Experiments and Monte Carlo Simulations on the Recombination Dynamics in Porous Silicon
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INTRODUCTION Recent experimentsl, 2 have provided evidence of anomalous relaxation behaviour of the luminescence of porous Silicon (p-Si), which follows stretched exponential decay for a variety of experimental conditions. Motivated by these experimental results, and by the fact that little is known about the recombination dynamics of the charge carriers in these nanomaterials, 3 ,4 we have initiated a detailed numerical study of the underlying transport behaviour in p-Si by means of Monte-Carlo (MC) simulations. 5 In this work, we present a simple model in which nanometer sized crystals (nano-crystals), characterised by a distribution of radiative and non-radiative recombination times, are assumed to be randomly placed on a three-dimensional percolation cluster slightly above the critical concentration. Charge carriers are allowed to hop between nearest-neighbour occupied sites. The competing effect between radiative and non-radiative transitions in a single Si nano-crystal, as well as the effects of geometrical constraints on transport due to the complex topology of the cluster and the additional disorder due to the random distribution of the particle sizes, are discussed. EXPERIMENTAL RESULTS Details regarding the sample preparation and the time resolved photoluminescence (PL) apparatus were reported elsewhere. 2 ,6 The etching parameters and emission energies (E) of the different samples used in this work are reported in Table I. E depends on the porosity, and shows a blue shift by increasing the porosity. Since higher porosity samples correspond to smaller mean size of the Si nano-crystals, the observed shift of the luminescence peak energy may well be correlated with the mean Si-particle size, supporting the idea of the quantum con7 finement model. 549 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
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d (tm) 7.1 7.9 7.0 7.8 4.5 2.0 2.6
peak (eV) 1.54 1.60 1.68 1.65 1.72 1.83 1.86
TABLE I Growth parameters and peak emission energies of the studied samples. [HF] is the HF concentration in the electrolyte, J is the current density, t is the time
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