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INTRODUCTION / MOTIVATION

For many years there has been great interest in building optoelectronic devices using the well established and economical silicon-based technology. Unfortunately, the material upon which this technology is based, bulk crystalline silicon, does not luminesce due to its indirect bandgap which prevents radiative recombination of the electrons and holes. This property prevents bulk crystalline silicon from being used for such devices, and has motivated basic research toward developing other silicon-based materials that will efficiently luminesce. One approach has been to construct nanometer size structures, or "quantum dots", since quantum confinement in such structures should modify the bulk silicon band structure and result in electron energy levels with allowed radiative transitions of desired energies. Canham [1] examined the photoluminescence of porous silicon (PS), which contains structure on this length scale, and reported efficient visible photoluminescence. The immediate explanation of this PL was based on quantum confinement. This explanation has since been questioned, and many people have proposed that the radiative recombination occurs in an unidentified compound, composed of silicon, oxygen and hydrogen, located at the surface of the PS structure [2]. For several reasons, it has proven difficult to demonstrate through the study of PS which of these explanations, or a combination of both, is correct. The electrochemistry of PS formation is complex and hard to control. The structure of PS has many different forms which depend on the conditions of preparation and are not easily characterized [3]. Because PS is prepared in liquid etches, the effects of liquid and atmosphere exposure are not easily isolated. Morisaki and coworkers reported efficient blue [4] and infrared [5] photoluminescence from oxidized ultrafine silicon particles (50 nm to 100 nm) produced by evaporating Si in an atmosphere of Ar (and 02). These particles are an attractive alternative to PS. While these particles are composed of a silicon-based material that luminesces, they have the added advantage that one may more easily control their growth and identify their structure, avoiding many of the experimental difficulties cited above for PS. For these reasons, we have chosen to pursue a detailed examination of similarly prepared material. 127 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society

This paper reports the preliminary results of our investigation of Si and SiOx nanoclusters (between 7 nm and 17 nm in size) produced by evaporation. We identify two strong room temperature photoluminescence (PL) bands centered around 1.65 eV and 2.12 eV. The PL is dramatically affected in a reversible fashion when the nanoclusters are exposed to a variety of gases. Structural measurements are presented along with the PL results to help characterize the clusters. II.

EXPERIMENTAL DETAILS

The clusters are produced by thermal evaporation in a gas at 0.60 Torr. Three types of materials are being studied: A)