultraviolet optics for the next generation of lithography in the semiconductor industry.

image

 

Figure 2.8.1 Transmission electron micrograph of a 6.9 nm period Mo2C/Si multilayer x-ray mirror (top) and the experimental and calculated reflectivity as a function of x-ray wavelength (bottom). The experimental reflectivity is 93.5 percent of the calculated values. [Reprinted with permission from T.W. Barbee, Jr., and M.A. Wall, ''Interface reaction characterization and interfacial effects in multi-layers,'' Proceedings of the SPIE 3113-20, 204 (1997). Copyright © 1997 SPIE.]

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.12
Atomic force micrograph of self-assembled InAs islands deposited by 
molecular beam epitaxy on a patterned GaAs(001) surface. The valleys 
and hills, which have been defined by optical lithography and etching, 
have a period of ~240 nm. The InAs islands, which are formed by 
deposition of about 1.5 monolayers of InAs on the corrugated GaAs 
surface, are preferentially located in the valleys of the surface. The 
height and diameter of the InAs islands are 10 nm and ~20 nm, 
respectively. These islands will be transformed into quantum dots 
by in situ overgrowth of a GaAs cladding layer. 
(Courtesy of the University of California at Santa Barbara.)

stages of the transition in order to avoid or suppress three-dimensional growth. On the other hand, the strain-driven transition is beneficial for the self-assembly of quantum dots, in which it is necessary to control the size distribution and self-organizing behavior of the islands. It is critical to understand the kinetic pathways to island formation.

Recent theoretical investigation suggests that systems with tensile stress could be more resistant to roughening than those with compressive stress. Recent work has supported this prediction in, for example, the Si-Ge system. Molecular dynamics modeling has attributed this observation to an increase in the energy of certain types of surface steps under tensile strain, which makes it energetically favorable for the surface to remain planar.

One of the major contributions of scanning-probe microscopies to heteroepitaxy has been improved understanding of morphological evolution in heteroepitaxy. A general trend in all experiments is the decreasing size of typical morphological features with increasing misfit stress. In many highly mismatched systems, surface ripples exhibit a strong tendency to facet along inclined planes,

Suggested Citation:"2 New Materials and Structures." 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.
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image

Figure 2.13
Atomic force microscope image of an initially planar 2-nm thick 
Si0.5Ge0.5 alloy layer on Si(001) after annealing to produce hut-
shaped islands caused by strain in the layer. [MRS Bulletin 21, 31 (1996).]

as in the epitaxial germanium "hut" clusters in Figure 2.13. The presence of {501} facets appears to be a general feature of strained-layer growth in the Si-Ge system. It is not understood, however, why such facets are stable and what role they play in the growth of coherently strained islands. The picture is emerging that {501} facets are the natural result of the desire to release as much elastic energy as possible without unduly creating energetically costly surface-step configurations.

During growth, strain relaxation in heteroepitaxial systems can lead to changes in island shapes. In semiconductor systems, coherent islands are often faceted and characterized by large aspect ratios [up to a 10:1 base-to-height ratio in the case of the germanium "huts" and up to 50:1 in silver on Si(100)]. Recent calculations have shown that strained islands are likely to undergo a shape transformation during growth. Below a critical island size, the energy balance favors compact, symmetric islands; for large islands, elongated shapes with high aspect ratios are preferred. This suggests one approach to the challenge of producing quasi-one-dimensional quantum wire structures.

Turning now to the other major strain-relief mechanism—defects such as

Suggested Citation:"2 New Materials and Structures." 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.
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dislocations—we enter an area of research with much longer antecedents. Traditionally, misfit dislocations have been avoided simply by keeping the film thickness at less than the "equilibrium critical thickness." More recently, other strategies have been introduced, such as reduced temperature growth or substrate patterning. When substrate patterning is used to isolate regions of the sample, relaxation is greatly retarded because most isolated regions contain no heterogeneous nucleation sites. These regions can be remarkably stable during thermal anneal, which shows that controlling nucleation may be the key to controlling, or even suppressing, relaxation.

One of the most successful methods to date of fabricating quantum dots uses self-assembly that results from growth kinetics controlled by strained-layer epitaxy. A strain-induced transition from two- to three-dimensional growth results in the formation of coherently strained islands on the surface of the semiconductor. Using these principles, islands <<20 nm in diameter can be fabricated with size distributions within ± 10 percent. The resulting islands are pseudomorphically strained and dislocation free.

The random distribution of islands can be modified by appropriate control of their nucleation and growth, kinetics. It was readily apparent early on that preferential nucleation of islands takes place at surface steps. This effect could be used to order islands. Recent progress in making extremely perfect kinkless steps over micron distances on Si(111) offers hope of ultimately achieving ordering of island assemblies by this approach.

Another strategy for ordering the islands is based on the very sharp transition from two- to three-dimensional growth. A corrugated substrate with concave and convex areas will tend to flatten and minimize its surface energy during epitaxy through faster growth of the convex areas. Thus, when depositing a strained layer over such a surface, the critical thickness will be reached sooner in these areas, and quantum dots will nucleate preferentially in these regions.

Another promising approach based on strain-induced nucleation allows for regulating the size and ordering of the islands in the growth direction. If two or more layers of quantum dots are grown sufficiently close to each other (closer than 10 nm for InAs/GaAs), it is possible to obtain self-alignment of the islands in the growth direction.

Future Directions And Research Priorities

The examples of new materials and structures presented in this chapter point out the major themes in the search for new and improved materials and properties that have characterized the past decade. The themes include the discovery of new and unexpected materials with novel properties and the use of new tools to provide improved understanding and control in well-known materials. These developments foreshadow many of the advances that are likely in the coming years. It is critical to emphasize, however, that many of the most exciting devel-

Suggested Citation:"2 New Materials and Structures." 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.
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opments have been complete surprises; this will almost certainly be true for the foreseeable future as well.

Materials Properties by Design: Complexity

The ability to tailor materials and structures to obtain a desired set of properties is in its infancy (see Box 2.9). Band-gap engineering, which has been achieved by judicious use of strain, alloying, and quantum-size effects, is one area that has had considerable impact. Control of the microstructure of a material, through processing and judicious choice of geometry or neighboring materials, has been used to control the physical properties of some materials. Artificially layered materials have been given unusual dielectric properties and have contributed to the search for new high-temperature superconductors.

Future progress will build on these recent accomplishments. Eventually, we can expect to be able to tailor materials at the molecular level, building up materials molecule by molecule in three dimensions. This capability will enable truly three-dimensional designs with as much or as little symmetry as needed. In the future, a design will be able to incorporate structure at multiple length-scales, enabling the optimization of multiple properties that involve phenomena operating at very different dimensions. Today, the most advanced artificially structured materials utilize individual material constituents of the same or very similar classes: III-V semiconductors, for example. We can look forward to being able to use a much more colorful palette, not limited to a single class of materials or even just to inorganic materials. Polymers, organic molecules, and even biological molecules are likely to become integral parts of increasingly complex structures as we learn more about how to manipulate molecules individually. A glimpse into the possibilities is described in Box 2.10.

As the structures that we are able to build become more complex, we will need the tools to be able to see, characterize, and manipulate them. We will need to be able to work with these structures on all of the length scales that are relevant for the properties we desire. The scanning-probe microscopies are an important step in this direction, but more, equally revolutionary advances will be required to truly take advantage of this new regime of materials design.

Finally, we will need to be able to make predictions about these new materials. Our ability to predict the existence of new materials with interesting properties is extremely limited. Witness the complete surprise presented by the discovery of high-temperature superconductors or the lack of theoretical guidance concerning other interesting avenues of inquiry in the search for other high-temperature superconducting families. Theoretical guidance regarding promising synthetic routes for fabricating new materials would be most useful to experimentalists trying to prepare them. Finally, improved communication between theorists and experimentalists might help to shorten the gap between prediction and fabrication—several decades in the case of C60, for example.

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.9 Combinatorial Chemistry and the Search for New Materials

Systematically varying the composition of a multicomponent system to optimize its properties is a time-honored empirical method in materials synthesis. For example, "phase spreads" of thin films proved powerful in the study of metalinsulator transitions almost 2 decades ago. Scaling up the approach to allow the fabrication and testing of tens or hundreds of compositional variants requires the ability to prepare small volumes of precisely controlled composition under known preparation conditions as well as the capability to test these miniature samples for desired properties. Known more recently as combinatorial chemistry, this approach is used to make a large number of chemical variants in parallel, to screen them quickly and reliably for chemical activity, and to build a library of information about the resultant chemical diversity. Until recently, combinatorial chemistry has been used primarily to transform the way new drugs are discovered, but in the coming decade, it may have an impact on the search for other classes of new material as well. The most notable forays of combinatorial chemistry into non-medical arenas are in the areas of superconducting compounds and phosphors (see Figure 2.9.1). The applicability of the technique to the search for catalysts is also being investigated. All these materials share the property of being very complex, containing many elements and eluding prediction of their properties or even existence using any currently available theories or models. As the entire field of condensed-matter and materials physics moves toward increasingly complex systems, techniques such as combinatorial chemistry are likely to make a home for themselves alongside more traditional techniques such as bulk crystal growth or physical and chemical vapor deposition. For this promise to be realized, however, new tools need to be developed that can analyze and sort the large number of samples that are produced by this powerful technique.

image

Figure 2.9.1
An array of different combinations of
phosphors being screened for brightness
in ultraviolet light. [Reprinted with permission
from E.D. Isaacs, M.A. Marcus, G. Aeppli,
X.-D. Xiang, X.-D. Sun, P. Schultz, H.-K.
Kao, G.S. Cargill Ill, and R. Haushalter,
"Syncrotron x-ray microbeam diagnostics
of combinatorial synthesis," Applied
Physics Letters 73, 1820 (1998).
Copyright © 1998 American Institute of Physics.]

Suggested Citation:"2 New Materials and Structures." 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.
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BOX 2.10 Polymers Enable Porous Inorganic Materials Synthesis to Order

Block copolymers self-assemble into ordered nanostructures consisting of three-dimensional arrays of spheres, cylinders, lamellae, and even bicontinuous domains depending on the lengths of the blocks. Polymer physicists have achieved a remarkably detailed understanding of the interplay between chain architecture and thermodynamics that leads to these nanostructures. As useful as this insight has been for all-polymeric materials, it now promises to be equally important for the synthesis of inorganics. A long-standing need has been for porous ceramics that have well-defined but large pore sizes (>5 nm). These can now be achieved using block copolymers as a template. A copolymer with hydrophilic and hydrophobic blocks is ordered into a nanostructure in which the hydrophobic block forms aligned cylinders. A ceramic precursor is absorbed preferentially into the hydrophilic matrix surrounding the cylinders and allowed to condense into a robust inorganic oxide network. The block copolymer can be extracted, leaving a ceramic matrix surrounding ordered uniform cylinderical pores, as shown in Figure 2.10.1. Pore size can be controlled between 5 and 30 nm simply by changing the length of the copolymer, leaving the ratio of blocks the same.

image

 

Figure 2.10.1
Transmission-electron microscope micrographs
of mesoporous silica with pores sizes 6 and
9 nm. [Reprinted with permission from D.
Zhao, D. Feng, Q. Huo, N. Melosh, G.H. Fredrickson,
B.F. Chmelka, and G.D. Stucky, "Triblock copolymer
syntheses of mesoporous silica with periodic
50 to 300 angstrom pores," Science
279, 550 (1998). Copyright © 1998 American
Association for the Advancement of Science.]

Suggested Citation:"2 New Materials and Structures." 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.
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Synthesis and Processing: Control

To make the increasingly complex materials and structures of the coming decades, tremendous advances in processing will be required. One can look forward to the day when arbitrarily complex materials and material combinations can be made with the same level of control as is possible for semiconductors today. One of the first classes of material that is likely to see the benefits of improved processing is the complex oxide family, including high-temperature superconductors. The complexity of these materials, however, pales in comparison with some of the other structures involving vastly different classes of material such as biological and inorganic materials. Although demonstrations of increasingly complex structures designed on the molecular level may be made using scanning-probe techniques, fabricating structures that can be studied intensively will require faster techniques that can make multiple samples. This almost certainly calls for a dramatic increase in our understanding of and ability to use self-assembly and biomimetic techniques to produce and process materials.

In the past decade there has been impressive progress in the understanding and control of defects—what they are, where they come from, and how to eliminate them or control their placement when they serve a defined purpose—in some materials systems, especially semiconductors. For other materials to reach the same level of perfection and processing control, the same level of understanding will be required.

Nanoscale fabrication and processing, wherein molecular chemistry and condensed-matter physics merge, will be key to achieving the level of control that will be needed to realize many of the exciting possibilities posed in this report.

Physics: Understanding

The materials and structures on the horizon offer rich possibilities for condensed-matter and materials physicists. More perfect materials will enable us to move toward developing a full understanding of the relationship between the detailed structure of a material and its properties. The ability to control defects will enable them to be studied themselves—how they interact with the material they inhabit and even how judiciously assembled collections of them interact with one another and with different defects. Advances of the past decade in probing surfaces and interfaces on the atomic scale offer the possibility that a full understanding of the initial stages of growth in systems more complex than silicon may one day truly be possible. Control of the structure of materials on various length scales simultaneously offers the opportunity to look for effects that result from the interplay of structure on these different length scales.

Technology: Relevance

Advances in new materials and structures have dramatically improved our

Suggested Citation:"2 New Materials and Structures." 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.
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lives in the past, and there is every reason to expect that new advances will have comparably great impact in the years to come. For this to happen, sustained research will be needed over many years. This research will need to have a balance between fundamental investigations into the physical mechanisms at play and research and engineering aimed at investigating the numerous questions that must be answered before a material can enter the technological mainstream: What can the material be used for? Is there a potential market of sufficient size to pay for the needed research and development? Is the advance so revolutionary, with improvements in customer capability so great, that it can found a new industry? If the improvement is in an area already occupied by an existing technology with significant infrastructure, can the material be integrated with the existing technology? And if so, is the improvement worth the development cost?

Just as revolutionary advances in new materials and processes enabled the transistor, the optical fiber, the solid-state laser, and many other technologies that have improved our lives and strengthened the economy, new developments in materials and structures hold out the promise of revolutionary breakthroughs in the twenty-first century.

Outstanding Scientific Questions

• Can we complement empiricism with predictability in our search for new materials and structures with desired properties? Can we predict the composition and structure of a new material, its properties, and how to synthesize it?

• Can we develop a full understanding of the initial stages of growth?

• Can we develop a full understanding of the relationship between the detailed structure of a material and its properties? Can we truly control defects?

Research Priorities

• Tailor materials at the molecular level.

• Use more complex combinations of materials: polymers, organic molecules, biological molecules, etc.

• Develop new tools to synthesize, visualize, characterize, and manipulate new materials and structures.

• Make increasingly complex materials and combinations with as much control as is currently possible in the making of semiconductors.

• Increase our understanding of and the ability to use self-assembly and biomimetic techniques to produce and process materials.

• Merge molecular chemistry and condensed-matter and materials physics to understand and control fabrication and processing on multiple length-scales.

• Integrate processing of new materials and structures with existing technologies.

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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.

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