A semiconductor structure in which a thin layer of one semiconductor is sandwiched between layers of a different semiconductor material. Materials are chosen so that electrons available to provide conduction in the middle layer are at a lower energy than those in the outer layers, creating an energy well confining the middle-layer electrons. Crucially, the middle layer is only a few atoms thick, comparable with the wavelength of the middle-layer electrons. The thinness of the layer dramatically modifies electron behaviour, since they experience an environment which is a two-dimensional plane, rather than three-dimensional bulk. Though the energies of electrons in the bulk form a smooth continuum, those in the middle layer are thus restricted to fixed energy levels. A large current will flow when an applied voltage is such that the energy of electrons approaching the sandwich-layer region matches that of an electron energy level in the middle layer; otherwise, only a small current will flow. The characteristics of the structure can be tailored using the layer thickness and the material composition. Further modification to electron behaviour is possible using multiple quantum wells, made from alternating layers of semiconductor material; these are a type of superlattice, a structure comprising alternating layers of different materials. Quantum well devices are fabricated using molecular beam epitaxy; the first experimental device being constructed in 1974. Reducing the middle layer of a quantum well to a single line, a one-dimensional system, gives a quantum wire and introduces new electron behaviour; a quantum dot, a zero-dimensional system, is a small, isolated region of semiconductor (or metallic) material that behaves like a custom-built atom containing tens of thousands of real atoms. Quantum well systems can be used to create compact, fast computer chips, highly efficient microscopic lasers, and optoelectronic devices; they form the basis of the lasers in CD players, and microwave receivers in satellite dishes. The quantum cascade laser (1994) relies on quantum wells as does the blue light semiconductor laser (1995).
A quantum well is a potential well that confines particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy values.
Fabrication
Quantum wells are formed in semiconductors by having a material, like gallium arsenide sandwiched between two layers of a material with a wider bandgap, like aluminium arsenide.
Applications
Because of their quasi-two dimensional nature, electrons in quantum wells have a sharper density of states than bulk materials. Quantum well infrared photodetectors are also based on quantum wells, and are used for infrared imaging.
By doping either the well itself, or preferably, the barrier of a quantum well with donor impurities, a two-dimensional electron gas (abbreviated 2DEG) can be formed.
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