Diffusion Length Measurements of Minority Carriers in Si-SiO 2 Using the Photo-Grating Technique
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DIFFUSION LENGTH MEASUREMENTS OF MINORITY CARRIERS IN Si-SiO2 USING THE PHOTO-GRATING TECHNIQUE Y. POSADA1, I. BALBERG2, L.F. FONSECA1, O. RESTO1 AND S.Z. WEISZ1, 1 Department of Physics, University of Puerto Rico, Rio Piedras, 00931 PR 2 The Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel ABSTRACT We have studied the microstructure, the transport and the phototransport properties of the Si crystallites network in Si-SiO2 composites. We have found that in our co-sputtered samples the average crystallite diameter, d, decreases from 40 to 5 nm as the content of the silicon, x, decreases from 80 to 40 volume%, and that the percolation of the network sets is at x ≈ 40 vol%. A simultaneous study of the photoluminescence (PL) shows the, quantum confinement, expected red shift of its peak with increasing d. On the other hand the very strong observed decrease of the PL intensity with x is interpreted here as due to a deconfinement effect that is dominated by the increase in the cluster size of connected Si crystallites. The results suggest that a closed random packing of the Si crystallites will be the preferred network for high intensity electroluminescence. INTRODUCTION There is a wide interest in developing electro-optical systems that are compatible with microelectronic silicon technology. Several studies have been devoted to the luminescence of such systems[1-3] and to the electro-luminescence (EL)[4], but there were fewer[5] studies on the transport properties[6]. One of the most intensively studied Si-based systems is the Si-SiO2 composites[1], although there are only a handful of studies on its transport[7,8] and phototransport[9] properties. This system is of physical interest because it shows quantum confinement effects[10,11] and Coulomb blockade effects[8] related to the bias dependence of the transport. However, no other aspects of the transport or its correlation with the PL have been reported. In this paper we focus on the effect of the microstructure of the nanocristallites’ network on the transport and PL properties. The knowledge derived from this study has potential application for light emitting diodes (LED’s). We describe the preparation procedure and the experimental techniques used for the characterization of our composites. Then, we present our experimental results and discuss the physical picture and the practical implications that emerge from the present study. EXPERIMENTAL DETAILS The samples were prepared in a sputtering system in which the targets are at the bottom and the substrates are located 5 cm above them. Our sputtering targets consisted of two attached semicircles of Si and SiO2 yielding a 13.2 cm disc. The substrates were quartz slides having a cross section of 1.1x0.09 cm2 and a length of 12.6 cm. The latter configuration enables to get, in a single run, composites with variable vol% of Si from practically x = 0% to practically x = 100%. A sputtering duration of 12 hours yielded a film with an average thickness of 9 µm. The substrate reached a maximum tempera
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