Using Molecular - Beam Epitaxy to Fabricate Quantum - Well Devices

Recent advances in thin-film crystal-g:rowth techniques such as molecular-beam epitaxy (MBE) have enabled the fabrication of quantum-well devices, which consist of alternating layers of various crystalline solid materials so thin that the materials' combined quantum-mechanical properties override their individual bulk properties. By using MBE, we constructed a number of quantum-well devices that have applications in ultrahigh-speed analog, digital, and electro-optical integrated and hybrid systems.. Scientists discovered tlJ.e basic principles of quantum mechanics more than half a century ago. Only recently, however, have researchers been able to fabricate devices that exploit the quantum-mechanical behavior of carriers in epitaxially grown ultrathin semiconductor layers. These devices couple many basic quantum-mechanical principles with recent advances in the control of the doping, the thic1messes, and the bandgaps of epitaxial semiconductor layers. Although bulk-semiconductor technologies use a few micrometers of a wafer's thickness to form devices, quantum-well technology sometimes requires as little as 7 nm or less. Attaining such minuscule geometries was impossible until the recent development of thin-film semiconductor crystal-growth techniques such as molecular-beam epitaxy (MBE). MBE is an ultrahigh-vacuum epitaxial-growth technology. In MBE, molecular beams ofspecific materials impinge upon an appropriately prepared wafer placed in an ultrahigh-vacuum environment. The reaction results in the growth of a thin epitaxial film on the wafer's surface. By manipulating the arrival rates of the beams and tlle temperature of the wafer surface, the process can be controlled so that layers of specified materials are grown to atomic-layer thicknesses , which makes the process suitable for quantum-well device engineering. This article will briefly review soine ofthe basic principles underlying quantum-well semiconductors , discuss the use of MBE to fabricate GaAs/AlxGa1_xAs semiconductor materials, and describe several quantum-well devices that have been developed with MBE at Lincoln Laboratory. (Editor's note: Readers who wish to leam in more detail about quantum-well deVices, bandgap engineering, and modem thin-film crystal-growth techniques can find excellent review articles in Refs. The energy bands of semiconductors are complicated. Figure 1 shows the band diagram of GaAs. The point of lowest energy in the conduction band is called the conduction-band edge, and the point of highest energy in the valence band is called the valence-band edge. The energy difference of these two points is the semiconductor's bandgap, denoted as E in gap Fig. 1. If the two band-edge points occur at the same lattice-momentum value, the gap is a direct gap. Otherwise, it is an indirect gap. From …