A Quantitative Model of the Electrical Activity of Metal Silicide Precipitates in Silicon Based on the Schottky Effect
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A Quantitative Model of the Electrical Activity of Metal Silicide Precipitates in Silicon Based on the Schottky Effect Teh Y. Tan and Pavel S. Plekhanov Department of Mechanical Engineering and Materials Science, Duke University Durham, NC 27708-0300 ABSTRACT A quantitative model of the electrical activity of metallic precipitates in Si is presented. An emphasis is placed on the properties of the Schottky junction at the precipitate-Si interface, as well as the carrier diffusion and drift in the Si space charge region. Carrier recombination rate is found to be primarily determined by the thermionic emission charge transport process across the Schottky junction rather than the surface recombination process. It is shown that the precipitates can have a very large minority carrier capture cross-section. INTRODUCTION Metal and metal silicide precipitates are abundant in multicrystalline Si used for manufacturing of low cost solar cells. In order to evaluate the effect of such impurities on solar cell performance, it is necessary to know the minority carrier capture cross-section of the impurity atoms as well as that of the precipitates. For most transition metals, the capture cross-section of individual atoms dissolved in Si has been measured [1,2], but much less is known regarding the precipitates. These precipitates mainly form at crystal imperfections (dislocations and grain boundaries), act as very active recombination centers to reduce minority carrier lifetimes in Si [3-8]. In electron beam induced current (EBIC) experiments it was found that the recombination activity of metallic precipitates is very high [3,5,7]. This can be explained by the presence of an electric charge on the precipitates due to Schottky effect [9,11]. Deep level transient spectroscopy (DLTS) study of metallic precipitates confirms this conjecture [12] and also indicates that precipitates form bandlike states in the semiconductor bandgap [4,12]. Moreover, the recombination activity due to precipitates decreases with the increase of the generation rate [10]. There exists several theoretical calculations of the recombination activity due to precipitates [13,14] in the formation of EBIC contrast, but there was no attempt to predict the precipitate capture cross-section (CCS) of charges based on the precipitate size and materials properties. Also, the issue of the recombination mechanism was not addressed. Understanding the mechanism of recombination at the precipitates and obtaining the associated CCS values are necessary to evaluate the effect of precipitated metallic impurities on the minority carrier lifetime. THEORY A metal or silicide precipitate embedded in the Si bulk forms Schottky junction with Si. The precipitate is charged and surrounded by a Si space charge region. When the semiconductor is illuminated, generation of non-equilibrium carriers occurs, and their distribution is characterized by quasi Fermi levels, separate for electrons and holes. These quasi Fermi levels are flat in the absence of recombination at the precipitate, whic
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