Condensed-Matter and Materials Physics Basic Research for Tomorrow's Technology (1999) / Chapter Skim
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2 New Materials and Structures
2 New Materials and Structures
Pages 93-136
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From page 93... ...
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.
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From page 94... ...
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.
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From page 95... ...
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.
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From page 96... ...
. 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.
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From page 97... ...
(Courtesy of Sandia National Laboratories.) FIGURE 2.2.3 A pictorial representation of the many-particle state that underlies the fractional quantum Hall effect.
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From page 98... ...
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.
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From page 99... ...
The crystal structures of these matenals are dramatically more complicated and have lower symmetry than those of low-temperature superconductors or semiconductors. The physical properties are similarly anisotropic.
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From page 100... ...
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.
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From page 101... ...
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.
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From page 102... ...
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.
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From page 103... ...
(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.
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From page 104... ...
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 ~ 1 25000 I 3 1 ooooo 200 75000 ct)
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From page 105... ...
105 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 memones.
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From page 106... ...
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.
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From page 107... ...
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.
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From page 108... ...
This suggests that electrodes with a large tolerance for oxygen deficiency, such as some of the metallic oxides, should offer better fatigue charactenstics 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.
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From page 109... ...
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.
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From page 110... ...
. The switch back of the central grain into the polarization down direction starts mainly at the grain boundaries with the surrounding grains.
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From page 111... ...
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.
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From page 112... ...
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.
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From page 113... ...
Tans, A.R.M. Verschueren, and Cees Dekker, "Individual single-wall carbon nanotubes as quantum wires," Nature 386, 474 (1997~.
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From page 114... ...
A C60-TDAE salt has been formed that exhibits a ferromagnetic state below 16 K This material, which has a low-symmetry monoclinic structure because of the highly nonspherical nature of TDAE, holds the record for the highest Curie temperature of any purely organic molecular solid.
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From page 115... ...
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)
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From page 116... ...
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.
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From page 117... ...
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.
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From page 118... ...
(e) Transmission electron micrograph of InAs quantum dots in a GaAs matrix, prepared by molecular beam epitaxy.
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From page 119... ...
Quantum dots are so small that they tightly confine normally mobile electrons, so the charges spend less energy on their wandenngs. Thus more energy is released when the electron and T I I I I \1 1 1 I T 1 7 1 1 1 1 7 T -\ U)
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From page 120... ...
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.
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From page 121... ...
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.
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From page 122... ...
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.
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From page 123... ...
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)
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From page 124... ...
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~.
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From page 125... ...
Legroves, and R.M. Tromp, Applied Physics Letters 62, 2962 (1993~.
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From page 126... ...
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.
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From page 127... ...
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.
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From page 128... ...
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.
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From page 129... ...
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.
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From page 130... ...
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 )
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From page 131... ...
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.
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From page 132... ...
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.
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From page 133... ...
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.
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From page 134... ...
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.
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From page 135... ...
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.
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From page 136... ...
· Merge molecular chemistry and condensed-matter and materials physics to understand and control fabrication and processing on multiple lengthscales. · Integrate processing of new materials and structures with existing technologies.
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