Quantum-confined structures

Fundamentals and Applications of Quantum-Confined Structures (a) Multiple Quantum Wells... 

Many-body luminescence from highly excited quantum-confined structures... 

Figure 2 In highly-excited quantum-confined structures, photoexcited carriers form electron and hole Fermi seas in conduction and valence bands, respectively Uth is the energy distance between the corresponding Fermi levels. Recombination of electron-hole pairs, belonging to space-quantization energy levels, gives rise to discrete lines in the luminescence spectrum.

In 1990, Canham observed intense visible photoluminescence (PL) from PSi at room temperature. Visible luminescence ranging from green to red in color was soon reported for other PSi samples and ascribed to quantum size effects in wires of width 3 nm (Ossicini et al, 2003). Several models of the origin of PL have been developed, from which we chose two. In the first (the defect model), the luminescence originates from carriers localized at extrinsic centers that are defects in the silicon or silicon oxide that covers the surface (Prokes, 1993). In the second model (Koch et al., 1996), absorption occurs in quantum-confined structures, but radiative recombination involves localized surface states. Either the electron, the hole, both or neither can be localized. Hence, a hierarchy of transitions is possible that explains the various emission bands of PSi. The energy difference between absorption and emission peaks is explained well in this model, because photoexcited carriers relax into surface states. The dependence of the luminescence on external factors or on the variation of the PSi chemistry is naturally accounted for by surface state changes. 

An important concern is two-photon absorption which can also become a significant problem at high power densities, especially in the guided wave geometry. These excitations are even more of a problem in multiple quantum well devices and quantum confined structures because direct two-photon absorption can create free carriers which decay very slowly, giving rise to a slow nonlinear response. Molecular and polymeric materials offer additional flexibility to shift the two-photon resonances by chemical modifications. 

By interrupting a bulk electrodeposition process in the initial stages of nucleation and growth, an electrode surface can be covered by well-separated nanocrystals. This makes electrodeposition a low-cost and easy-to-use approach for the formation of semiconductor nanocrystals in which quantum confinement leads to an atomic-like energy spectrum [199]. Electrodeposition of quantum-confined structures could be of... 

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