Quantum well

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. These structures can be grown by molecular beam epitaxy or chemical vapor deposition with control of the layer thickness down to monolayers. This is now common in industry, in research, and even for academic students [A book about these techniques (in Italian)].

Applications
Because of their quasi-two dimensional nature, electrons in quantum wells have a sharper density of states than bulk materials. As a result quantum wells are in wide use in diode lasers, specifically blue lasers. They are also used to make HEMTs (High Electron Mobility Transistors), which are used in low-noise electronics. 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 (2DEG) may be formed. This quasi-two dimensional system has interesting properties at low temperature. One such property is the quantum Hall effect, seen at high magnetic fields. Acceptor dopants can lead to a two-dimensional hole gas (2DHG).