Optical properties of silicon nanocrystal LEDs

In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO2 oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson-Schrodinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV. However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV. On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I-V measurements confirm that the current is related to a pure tunnelling process. A modelling of I-V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm. The Fowler-Nordheim process is not observed during light emission for electric fields below 5 MV/cm. Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability. (C) 2002 Elsevier Science B.V. All rights reserved.