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82 Herb Goronkin, Paul von Allmen, Raymond K. Tsui, and Theodore X. Zhu (thickness), provides a means of “tuning” the resulting wavelength of the material. The critical feature size—in this case, the thickness of the quantum well—depends on the desired spacing between energy levels. For energy levels of greater than a few tens of millielectron volts (meV, to be compared with room temperature thermal energy of 25 meV), the critical dimension is approximately a few hundred angstroms. Although the first quantum well laser, demonstrated in 1975, was many times less efficient than a conventional laser (van der Ziel et al. 1975), the situation was reversed by 1981 through the use of new materials growth capabilities (molecular beam epitaxy), and optimization of the heterostructure laser design (Tsang 1982). Even greater benefits have been predicted for lasers with quantum dot active layers. Arakawa and Sakaki (1982) predicted in the early 1980s that quantum dot lasers should exhibit performance that is less temperaturedependent than existing semiconductor lasers, and that will in particular not degrade at elevated temperatures. Other benefits of quantum dot active layers include further reduction in threshold currents and an increase in differential gain—that is, more efficient laser operation (Asada et al. 1986). Figures 5.16 and 5.17 illustrate some of the key concepts in the laser operation. Stimulated recombination of electron-hole pairs takes place in the GaAs quantum well region, where the confinement of carriers and of the optical mode enhance the interaction between carriers and radiation (Fig. 5.16). In particular, note the change in the electronic density of states, as a function of the “dimensionality” of the active layer, shown in Figure 5.17. The population inversion (creation of electrons and holes) necessary for lasing occurs more efficiently as the active layer material is scaled down from bulk (3-dimensional) to quantum dots (0-dimensional). However, the advantages in operation depend not only on the absolute size of the nanostructures in the active region, but also on the uniformity of size. A broad distribution of sizes “smears” the density of states, producing behavior similar to that of bulk material. Thus, the challenge in realizing quantum dot lasers with operation superior to that shown by quantum well lasers is that of forming high quality, uniform quantum dots in the active layer. Initially, the most widely followed approach to forming quantum dots was through electron beam lithography of suitably small featured patterns (~300 Å) and subsequent dryetch transfer of dots into the substrate material. The problem that plagued these quantum dot arrays was their exceedingly low optical efficiency: high surface-to-volume ratios of these nanostructures and associated high surface recombination rates, together with damage introduced during the fabrication itself, precluded the successful formation of a quantum dot laser.
5. Functional Nanoscale Devices 83 e e e p-type AlGaAs e e e conduction band photon of light h h h n-type AlGaAs undoped GaAs h h h valence band Figure 5.16. Schematic of a semiconductor laser. 3D Bulk Semiconductor 2D Quantum Well N(E) 1D Quantum Wire 0D Quantum Dot E Figure 5.17. Density of electronic states as a function of structure size. With the demonstration of the high optical efficiency self-assembled formation of quantum dots (see Chapter 2), formed without need of external processing and having the natural overgrowth of cladding material (which addressed issues of surface recombination), there ensued a marked increase
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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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1. Introduction and Overview 3 •
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1. Introduction and Overview 5 worl
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1. Introduction and Overview 7 grow
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1. Introduction and Overview 9 lead
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1. Introduction and Overview 11 2.
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1. Introduction and Overview 13 It
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Chapter 2 Synthesis and Assembly Ev
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2. Synthesis and Assembly 17 transi
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2. Synthesis and Assembly 19 probe
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2. Synthesis and Assembly 21 by adj
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2. Synthesis and Assembly 23 blocks
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2. Synthesis and Assembly 25 Self-A
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2. Synthesis and Assembly 27 Figure
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2. Synthesis and Assembly 29 us to
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8. Research Programs on Nanotechnol
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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 D. Site Reports—Japan OV
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Appendix F. Glossary 2DEG A/D AAAR
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Appendix F. Glossary 329 ESPRIT ESR
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Appendix F. Glossary 331 MA MBE mCP
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Appendix F. Glossary 333 PVDF QCA Q
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Appendix F. Glossary 335 WTEC XAS X
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