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                            SEMICONDUCTOR NANOSTRUCTURES

                                In 1970, L. Esaki and R. Tsu proposed the fabrication of an artificial heterostructure of nanometric layers of
                                two semiconductors materials with different energy gaps: a semiconductor superlattice. The energy potential
                                arising from the periodic alternance of each kind of semiconductor is a succession of quantum wells where
                                the energy can only take discrete values. When an external electric field is applied to the heterostructure, the
                                potential bends and electrons can move according to three transport regimes: miniband transport (strongly
                                coupled superlattices), resonant sequential tunneling (in weakly coupled superlattices), and Wannier-Stark
                                hopping (not considered here).

                                  A quantum well (AT & T Bell Labs)           A periodic superlattice, each period made of a barrier and a well

                                Miniband transport: when potential barriers are thin (≤ 3 nm) the discrete energy levels overlap leading to
                                the creation of extended quantum states with energetic width Δ (strongly coupled superlattices).

                                Resonant sequential tunneling: when potential barriers are large, electrons move from one quantum well
                                to the next by tunneling to the same energy level, after what they fall, by scattering, to a lower energy level.
                                (weakly coupled superlattices).
                                                                                                                      Resonant sequential tunneling under a dc bias

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