Other meanings of Quantum well
Semiconductor physics
A quantum well is a semiconductor structure confining particles in a narrow potential well with quantized energy levels. It is usually formed by placing a thin, lower-band-gap semiconductor layer between wider-band-gap barrier layers, so electrons and holes are restricted in one spatial direction while remaining mobile in the other two. This quantum confinement changes the material’s optical absorption, emission, and electronic transport, enabling devices including semiconductor lasers, light-emitting diodes, infrared detectors, and quantum cascade lasers.1
A quantum well confines carriers through a band-energy difference between a narrow-layer material and its surrounding barriers. In a common GaAs/AlGaAs heterostructure, the GaAs layer has lower conduction- and valence-band energies, producing potential wells for electrons and holes.1 When the layer is thin enough, the carrier wavefunction cannot vary continuously in the growth direction; only discrete subband energies are allowed. Motion parallel to the interfaces remains approximately free, giving a two-dimensional electron or hole gas.
The confinement energy depends strongly on well width, effective mass, and barrier height. Narrower wells generally produce larger separations between subbands, while finite barriers allow wavefunctions to leak into adjacent layers. Coulomb attraction can bind an electron and hole into an exciton, whose optical properties are also modified by confinement.
Precisely controlled epitaxial growth makes quantum wells practical semiconductor devices. Molecular-beam epitaxy and metalorganic vapour-phase epitaxy deposit atomically ordered layers whose thickness and composition determine the well and barrier potentials. Multiple wells can be stacked with barriers between them to increase optical gain or absorption while preserving controlled coupling.
Electrical behavior is often improved by modulation doping: dopant atoms are placed in a barrier rather than directly in the well, allowing carriers to occupy the well while reducing ionized-impurity scattering. This arrangement helped establish high-mobility two-dimensional electron gases and enabled studies of phenomena such as the quantum Hall effect.2 In optical devices, injected electrons and holes recombine across the well, producing photons whose energy is set largely by the subband separation and band gap.
Quantum wells are central to efficient semiconductor light sources because confinement increases the overlap of electron and hole states and concentrates the electronic density of states near useful transition energies. Separate-confinement heterostructure lasers place one or more wells inside a broader optical waveguide, reducing the threshold current and permitting wavelength engineering through composition and thickness.1
In infrared technology, engineered intersubband transitions can emit or detect photons without requiring electron-hole recombination. The quantum cascade laser uses repeated quantum wells so one electron can generate multiple photons as it traverses a layered structure; its emission wavelength is selected by the designed subband spacing rather than solely by the bulk band gap.3 Quantum-well infrared photodetectors similarly exploit transitions between confined conduction subbands.
Quantum wells are not limited to simple rectangular potential profiles or to electrons and holes. Graded-composition layers create approximately triangular wells, electric fields can shift optical transitions through the quantum-confined Stark effect, and coupled wells permit tunnelling between otherwise separate subbands. These effects are useful for electro-absorption modulators, resonant tunnelling structures, and studies of coherent carrier dynamics.
A well can also be designed for unusual carrier systems. In semiconductor quantum wells, excitons may remain optically significant at temperatures where their binding is weakened, while magnetic fields, strain, and strong light fields produce additional control over the levels. At very small dimensions, interface roughness and alloy disorder become major limits: a fluctuation of only a few monolayers can change the transition energy and broaden the optical spectrum. Thus fabrication precision is part of the quantum design itself.
Thicknesses, confinement energies, and device wavelengths vary substantially with material composition, barrier design, strain, and operating conditions.
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