Si-Cluster Luminescence
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Si-CLUSTER LUMINESCENCE
WINSTON A. SAUNDERS*, HARRY A. ATWATER*, KERRY J. VAHALA*, RICHARD C. FLAGANt, AND PETER C. SERCEL*1 *Applied Physics, M. S. 128-95 fChemical Engineering, M. S. 210-41 California Institute of Technology, Pasadena, CA 91125 ABSTRACT We report on a technique for generating Si-clusters in the nanometer size regime. The cluster source is capable of generating micron-thick cluster films in a few minutes. The cluster films luminesce in the visible and infrared. Source construction is discussed. Electron micrographs of the clusters are shown. Luminescence and optical absorption spectra for the clusters are presented. INTRODUCTION The recent observation of luminescence from porous Si strongly suggests that under appropriate conditions nanometer-scale Si can produce visible luminesce.1 While many details of the microstructure of porous Si remain unclear, 2 wire-like and dot-like 3 structures are the strongest of the proposed candidate structures. Here, we report experimental results pertaining to the generation of luminescent nanometer-scale Si clusters. In the first part of the paper we discuss the cluster source, a spark discharge source, which uses a high energy electric spark to vaporize material from crystalline Si electrodes. In the second second part of the paper, optical measurements on the clusters are presented. Luminescence spectra are similar to those of porous Si. The measured optical absorption of the clusters closely follows the luminescence spectrum. CONTINUOUS FLOW SPARK-DISCHARGE SOURCE The clusters used -in these experiments are formed using o continuous-flow spark-discharge (spark) source, shown schematically in Figure 1. The source vaporizes material from a crystalline silicon electrode using a high energy electric spark. This vapor subsequently nucleates in the flowing Ar jet, forming nanometer-scale clusters. The source body is a single piece of Delrin, drilled to accommodate two 3/8" diameter electrodes and a 1/8" gas inlet. The electrodes are fitted with screw end caps which tighten an o-ring seal between the electrodes and the source body. The gas inlet is press-fit into a reamed 1/8" hole. The electrodes are insulated with heat-shrunk Teflon to minimize discharges between the electrodes and the vacuum chamber. A simple flashlamp circuit is used to drive the spark source. Typically, the spark energy is between 50 and 150 mJ. This energy range is similar to that used in laser vaporization cluster sources. 4 The components of the circuit are chosen to give a pulse width of 2 lisec and a repetition rate of about 200 Hz. 1. Present address: Physics Dept.. University of Oregon, Eugene, OR 97403 Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
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Fig. 1. A schematic diagram of the continuous flow spark discharge (CFSD) source. A flashlamp driver circuit is utilized to extract the energy stored in the capacitor. Typically, the spark duration is 2 gsec. The source is mounted to a vacuum chamber exhausted by a zeolyte-trapped mechanical pump. Before a
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