Sputtering Effects and Two Dimensional Arrangement of Nanoparticles in Insulators Under High Flux Cu Implantation
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ABSTRACT Application of negative heavy ions, alleviating surface charging on insulators, enables us to conduct low-energy and high-flux implantation, and leads to a well-defined tool to fabricate near-surface nanostructures. Negative Cu ions of 60 keV, at high doses, have generated nanocrystals in amorphous(a-)Si0 2 with a size (-10 nm) suitable for nonlinear optical devices. The kinetic processes, inside the solid and at the surface, are studied by cross-sectional TEM and tapping AFM, respectively. In a-SiO 2 , nanoparticles spontaneously grow with dose rate, being controlled by the surface tension and radiation-induced diffusion. Furthermore, the nanospheres give rise to a two-dimensional (2D) arrangement around a given dose rate. The 2D-distribution occurs in coincidence with enhanced sputtering where a considerable Cu fraction sublimates from the surface. The dose-rate dependence of nanoparticles indicates that the surface-sputtering process influences the intra-solid process and contributes to the 2D-distribution. A self-assembling mechanism for 2D-arrangement of nanospheres is discussed taking into account contribution of the surface sputtering. INTRODUCTION Metal nanoparticles embedded in insulators exhibit nonlinear optical susceptibility and the fast response of pico-seconds with the surface plasmon resonance [1,21, and are demanded for photonic applications. We have focused on a nanoparticle system distributed within a thin thickness, ideally a two-dimensional (2D) configuration, which may provide an electronic and/or optical element of the integrated devices in future- To seek such a special nanophase,
we have employed negative-ion implantation of a relatively low energy, e.g., 60 keV[3-5]. Negative ions cause the same intra-solid processes as positive ions, but have a ment of accurate implantation by virtue of little surface charging on insulators[6]. In comparison with conventional methods, e.g., glass forming or molecular-beam epitaxy, advantages of the present method are the kinetic variety with ion energy deposition and the spatial controllability of implants. In a common insulating substrate of amorphous (a-)SiO 2, kinetic variations of nanoparticles have emerged in significant dose-rate dependence of optical absorption and depth-oriented migration of implants, e.g., bimodal distribution in depth [7,8]. Particularly, it has been occasionally found [9] that strong depth-oriented rearrangement of nanoparticles gives rise to a 2D-configuration of nanoparticles at a certain irradiation condition. As well as those ion-induced kinetics, a variety of the nanoparticle morphology may be contributed by the metastable natures of a-SiO2 , i.e., less densely packed structure, radiolysis, a variety of defects, ring structures[ 10] and radiation-induced viscous flow [11]. Since stochastic collision events of implants lead to a gaussian-like broad profile of implants, a normal precipitation process is unlikely to cause a 2D-distribution and there must be a centripetal force to a certain depth which causes self-as
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