Progress and prospects of advanced quantum nanostructures and roles of molecular beam epitaxy
Introduction
Over the last 50 years, semiconductor electronics have made immense progress and revolutionized our society and everyday life. In particular, transistors, large scale integrated circuits, lasers, sensors, and other devices have provided us powerful tools and means, with which we can acquire, store, process, transmit and display a variety of information. In many of these devices, the two-dimensional (2D) electrons confined in 10 nm-scale film structures are used (See Fig. 1(a)). For example, the conductive channels of heterojunction field effect transistors (FETs) and MOSFETs and the active layer of quantum well (QW) lasers all consist of 10 nm-scale films formed either by fine epitaxial methods such as molecular beam epitaxy (MBE) or by the electronstatic confinent of insulated gate FET geometries [1], [2], [3], [4].
Furthermore, Esaki and Tsu proposed in 1969 the concept of semiconductor superlattices (SLs), where confined electrons are stacked to form resonantly coupled states, so as to exploit unique properties of these structures (See Fig. 1(b))[5]. Indeed a series of works on QWs, SLs, and related layered structures have shown that these systems possess a series of new properties and functions, originating mainly from the quantum confinement of electrons. Key examples include the integer- or fractional- quantum Hall effects [6], [7], the quantum-confined Stark effect [8], the resonant tunneling transport, the Bloch oscillation [9], and the emission [10] and detection [11] of mid-infrared photons based on inter-subband transitions of 2D electrons. In the first part of this paper, we discuss how MBE and related epitaxial techniques are refined to form nearly ideal QWs, SLs, and selectively doped heterojunctions. We emphasize the importance of precisely controlling various microscopic processes of epitaxial growth.
To expand the forefront of nanostructure research, Sakaki et al. proposed, and analyzed in 1975 and 1980 the concepts of semiconductor quantum dot (QD) and quantum wire (QWR) structures (See Figs. 1(c) and (d)) and their possible use for new devices, such as gate-controllable non-linear transport devices [12] and QWR FETs [13], respectively. These concepts were extended in 1982 by Arakawa and Sakaki [14], when the use of QWR/QD for injection lasers was proposed. Despite the early difficulty, methods to form wires and dots have been developed and unique properties and functions of 1D and 0D electrons are intensively investigated [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In particular, self-assembled QDs on mismatched substrates [15], [16], the selective growth of QWRs on patterned or stepped substrates [17], [18], [19], [20], [21], [22], [23], [24], [25], the overgrowth onto the exposed edge of a QW [13], [27], [28], [29], [30], [31], [32], and the self-organized formation of nanoparticles and nanotubes in vapor or liquid phase have greatly facilitated the realization and applications of 1D/0D electron systems. In the latter part of this paper, we review the epitaxial synthesis of these QD/QWR structures and discuss their electronic and photonic properties. We examine also several QD- and QWR-devices, including QD-based memories [21], detectors [22], [23], LEDs, and lasers [24], [25], and QWR-based FETs [13], [29] and other exploratory field-effect devices, and discuss their roles in the future electronics.
Section snippets
QWs, and superlattices and importance of interface qualities and material purities
As stated earlier, many of the important achievements in semiconductor physics and electronics have been made over the last 30 years by using QWs, SLs and selectively doped heterojunction systems. Their impressive properties and functions were achieved by using the full power of MBE. Hence, we discuss here the importance of not only clarifying but also controlling various growth processes, which determine the interface morphology and purities of these structures.
The energy Eze (n=1) of the
Quantum dots and wires: their epitaxial synthesis and device prospects
As stated earlier, the quantum manipulation of electrons and its applications were tried first by using QWs, SLs, and other layered nanostructures with respect to their motion normal to the layer. To expand the forefront of research, Sakaki proposed the quantum control of the in-plane motion of electrons by the use of QDs and QWRs and related structures and pointed out their possible device applications for novel non-linear transport devices and QWR FETs [13], [14]. These works were then
Conclusions
Marvelous developments of molecular beam epitaxy achieved over the last three decades have opened and established a new and fertile research field of quantum wells, selectively doped heterojunctions, and superlattices, where the quantum mechanical manipulation of quasi-2-dimensional electron gas has induced the births of various high performance devices and a series of discoveries of new and important physical phenomena. Although the levels of our current technology to form quantum wires and
Acknowledgements
The author wishes to express his deep thanks to Drs. A. Y. Cho, L. Esaki, H. Kroemer, A. C. Gossard, P. Petroff, B. A. Joyce, T. Foxon, K. Ploog, J. Harris and other early explorers of MBE for providing intellectual stimulations and hearty encouragements for more than 20 years. He is also grateful to all the members of his research group, whose name can be found as co-authors in some of the references listed below. This work is supported by the grant in aid for scientific research from the
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