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Quantum well
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== 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 [[aluminum arsenide]]. (Other examples: a layer of [[indium gallium nitride]] sandwiched between two layers of [[gallium nitride]].) These structures can be grown by [[molecular beam epitaxy]] or [[chemical vapor deposition]] with control of the layer thickness down to [[monolayer]]s. Thin metal films can also support quantum well states, in particular, thin metallic overlayers grown in metal and semiconductor surfaces. The vacuum-metal interface confines the electron (or hole) on one side, and in general, by an absolute gap with semiconductor substrates, or by a projected band-gap with metal substrates. There are 3 main approaches to growing a QW material system: lattice-matched, strain-balanced, and strained.<ref name=":4">{{Cite journal|last1=Sayed|first1=Islam|last2=Bedair|first2=S. M.|date=2 March 2019|title=Quantum Well Solar Cells: Principles, Recent Progress, and Potential|journal=IEEE Journal of Photovoltaics|volume=9|issue=2|pages=402β423|doi=10.1109/JPHOTOV.2019.2892079|s2cid=67874610|issn=2156-3381|doi-access=free}}</ref> * Lattice-matched system: In a lattice-matched system, the well and the barrier have a similar lattice constant as the underlying substrate material.<ref name=":4" /> With this method, the bandgap difference there is minimal dislocation but also a minimal shift in the absorption spectrum. * Strain-balanced system: In a strain-balanced system, the well and barrier are grown so that the increase in lattice constant of one of the layers is compensated by the decrease in lattice constant in the next compared to the substrate material. The choice of thickness and composition of the layers affect bandgap requirements and carrier transport limitations. This approach provides the most flexibility in design, offering a high number of periodic QWs with minimal strain relaxation.<ref name=":4" /> * Strained system: A strained system is grown with wells and barriers that are not similar in lattice constant. A strained system compresses the whole structure. As a result, the structure is only able to accommodate a few quantum wells.<ref name=":4" /> [[File:MCM GaAs AlGaAs structure.svg|thumb| A heterostructure made from semiconductors AlGaAs (large band-gap) and GaAs (smaller band-gap) in a quantum well configuration. In the central GaAs region of length d, the conduction band energy is lower, and the valence band energy is higher. Therefore both electrons and holes can be confined in the GaAs region. ]]
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