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layers. The resultant epitaxial structure is an array, or superlattice, of quantum wires [11]. The periodicity of the superlattice is determined by the spacing between the terrace steps. The limits on the perfection of the quantum wire superlattices are the nonuniformity of the terrace step spacing, kinks in the terrace steps, and possible interdiffusion during growth.

When epitaxial structures are grown in which a material with a somewhat different crystal lattice spacing is grown on an original material, another kind of self-ordering can occur that provides an effective way to make quantum dots. In this case, adatoms with larger radii than the substrate atoms can relieve their compressive strain by forming islands in which they can spread out. This layer/island growth mode (known as Stranski-Krastanow growth) enables InAs depositions on GaAs substrates to organize themselves into islands [12]. InAs has a lattice constant that is 7 percent larger than GaAs. The islands are quantum dots whose sizes can be just a few nanometers. This is far smaller than can yet be achieved by lithography. Furthermore, the islands can be embedded by an overgrowth process. Then the process can be repeated to make stacked layers of these self-assembled quantum dots. The self-assembled quantum dot islands can be grown by both MBE and MOCVD. They form the basis for a rich and active pursuit of the science and technology of quantum dots. The localization of charge that they produce within semiconductor lasers can be beneficial to laser operation. Quantum dot lasers have been made with these materials, and it is believed that quantum dot confinement may be playing a major role in the successful operation of blue gallium nitride lasers, which typically contain InGaN active layers [13]. Electrical charging of the dots, one electron at a time, can be observed. Quantum-confined luminescence from the quantum states of the dots can also be seen. The major limitations on the dot structures are nonuniformity in their shape, size, spacing, and arrangement. Each of these issues is critically dependent on growth technique and technology. The ultimate impact of quantum dots on science and technology will depend strongly on the resolution of these issues.

In addition to quantum structures in semiconductors, it is interesting and useful to form metal/semiconductor composite nanostructures. A path to such structures has been provided by low-temperature epitaxy of compound semiconductors. It was discovered that epitaxial growth of crystalline GaAs could be performed at lower than conventional growth temperatures, with the result that excess As could be incorporated in the growth [14]. GaAs can be grown epitaxially at temperatures down to 200 °C, compared to more conventional growth temperatures of 500 °C to 700 °C. With an appropriate excess flux of arsenic relative to gallium, 1 to 2 percent of excess arsenic (relative to exactly stoichiometric GaAs) can be incorporated with low temperature growth. The excess arsenic provides electron traps (deep impurity states that pin the Fermi level in the gap of the GaAs), producing high-resistivity material and rapid recombination of electrons and holes. Annealing the material leads to diffusion of the excess arsenic to form metallic arsenic nanoparticles with diameter approximately 10 nanometers. The composite GaAs-As semiconductor-metal material can be applied to growing nonconductive insulation layers for application in field effect transistors to isolate semi-insulating substrates from transistor channel layers. The deep trap states and the metal particles in low-temperature-grown GaAs also provide efficient trapping of photo-induced carriers and rapid recombination of photo-induced electrons and holes. They are attractive for making fast photodetectors and phototransistors, eliminating long-lived carriers that would slow response and add dark current [15].

A significant application of low-temperature-grown GaAs has been the production of fast photoconductive materials for use in generation of terahertz-frequency electromagnetic waves [16]. The low-temperature-grown GaAs is the active element in photoconductive structures that generate electromagnetic radiation at the difference frequency between laser pumping beams that are striking the material. These photomixers produce levels and coherence of far-infrared terahertz-frequency radiation that can be the basis of a semiconductor far-infrared terahertz technology. These radiation sources are needed in astrophysics for the sensitive heterodyne detection of radiation from gases