Radiative and Non-Radiative Processes for the Light Emission from Porous Silicon
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RADIATIVE AND NON-RADIATIVE PROCESSES FOR THE LIGHT EMISSION FROM POROUS SILICON J.C. VIAL+, A. BSIESY, G. FISHMAN, F. GASPARD, R. HERINO, M. LIGEON, F. MULLER, R. ROMESTAIN L.S.P. CNRS-Universite J. Fourier de Grenoble, B.P. 87 -38402 St Martin d'HWres -France and R.M. MACFARLANE+ I.B.M. Almaden Research Center, 650 Harry road, San Jose California. 95120-USA
ABSTRACT Highly porous silicon, well passivated via an anodic oxidation process, is a stable and efficient visible light emitter showing a 3% photoluminescence efficiency at room temperature. Luminescence decay times are on the order of 100 tps at room temperature and 10 ms at low temperature. Above room temperature the de-excitation is dominated by nonradiative processes well describe by a tunnelling escape of carriers from confined regions. The "anomalous" luminescence behaviour showing a dramatic increase of the lifetimes upon cooling associated with a decrease of the intensity is explained by the temperature dependence of the effective radiative recombination rates due to a population redistribution among two excited states with very different radiative relaxation rates.
I-INTRODUCTION Silicon, the most studied semi-conductor, with an indirect band gap transition in the infrared (1.1eV) is not expected to be a good light emitter, especially in the visible range. Nevertheless, triggered by the recent discoveries of bright visible photo- and electroluminescence from porous silicon1 - 3 , the interest on the visible light emission from various silicon nanostructures 4 - 6, has been renewed. In addition to their nanometer sized structures these materials have similar spectral behaviors, the most prominent are a rather high luminescence quantum efficiency and unexpected long luminescence lifetimes 7 , particularly at low temperature as we show here. Previous work 7 has shown that efficient visible light emission could be obtained from as-formed high porosity samples without further chemical dissolution and that enhanced light emission is observed after anodic oxidation. It has been demonstrated by the observation of the photoluminescence evolution, as well as of the so called "wet electro-luminescence"' 8, that anodic oxidation, which has the advantage of bestowing good mechanical properties on the porous layer, directly controls the non-radiative processes via the passivation of the silicon nanocrystallite. Concerning the radiative processes several theoretical models have been developed. At present, the effective mass approximation models 9 10 as well as a more elaborate one 1 1 are able to explain that there is an increase of the radiative recombination rates as confinement increases but the predictions are several orders of magnitude higher than the experimental 12 observations. In addition the "Anomalous temperature dependencies of photoluminescence" 13 remain unexplained within these theoretical models. The aim of this paper is to show that the use of well controlled anodically oxidised porous silicon together with quantitative measurements of luminesce
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