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The potential wells formed by sandwiches of epitaxial semiconductor layers are illustrated in Figure 1. Quantization of the energy levels of the confined states of the electrons in the layers occurs as illustrated in the figure. Optical transitions between the confined electron energy states are allowed; consequently, electrons can emit photons as they drop from one energy level to another. Transitions from the lowest-conduction-band electron state to the highest-valence-band hole state produce the luminescence that is the basis of the light emission in quantum well lasers [4]. Today, quantum well lasers are the standard commercial semiconductor lasers and are used in applications as varied as fiber-optic communication; laser printing; materials processing; compact disc, CD-ROM, and digital video disc data storage; scientific instruments; optical pumping; medical treatment; and pointing and alignment. Recently developed quantum well lasers formed with gallium nitride epitaxy produce blue and white light previously impossible to produce with semiconductor devices. The greatly improved efficiency of semiconductor lasers and LEDs over traditional lightbulbs could revolutionize lighting [5]. Vertical-cavity, surface-emitting lasers (VCSELs) are a new generation of quantum well lasers that emit light from the surface of the epitaxial structures (as opposed to the edges). They can be formed into arrays of light emitters on a single chip with high efficiencies and low power usage.

In addition to enabling much of today's light wave technology, the flexible architecture and control of epitaxial growth are also powerful in allowing new, high-performance electronic devices and structures to be designed and fabricated. A particularly significant structure is the modulation-

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FIGURE 1 Potential wells for electrons and holes in epitaxial semiconductor sandwich. Wells are formed by layers of semiconductor materials with different electron affinities and band gaps. Lowest quantized electron energies and highest quantized hole energies are shown.