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Quantum Wells

If one makes a heterostructure with sufficiently thin layers, quantum interference effects begin to appear prominently in the motion of the electrons. The simplest structure in which these may be observed is a quantum well, which simply consists of a thin layer of a narrower-gap semiconductor between thicker layers of a wider-gap material [37]. The band profile then shows a ``rectangular well,'' as illustrated in Fig. 11.

Figure 11: Energy-band profile of a structure containing three quantum wells, showing the confined states in each well. The structure consists of GaAs wells of thickness 11, 8, and 5 nm in AlGaAs barrier layers. The gaps in the lines indicating the confined state energies show the locations of nodes of the corresponding wavefunctions.

The electron wavefunctions in such a well consist of a series of standing waves, such as might be found in a resonant cavity in acoustic, optical or microwave technologies. The energy separation between these stationary states is enhanced by the small effective mass of electrons in the conduction bands of direct-gap semiconductors. With advanced epitaxial techniques, the potential profile of the quantum well need not be rectangular. Because the band-edge energy is usually linear in the composition, will follow the functional form of the composition. The quantum states in two parabolic wells [39] are illustrated in Fig. 12.

Figure 12: Energy band profile of a structure containing two parabolic quantum wells. The composition is similar to that of Fig. 5, and the overall width of the wells are 20 and 8 nm.

Quantum well heterostructures are key components of many optoelectronic devices, because they can increase the strength of electro-optical interactions by confining the carriers to small regions [1,2].

William R. Frensley
Sun May 21 16:29:20 CDT 1995