Condensed-Matter and Materials Physics Basic Research for Tomorrow's Technology (1999) / Chapter Skim
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1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology
1 Electronic, Optical, and Magnetic Materials and Phenomena: The Science of Modern Technology
Pages 31-92
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The technological impact of such advances is perhaps best illustrated in the areas of condensed-matter and materials physics discussed in this chapter, which will examine selected examples of electronic, magnetic, and optical materials and phenomena that are key to the convergence of computing, communication, and consumer electronics. Technology based on electronic, optical, and magnetic materials is driving the information age through revolutions in computing and communications.
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32 CONDENSED-MATTER AND MATERIALS PHYSICS interplay between and interdependence of science and technology. Perhaps in no other area are advances in technology more closely linked to advances in understanding.
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Advances in magnetic materials and their applications are not limited to bulk materials with high permanent magnetization and magnetic materials used in information storage. Improvements in soft bulk magnetic materials play an important role in transformers used in the electric power distribution industry.
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. If the silicon integrated circuit is the engine that powers the computing and communications revolution, optical fibers are the highways for the information age.
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Compound semiconductor diode lasers provide the laser photons that transport information along the optical information highways. Semiconductor diode lasers are also at the heart of optical storage and compact disc technology.
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Both magnetic and optical storage are in wide use. Recently, the highest performance magnetic storage/readout devices have begun to rely on giant magnetoresistance (GMR)
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1973 (14% at 4.2K) Large MR observed in ultra-perfect single crystal Fe whiskers at low temperatures 1968 Very large MR in La-Ca-Mn-O perovskites in large fields below room temperature 1955 (now described as Colossal MR or CMR)
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ELECTRONIC MATERIALS AND PHENOMENA Materials and Physics That Drive Today's Technology Silicon Technology As noted in the introduction, semiconductor technology is the key enabler of the information age. The science of materials as a specific discipline is a relatively
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If the scaling trend continues as indicated by Moore's Law, which the industry has followed since its inception, integrated circuits with minimum feature sizes of 50 rim will be manufactured in high volumes within 15 years (see Box 1.2~. Continuing to advance this technology requires that the industry invest in expensive new manufacturing facilities and an ever-increasing scientific understanding and control of semiconductor 3Semiconductor Industry Association, National Technology Roadmap for Semiconductors, SEMATECH, Austin, Tex.
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Manufacturing complex circuits that rely on devices with these feature sizes will require several hundred processing steps with atomiclevel control. However, the performance of complex integrated circuits with tens of millions of transistors may be degraded because of nonuniform operating
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Even with these advances, as feature sizes continue to decrease, integrated circuits based on field-effect transistors will eventually encounter fundamental limits, such as interconnect delays caused by the ever-increasing number of interconnects, heat generation, or quantum limits of transistors that are too small to confine the electrons in the channels. Today's approach to the design and manufacture of integrated circuits will no longer be extendible to smaller feature sizes and higher densities.
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These techniques allow atomic layer control in the growth of one material on another in single atomic layers to produce materials not found in nature. The use of novel forms of microscopy for fabrication and testing, and the development of comprehensive modeling techniques that take into account all of the materials physics and carrier dynamics of the structures, will determine our ability to design and build such structures on the atomic scale with feature sizes comparable to the quantum de Broglie wavelength of the electrons.
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As feature sizes decrease, quantum mechanical effects become observable. Combining the small feature size available with electron-beam lithography with very high quality artificially structured materials that can be grown in compound semiconductors led to the discovery of entirely new physical effects, such as the fractional quantum Hall effect (FQHE)
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Figure 1.3.1 shows schematically a BEEM measurement of ~50 nm-diameter GaSb self-assembled quantum dots located ~7.5 nm below an Au/GaAs interface. Because the dots are close to the surface, their profiles can be seen in the STM topography, so that the tip can be located on and off the dot for comparison.
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Narayanamurti, "Local conduction band offset of GaSb self-assembled quantum dots on GaAs," Applied Physics Letters 70, 1590 (1997)
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The study of electron transport through these minuscule conducting regions has revealed a variety of fascinating phenomena including observable effects caused by individual electrons. Current versus voltage measurements for a quantum dot show discrete staircases where each successive plateau represents the addition of one electron to the quantum dot (see Figure 1.4.1)
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. The peaks occur when the energy levels in the quantum dot coincide with the Fermi level of the electrons in the leads.
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Reno, and M.J. Hafich, "Planar quantum transistor based on 2D-2D tunneling in double quantum well heterostructures," Journal of Applied Physics 84, 5626 (Nov.
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Reno, and M.J. Hafich, "Planar quantum transistor based on 2D-2D tunneling in double quantum well heterostructures," Journal of Applied Physics 84, 5626 (Nov.
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and the Weizmann Institute in Israel has attracted additional interest because it shows the Kondo effect in a way that will allow one to explore the phenomenon in a system with many tunable parameters. Kondo-like effects in quantum dots are observable only under a very narrow set of conditions.
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In this region the quantum dot has an unpaired electron, which is free to form a singlet with the electrons in the leads. This singlet state couples electrons from the .$ ~09 ~0~0 0~5 0~0 0~5 ~ ds (,1~ ~s a\ FIGURE 1.6.1 Evidence for the Kondo effect in a single electron transistor.
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If the enhanced conductance that appeared between the two peaks were due to the Kondo effect, it would require a symmetric interaction of the unpaired electron on the quantum dot with electrons in both leads. But if one applies a voltage Vds across those two leads, separating the Fermi energy levels of the two reservoirs, that interaction is no longer symmetric, and the conductance must fall.
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Recently, nanometer-size SETs and single-electron memories (SEMs) have been demonstrated that have quantum dots sufficiently small to operate near room temperature (see Box 1.7)
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The crucial difference is that, in a SEM, the quantum dots sit between the channel and the gate electrode rather than replacing the channel as they did in the SET. A SEM structure based on silicon technology is shown in Figure 1.7.1.
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and How do we manufacture SETs and SEMs at reasonable operating temperature and cost? Recognizing that one cannot continue to scale silicon integrated circuits to smaller feature sizes indefinitely raises the question, What is beyond silicon?
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· Continue to develop the processing and characterization tools required to create and evaluate ever more-complex, ever-smaller, artificially structured materials. OPTICAL MATERIALS AND PHENOMENA Materials and Physics That Drive Today's Technology Optical Communications As mentioned in the introduction, fueled by the explosion of Internet use and the globalization of voice and data communications, lightwave communication systems capacity and installation are growing exponentially with a growth rate of about a factor of 10 every 6 years.
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These advances were accompanied by recent major advances in InP-based electrically modulated single-wavelength semiconductor diode lasers operating in the 1.3-,um and 1.5-,um wavelength regions, where the lowest loss in silica fiber occurs; in fast avalanche photodiode detectors; in erbium-doped fiber amplifiers and other fiber devices (see Box 1.8~; and in highpower semiconductor diode lasers used to pump the fiber devices. Current digital optical telecommunications networks typically use the NRZ (non-return-to-zero)
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Optical amplification can be achieved through stimulated emission from the excited states of Er atoms in the glass if a population inversion is created with pump light from a semiconductor diode laser. The optical properties of rare-earth impurities in a glass matrix were first studied in the 1960s.
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For example, lasers that can operate at elevated temperatures without active cooling are necessary, as are passive optical distribution systems and low-cost upstream communication devices such as optical modulators. Multiple wavelength optical transmission systems for long-distance networks (DWDM, or dense wavelength division multiplexing)
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A key enabler for today's optical recording technology was the invention of the compound semiconductor laser diode in the mid-1960s. Also necessary was the subsequent development of these tiny diode lasers into low-cost, robust, reliable, relatively high-power devices.
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Near-field technology has the potential for data storage density one to two orders of magnitude higher than conventional optical and magnetic storage projections to the year 2000. Another potential advantage of this technology is the ability to use very low mass optical heads mounted directly onto sliders that have been developed for magnetic storage to reduce seek times.
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Several groups have subsequently demonstrated a blue-green laser in the GaN/AllnGaAs system, and a worldwide race is now under way to achieve a reliable continuous wave laser at room temperature. Display, Printing, and Copying Technologies Exciting recent developments in synthesis of semiconducting organic materials have enabled researchers to demonstrate a variety of optoelectronic devices based on electronically active organics.
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This property, combined with the ability to rigorously exclude deep-charge transport traps, has made organic materials superior for photocopying applications.
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A number of FIGURE 1.4 Flexible light-emitting diode display based on evaporated organic materials. (Courtesy of University of California at Santa Barbara.)
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These rely on optical excitation of the organic films. It may turn out that the best way to make suitable solidstate organic lasers is a hybrid design using passive organic gain media that are optically pumped by electrically pumped inorganic semiconductor lasers such as those based on InGaN.
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Some of these opportunities may come as a result of photochemically modifying single domains or arrays of domains with a nearfield optical microscope, synthesizing materials with giant optical nonlinearities, linking organic materials to inorganic quantum dot structures, examining the phase diagrams of mixed polymeric systems, and designing functional polymers that interface to biological systems. Most likely, even more exciting but unanticipated phenomena will arise from this young and vital area of research.
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This is especially true at interfaces between materials, which are becoming increasingly important as device dimensions shrink. Overcoming challenges associated with making efficient electrical contact to organic materials is central to developing efficient lightemitting diodes, transistors, and electrically pumped lasers.
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. Confining the electrons into a zero-dimensional "quantum dot," a box comparable in size to the wavelength of the electron in all directions, produces a series of even sharper spikes that correspond to a series of confined quantum levels for the electrons in the box (Figure 1.6c)
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. ENERGY ~ENERGY ~ 20 NANOMETERS ,~ ~1 r ~ L14L ENERGY ~ENERGY ~ (7 z ZERO RESONANT VALLEY VOLTAGE VOLTAGE VOLTAGE ~ He I ~ L ZERO VOLTAGE RESONANT VOLTAGE VALLEY VOLTAGE \ FIGURE 1.5 Illustration of the effect of quantum confinement on the density of electronic states.
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The energy levels and wave functions, the optical matrix elements and the electron-phonon scattering times are designed to achieve population inversion and the desired wavelength along with other laser characteristics. The active regions of a QC laser are alternated with electron injectors from which electrons tunnel into the upper excited state of the laser transition (Figure 1.11.1 and see Figure 1.6)
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The phase diagram of the excitonic matter in semiconductors and its interaction with photons, the observation of lasing between sharp excitonic levels or lasing from Bose-condensed excitors is still controversial, however, and is a field of much current interest. Over the last decade four approaches for forming small optical devices have been used: 1.
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4. Solution chemistry, described in Chapter 5, was used to produce a monodisperse colloidal suspension of semiconductor quantum dots.
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In ordinary dielectric waveguides today, the bending losses caused by leakage from evanescent fields requires very smooth bends with bending radii of 10 cm thus the waveguides are large, making manufacture and packaging of integrated optical structures
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74 CONDENSED-MATTER AND MATERIALS PHYSICS Add: I: ~4 : ~ .: Ad.
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Some outstanding technology needs are low-cost all-optical communications network and consumer access components such as new fiber materials and devices; all-optical buffer memory, add-drop filters, amplifiers, semiconductor blue lasers, fast light switches, fast spatial light modulators; materials with tailorable optical properties such as better nonlinear optical materials, resists, and photonicband gap materials in the near IR communications wavelength region; and lowcost assembly and manufacturing techniques for optical components such as selfassembly, stamping, and printing. SCIENCE AND TECHNOLOGY OF MAGNETISM Beginning with the Ancient Mariner's compass and continuing with such applications as automobile starter motors, refrigerator magnets, and computer hard disk drives, the importance of magnetic materials (Figure 1.9)
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The section concludes by summarizing some of the major outstanding scientific questions and suggesting priority directions for future research. Technology Pull The industry of magnetic recording, in all its forms, constitutes an enterprise r n MR,.
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The United States "owns" about 40 percent of the magnetic storage business, the largest single component of which is hard disk drives (see Box 1.12~. This component alone is a $30 billion a year business.
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The evolution of the areal storage density in leading-edge products was illustrated in the introduction. This exponential growth in storage density has led to an associated growth in the number of hard-disk drive bytes shipped per year, as shown in Figure 1.12.2.
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Through the first half of the decade thin film inductive heads dominated the industry and became ever more difficult to scale. The first magnetoresistive heads were introduced in 1992 and played a pivotal role in increasing the compound annual growth rate of the areal storage density from 30 percent to 60 percent (with a 40 percent annual reduction in cost per bit!
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The rapidly growing NdFeB segment of the bulk magnetic materials market is proected to reach $4 billion by the year 2005. "Soft" bulk materials play key technological roles in radio frequency (rf)
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Applications range from sonar pulse generation to high-reliability replacement of hydraulic systems in aircraft and even tanks. A final area is magnetoelectronics (exclusive of magnetic storage)
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82 CONDENSED-MATTER AND MATERIALS PHYSICS FIGURE 1.11 Micrograph of prototype 256-kb nonvolatile magnetic random access memory chip. (Courtesy of Honeywell.)
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The advancing march of magnetic technology makes investigation of these limits inevitable, but probing these limits raises some of the most challenging questions for condensed-matter physics and materials science, such as, What is the smallest size magnetic element stable against external perturbations such as temperature fluctuations? and, Given that quantum mechanics sets bounds on the lifetime of any magnetic state, how do such bounds ultimately establish limits on the size of the smallest possible magnetic entities useful for technological applications?
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High magnetic field experiments have shown that the Fe3+ ions exhibit antiferromagnetic behavior; neighboring spins prefer to be antiparallel. The spin structure of the molecule passes through a rich sequence of phase transitions resembling those in bulk layered antiferromagnets.
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molecule: at low temperatures, this structure shows a propensity to switch from up to down at a sequence of regularly spaced magnetic fields. Some evidence suggests that these magnetic fields coincide with resonances between quantum levels of the up and down wells, resulting in enhanced tunneling.
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Work on magnetic multilayers is also stimulating new thinking concerning novel devices that can be made by integrating magnetic materials with standard semiconductor technology. An example of this is shown in Figure 1.14 where a GMR (Co/Cu)
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. The magnetoresistance of doped manganite structures such as Lay xSrxMnO3 changes by a factor of 2 or 3, although not at temperatures and magnetic fields suitable for practical device applications.
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, magnetic force microscopy (MFM) , magneto-optic Kerr imaging, and scanning electron microscopy with polarization analysis (SEMPA)
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A number of other techniques based on such effects as spin-polarized photoemission, magnetic circular dichroism, and second harmonic generation are becoming increasingly prevalent, while many others are in the initial stages of demonstration. Major Outstanding Materials and Physics Questions and Issues in Magnetism Many outstanding scientific questions remain in the field of magnetism.
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If those needs are met, it is anticipated that these technology areas will continue to follow their historical exponential growth in capability per unit cost for the next few years. Silicon integrated circuits are expected to continue to follow Moore's Law at least until the limits of optical lithography are reached; transmission bandwidth of optical fibers is expected to grow exponentially with advances in optical technology and the development of soliton propagation; and storage density in magnetic media is expected to continue to grow exponentially with the
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As device and feature sizes continue to shrink in integrated circuits, scaling will encounter fundamental physical limits. The feature sizes at which these limits will be encountered and their implications are not understood.
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· What is beyond today's FET-based silicon technology? · Can we create an all-optical communications/computing network?
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