A Led Based on Porous Polycrystalline Silicon
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ABSTRACT Previous studies on electroluminescence in porous silicon were based on crystalline wafers. In this paper, we shall report the characteristics of a LED based on porous effects in a cast polycrystalline silicon substrate. A layer of porous region was first formed on a cast polycrystalline silicon substrate by anodization, followed by the deposition of a semitransparent Au layer. Under forward bias, the LED emits stable yellowish white light (with the presence of bright spots) for currents above 20 mA/cm 2. From the electroluminescence spectra measured, we suggest that the emission is due to the recombination of electron-hole pairs in a microplasma region. We propose a model where the microplasma is present in the depletion region of the heterojunction formed between the bulk polysilicon and the surface porous polysilicon. The defects and grain boundaries in a polycrystalline material facilitate the formation of such microplasma. The heterojunction model will also be used to explain the current characteristics of the LED. The effect on the LED characteristics due to indium coating on the porous substrate prior to Au deposition was studied, and the results agree with the heterojunction model. Our work shows that cast polycrystalline silicon substrates have potential for LED fabrication in cheap and large area applications.
INTRODUCTION Luminescence in porous silicon material has attracted great research interests due to its possible integration with the silicon fabrication technology. In the past, there have been many
reports on light emitting diodes (LEDs) based on porous crystalline silicon using Au, ITO Schottky diodes or porous p-n junctions [1-3]. The injection efficiency in porous silicon LEDs has increased significantly from 10-4 to 10-2. Some studies were also reported on the photoluminescence in porous polycrystalline silicon [4-6]. However as far as we know, there have been very few studies on the nature of electroluminescence produced in porous polycrystalline silicon substrate, which is of interest in large area and low cost applications. In this paper, we shall report on the characteristics of a LED formed between cast polycrystalline substrate and Au electrode.
EXPERIMENT The substrate used was p-type polycrystalline silicon of 2 0-cm formed by casting supplied by Wacker. The size of the grains was of a few mms, and the surfaces of the substrate were unpolished. To form an ohmic contact, the backside was first doped with boron to a resistivity of 1X10-3 f-cm, and then coated with Al by evaporation followed by further annealing in nitrogen at 450 0C for 30 minutes. A porous region was formed on the surface of the polycrystalline silicon substrate by anodization. The electrolyte used was HF:ethanol with 1:1 ratio, and anodization was carried out with a current density of 10 mA/cm2 for 15 minutes. During anodization, the sample was illuminated with a 500W halogen lamp. The thickness of the porous region was about 5 Rtm as determined from SEM micrograph. 163 Mat. Res. Soc. Symp. Proc. Vol. 486 ©1
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