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76 Herb Goronkin, Paul von Allmen, Raymond K. Tsui, and Theodore X. Zhu In summary, while significant progress has been made in nanofabrication techniques, the field of single-charge electronics is still limited in scope by the lack of a suitable architecture that fully utilizes the unique aspects of single-electron charging. Current approaches require many SED elements to achieve conventional functions such as adders, exclusive NORs, etc. Simulations of such circuits predict slow operating speed. The field seems to be stuck on applying conventional electronics to SEDs. Either a new architecture will be discovered or SEDs may find a niche home in those applications where measurements of single charges are needed. For the fabrication of nanoscale electronic devices, the self-organizing technique appears to be the most promising. The field of self-organized semiconductor QDs is quite active, but aside from optical emitters, very few practical electronic functions have been proposed. Of those, the singleelectron flash memory is attracting attention, but there has been no serious proposal as to how the device could operate under normal integrated circuit performance conditions and reliability specifications. Using the SED as a floating gate to a MOSFET has the same kinds of problems as applying conventional approaches to SED architectures; until someone comes up with a better idea, the future of these approaches remains to be determined. NANOMAGNETICS The discovery in 1988 of GMR in structures of alternating magnetic and nonmagnetic thin layers (Baibich et al. 1988) was the accumulation of several decades of intensive research in thin film magnetism (Shinjo and Takada 1987) and improvements in epitaxial growth techniques developed mainly in semiconductor materials. Not surprisingly, the first GMR structure was fabricated using molecular beam epitaxy (Baibich et al. 1988). The high quality magnetic and nonmagnetic metallic films provide electrons with a mean free path exceeding 100 Å; on the other hand, the epitaxial growth allows for each constituent layer of the structure to be as thin as a few atomic layers. The greatly enhanced spin-dependent scattering in these multilayered structures provides magnetoresistance changes as high as 50%. Table 5.3 shows some of the institutions involved in GMR research and development, based on various publications, patents, and WTEC visits. Two subsequent major developments from IBM enabled the application of GMR materials to hard disk heads, RAM, and sensors. The first development was the demonstration of equally good or better GMR materials using high throughput and production-worthy magnetron sputtering systems (Parkin et al. 1990). The other development was the invention of magnetically soft spin-valve structures, which allow low field and low power operation (Dieny et al. 1991a; Dieny et al. 1991b).
5. Functional Nanoscale Devices 77 TABLE 5.3. Giant Magnetoresistance Activities Memory: MRAM HD Heads Structures / Physics / Materials • Fujitsu • Hitachi • Honeywell • IBM • Motorola • Matsushita • Mitsubishi • Philips • Samsung • Toshiba • Seagate • Siemens • Sony • IMEC • L’Ecole Polytechnique Lausanne • IBM Zurich • Tohoku University • Nagoya University • NIST • UC Santa Barbara • UC San Diego • Carnegie Mellon • Princeton *Based on publications, patents or visits Industrial R&D efforts on GMR materials initially focused on high density read heads. The major U.S. players are IBM, Seagate, Quantum, ReadRite, and Applied Magnetics. In Japan, all the semiconductor companies are involved, in addition to strong magnetic media powerhouses such as TDK and Yamaha. Korea’s Samsung is also actively involved in the GMR race. In Europe, Thomson CSF, Philips, and Siemens seem to have fallen behind. All in all, IBM is in a commanding position to reap the benefits of the GMR phenomenon. In November 1997, it announced the volume production of the first generation of GMR read heads. In 1995, a different class of high magnetoresistive materials was discovered in which the nonmagnetic layer separating the two ferromagnetic layers is made with an ultrathin insulating material, such as an aluminum oxide layer < 20 Å thick (Miyazaki and Tezuka 1995; Moodera et al. 1995). With the switching of magnetization of the two magnetic layers between parallel and antiparallel states, the differences in the tunneling coefficient of the junction and thus the magnetoresistance ratio have been demonstrated to be more than 25%. A distinctive feature of this MTJ class of materials is its high impedance (> 100 kΩ-µm 2 ), which allows for large signal outputs. The gradual improvement of GMR and MTJ materials have made them attractive for nonvolatile magnetic random access memory (MRAM) applications. The potential to make MRAM a high density, high speed, and low power, general purpose memory prompted the Defense Advanced Reseat Projects Agency to fund three MRAM consortia beginning in 1995, led by IBM, Motorola, and Honeywell, respectively. Other companies engaged in MRAM research are Hewlett-Packard, Matsushita, NEC, Fujitsu, Toshiba, Hitachi, and Siemens.
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National Science and Technology Cou
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WTEC Panel on NANOSTRUCTURE SCIENCE
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ABSTRACT This report reviews the st
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Foreword Timely information on scie
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Foreword iii collaboration and thus
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Contents Foreword Table of Contents
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List of Figures 1.1 Organization of
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List of Figures ix 5.12 Granular GM
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List of Tables ES.1 Technological I
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Executive Summary Richard W. Siegel
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Executive Summary xix past decade,
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Executive Summary xxi TABLE ES.2. C
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Executive Summary xxiii blocks, and
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Chapter 1 Introduction and Overview
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Chapter 2 Synthesis and Assembly Ev
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APPENDICES Appendix A. Biographies
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Appendix A. Biographies of Panelist
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Appendix B. Site Reports—Europe S
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Appendix B. Site Reports—Europe 1
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Appendix F. Glossary 335 WTEC XAS X
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