Reactive Cluster Epitaxy: CoSi 2 Nanoparticles on (111) Si
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ABSTRACT The formation of epitaxial CoSi 2 islands of nanoscopic dimensions is reported using the technique of reactive cluster deposition. Co clusters in the size range 5-50nm were synthesized by sputtering a high purity Co target inside a UHV sputtering chamber. The clusters were then deposited on the reconstructed Si (111) surface. Upon annealing the particles reacted with the Si substrate to form epitaxial CoSi 2. Our observations were made using a JEOL 200CX transmission electron microscope modified for in-situ sputtering and ultrahigh vacuum conditions.
[NTRODUCTION Interfacial structures play a central role in determining the character and properties of a wide variety of technological materials. As the length scales of interest in state of the art technologies continue to decrease, increasing scientific and technical challenges are being encountered in the determination of the structures of materials. A variety of unusual physical phenomena have been observed in dimensionally constrained materials which include clusters, thin films, multilayers and other 'nanostructured materials'. Novel phenomena include non-bulk optical absorption and emission spectra [1], enhanced magnetoresistance [2] and strongly correlated electronic charge transport [3]. Potential applications of these and other phenomena in nanoscale electronics, optics and magnetics have prompted a surge of interest in understanding and controlling the synthesis and properties of nanostructured materials [4, 5]. The large surface area: volume ratios of nanocrystals (responsible for a number of their interesting properties) also increases their sensitivity to atmospheric contamination (for example by reaction with oxygen, organic molecules etc). Interpretation of experimental data is often complicated by concerns over the influence of such contamination. In our experiments we have sought to address these concerns by characterizing our samples in-situ, immediately following synthesis, and under ultrahigh vacuum conditions. The electron microscope was chosen since it facilitates characterization of particle morphology, buried interfacial structures and crystallographic orientation in a single experiment. Supported (macroscopic) particles have proved to be of scientific interest for a variety of reasons and provided a useful means of investigating, for example, the existence of preferred orientations in grain boundary structures [6-8]. We have previously studied the deposition of metal nanoparticles (Cu, Ag, Co) on metal substrates (such as Cu, Ag) and observed a variety of novel phenomena including 'contact epitaxy' [9] and 'burrowing' [10]. In this paper we have
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Mat. Res. Soc. Symp. Proc. Vol. 619 ©2000 Materials Research Society
extended this work to include reactive cluster epitaxy by depositing our metal clusters on the clean (111 )Si surface. We demonstrate that nanoscopic epitaxial CoSi 2 islands may be formed on the silicon surface by this technique. Nanoscopic silicide islands would be of potential technological significance for the fab
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