Visible Light Emitting Diode Employing Electrochemically Anodized Nanocrystalline Silicon Thin Film
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INTRODUCTION Extensive efforts have been carried out to develop a novel light emitting diode (LED) based on nanocrystalline silicon (nc-Si) formed by electrochemically anodized Si (porous Si) which shows strong visible photoluminescence (PL) [1,2]. Since the first observation of electroluminescence (EL) in Schottky barrier diodes [3] and p-n junction diodes [4], the emission color extends over most of the visible spectral region (including blue [5]) and the external quantum efficiency can reach about 0.2% [6]. Therefore it would be an attractive approach to fabricate light emitting nc-Si
thin films due to combining both fabrication techniques of microcrystalline (jtc-Si) thin films and porous Si for development of large- area light emitting devices. Little work has been done to obtain EL emission from electrochemically formed nc-Si thin films produced from plasma-chemical-vapor-deposition (CVD)-prepared uc-Si [7], as well as from polycrystalline Si fabricated by low pressure CVD [8], however EL emission has been obtained using an electrolyte junction in both cases. Microcrystalline Si deposited by plasma CVD have been successfully applied to optoelectronic devices such as solar cells, image sensors and thin film light emitting diodes (TFLEDs) due to the higher dark conductivity than that of amorphous Si (a-Si) by several orders of magnitude [9,10]. The device fabrication utilizes fully the benefit of the plasma CVD method (i.e., large area deposition at low temperature on several substrates involving glass, steel, and polymer films). In this article, we present a solid state p-i-n junction TFLED which incorporates p-type nc-Si electrochemically anodized from Jc-Si. Device performance of the nc-Si TFLED including EL and current-voltage characteristics is discussed. Raman and PL studies are also presented on the nc-Si thin films [10].
EXPERIMENTAL The structure of the TFLED is illustrated in Fig. 1. The p-type nc-Si thin film was electro675 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
chemically formed on a SnO 2coated glass substrate from boronAln a-SiC doped p-type pc-Si. The pc-Si was ( nm) deposited by the rf plasma CVD i a-Si method with a low deposition temperature of 180°C. The rf power was roughly 2.5 times higher than that for an a-Si film. The thickness of the pc-Si was 200 nm as deposSnO2 ited. A mixture of Sill4 , B2H 6 and H2 gases was used as a source gas. Glass The B2HdSiH,a flow rate ratio was varied 0.1 to 2.5 vol.% to change the crystalline volume fraction inthe film. The SiH 4/H 2 flow rate ratio was set to be less than FIG. 1. Structure of p-type nc-Si/ intrinsic a-Si/ n-type -3 vol.%. Electrochemical anoda-SiC heterostructure TFLED. The nc-Si layer was ization was performed in an HF electrochemically anodized from plasma-CVD-deposited pcaqueous solution. A 50% HF soluSi. The thickness of the p layer must decrease from the pction was further diluted by H2 0 into Si thickness of 200 nm as deposited. 1/20-1/50. The anodizing current density and time were changed in the ra
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