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Definition: thin layers confining carriers in one dimension
A quantum well is a thin layer which can confine (quasi-)particles (typically electrons or holes) in the dimension perpendicular to the layer surface, whereas the movement in the other dimensions is not restricted.
The confinement is a quantum effect. It has profound effects on the density of states for the confined particles. For a quantum well with rectangular profile, the density of states is constant within certain energy intervals.
A quantum well is often realized with a thin layer of a semiconductor medium, embedded between other semiconductor layers of wider bandgap. (Examples: GaAs quantum well embedded in AlGaAs, or InGaAs in GaAs.) The thickness of such a quantum well is typically ∼5-20 nanometers. Such thin layers can be fabricated with molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Both electrons and holes can be confined in semiconductor quantum wells.
Semiconductor quantum wells are used e.g. in the active regions of laser diodes or surface-emitting semiconductor lasers, where they allow to reach significantly lower pump thresholds than with thicker layers. (This holds particularly for double-heterostructures, where the quantum well region also acts as a waveguide.) Quantum wells are also used as absorbers in semiconductor saturable absorber mirrors (SESAMs), and in electroabsorption modulators. If a large amount of optical gain or absorption is required, multiple quantum wells (MQWs) can be used, with a spacing typically chosen large enough to avoid overlap of the corresponding wave functions.
Bibliography
| [1] | T. Makino, "Analytical formulas for the optical gain of quantum wells", IEEE J. Quantum Electron. 32, 493 (1995) |
| [2] | P. S. Zory, ed., "Quantum Well Lasers – Principles and Applications", Academic Press, 1993, ISBN-13: 978-0127818900 |
See also: quantum dots, laser diodes, electroabsorption modulators, waveguides
Categories: photonic devices, quantum optics
