Mechanisms of visible photoluminescence in porous silicon

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Abstract
We propose a model for porous silicon, an irregular structure obtained by removing some silicon atoms randomly from a perfect silicon crystal. It is shown by using two-dimensional clusters with a tight-binding model that this model exhibits the energy-gap widening and the nonexponential decay of the photoluminescence (PL), which are in good agreement with the observed properties of porous silicon. In this model, the energy-gap widening is due to the localization of eigenstates caused by the randomness of the structure. The distributions in both size and position of the localized eigenstates, i.e., a statistical effect, yield nonexponential PL decay which is describable by the stretched exponential function. A dynamical effect due to electron and hole hopping between the eigenstates explains the observed strong energy dependence of the PL lifetimes.