A quantum well is a nanometer-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 a 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 band gap (examples: GaAs quantum well embedded in AlGaAs, or InGaAs in GaAs). The thickness of such a quantum well is typically ≈ 5–20 nm. 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. In optically pumped semiconductor lasers (→ vertical external-cavity surface-emitting lasers), most pump radiation may be absorbed in the layers around the quantum wells, and the generated carriers are captured by the quantum wells thereafter.
If a quantum well is subject to strain, as can be caused by a slight lattice mismatch (e.g., for InGaAs quantum wells in GaAs), the electronic states are further modified, which can even be useful in laser diodes.
Semiconductor quantum wells are often used in the active regions of laser diodes, where they are sandwiched between two wider layers with a higher band gap energy. These cladding layers function as a waveguide, while electrons and holes are efficiently captured by the quantum well (separate confinement), if the difference in bandgap energies is sufficiently large. 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.
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