Effects of Amorphous Carbon Films on the Performance of Porous Silicon Electroluminescence
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Effects of Amorphous Carbon Films on the Performance of Porous Silicon Electroluminescence Bernard Gelloz and Nobuyoshi Koshida Dept of Electrical and Electronic Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184 8588, Japan. ABSTRACT Efficient electroluminescence (EL) is obtained at low operating voltages (1 %) has been reached thanks to electrochemical oxidation (ECO) [24]. ECO leads to the preferential oxidization of the coarser parts of PS. The result is a dramatic reduction of the leakage current [2,3]. High external power efficiency (EPE) (0.39 %) has also been achieved at an operating voltage as low as 5 V using the ECO technique in combination with a thin (~1 µm) PS layer [4]. The stability of the most efficient devices does not exceed a few days or weeks. Some studies aiming at reducing PS oxidation have been reported. The best results have been obtained using PS passivation by partial thermal [5] or electrochemical [4,6] oxidation of PS, or by protecting PS using a capping layer [7]. The capping function is to prevent water molecules from penetrating inside PS pores, thus preventing Si oxidation. The power efficiency, the stability and the reproducibility should be improved if one would like to consider PS in an application. With these purposes in mind, the effect of a carbon film as a buffer layer between PS and the top ITO electrode on the performance of PS-based diodes has been investigated. Due to its high chemical stability and its buffer function, the carbon film
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enhances both the stability and the power efficiency. In addition, high reproducibility is achieved even with devices including a thin PS layer, operating at very low voltages.
EXPERIMENTAL DETAILS All PS layers are formed from n+-type Si (100). A superficial compact PS layer is first formed under 5 mA/cm2 during 30 s in dark condition in HF (55wt.%): ethanol=1:4.5. In the same electrolyte, the optically active PS layer is then formed under 15 or 5 mA/cm2 during 10 min, under illumination from a 500W tungsten lamp. The total thickness of the porous layer is about 600 nm. Having been rinsed with ethanol for 2 min, and without being dried, ECO of the porous layers is achieved by anodically polarizing PS in an aqueous solution containing 1M sulfuric acid (H2SO4). ECO is carried out until the maximum of EL during the treatment. More details about this technique can be found in our original reports [2,3]. Then, the samples are rinsed with ethanol for 2 min, and dried. In some cases, a few nanometer thick amorphous carbon film is then deposited by sputtering (60 W) onto PS. Deposition time ranged from 15 min to 45 min. The color of the carbon film was brown with darkness increasing with the film thickness. Its light transmission at 600 nm was ranging from about 95 % to 70 % for sputtering time in the range 15 min to 45 min. Finally, a 400 nm thick ITO layer is then deposited by sputtering (50 W) onto the porous layer or the carbon film (if present) for use as the top electrode.
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