Recombination in compositional multilayers

Low temperature non-radiative recombination in bulk a-Si H occurs by tunneling to defects over a distance of about 100 A (Section 

The recombination is modified in a multilayer structiu e whose layer spacing is similar to the carrier tunneling distance and is observed in photoluminescence measurements (Tiedje 1985). Fig. 9.22(a) shows that the luminescence intensity of a-Si H/nitride multilayers decreases as the layer thickness drops below about 500 A. The interface states and bulk nitride defect states cause non-radiative recombination because the electron-hole pairs are never far from an interface. The model of non-radiative tunneling developed in Section 8.4.1 can be adapted for recombination in thin layers. When the layer thickness is less than the critical transfer radius, the luminescence efficiency is (see Eq. (8.52)). 

The opposite effect happens in multilayer films of materials in which the luminescence is strongly quenched by bulk defects, for example, a-Ge H. The luminescence intensity increases in a-Ge H/a-Si H 

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