Grain Boundary Characterization in Polysilicon by Light Beam Induced Current Topography and Image Processing
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GRAIN BOUNDARY CHARACTERIZATION IN POLYSILICON BY LIGHT BEAM INDUCED CURRENT TOPOGRAPHY AND IMAGE PROCESSING K.MASRI, J.P.BOYEAUX*, S.N.KUMAR, L.MAYET*, 0and A.LAUGIER Laboratoire de Physique de la Mati~re (U.A.- CNRS N 358 ), INSA de Lyon; 20, Av. A. Einstein, 69621 Villeurbanne Cedex; * Permanent address: Universitd Lyon I - ISIDT, Bat. 203, 69622 Villeurbanne Cedex; FRANCE
ABSTRACT
A high performance light-beam-induced-current (LBIC) analyser has been used to determine the recombination velocity at the grain boundary (S) and the minority-carrier diffusion length (L). For this purpose a Schottky diode ( Cr/Si ) was fabricated using a p-type silicon bicrystal (1 92 cm, 113 grain boundary). The contacts were obtained by a "cold" technology. The diffusion length, determined by the method proposed by Ioannou, was subsequently fitted into the model proposed by Marek to evaluate the recombination velocity by the curve-fitting of the experimental and theoretical photocurrent profiles. A value of S = 2.104 cm/s was thus obtained. The influence of the thin oxide layer at the Cr/Si interface is also discussed. INTRODUCTION Polycrystalline silicon is a potential candidate for large area solar-cell applications and cells with efficiencies comparable to that of single-crystal solar cells can be produced with low production costs. However, the polycrystalline material introduces large spatial inhomogeneities due to the presence of grain boundaries. Grain boundaries are regions infested with a large number of defects which detract the carriers from the transport of current, and also act as a potential barrier between adjacent grains. By acting as recombination centers for the minority carriers, grain boundaries reduce the solar cell performance. Consequently, the development of high quality polycrystalline solar cells necessitates the use of analytical techniques with a good spatial resolution for the topographical characterization of the cell performance. Primarily, the surface probing can be carried out either by an electron-beam or by a light-beam. The method of electron-beam-induced-current (EBIC) characterization has been widely used and several attempts have been made [1-2] to characterize the signal thus emitted. However, many of the experiments possible with the EBIC mode can be performed using a light-beam as the excitation source. Since the LBIC mode would not add any excess charge to the specimen under test, this can be a major advantage, especially for high resistivity materials. Recently, Marek [31 has developed a theoretical model for a quantitative evaluation of the LBIC profiles at grain boundaries. In the present work, we show that this model can be successfully applied for the case of p-type Czochralski (CZ) grown silicon (1 Q) cm, Y-13 grain boundary) to
calculate the minority carrier diffusion length, and the recombination velocity at the grain boundary, by fitting the experimental and theoretical photocurrent profiles. THEORETICAL CONSIDERATIONS
Determination of the recombination velocity Recently, Mare
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