Nanocrystal Growth at High Dose Rates in Negative Copper-Ion Implantation Into Insulators
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ABSTRACT Nanoparticles of Cu were fabricated by negative-ion implantation, leading to spontaneous formation at high beam fluxes. Negative ions, alleviating surface charging, exhibit significant merits in carrying out low-energy implantation at high dose rates. The kinetic processes were studied by measuring dose-rate dependence of colloid formation and resultant optical properties. Negative-Cu ions of 60 keV were implanted into silica glasses at high-current densities, up to 260 pA/cm2 , fixing the total dose at 3.0 x 1016 ions/cm2 . Spherical nanocrystals of Cu atoms formed within a narrow region, near the projectile range of Cu ions. Simultaneously, much smaller particles spread out beyond a depleted zone, deeper than the projectile range. The nanocrystal growth and optical properties were greatly dependent on the dose rate and the specimen boundary condition. The growth process is explained by a droplet-model based on surface tension and radiation-induced diffusion. Beam-surface interactions also play an important role in the mass transport from the beam flux to the interior solid. INTRODUCTION Nanoparticles of a metal, embedded in dielectrics, have recently attracted much attention to develop novel electronic and optical properties. The metal-colloid systems exhibit nonlinear [1] and fast optical response [2], which may become important for optoelectronics. Various fabrication methods, from conventional thermal precipitation to state-ofthe-art atom manipulation, have so far been applied for colloid formation. The fabrication methods now expand their variety, not only to improve structural control for various materials, but also to form colloids of compounds, etc. Our efforts have been directed to a colloid system distributed within a thin thickness, i.e., 2-dimension-like distribution. The 2Dconfiguration may become important for both electronic and optical devices in future. To attain such a special distribution, we have employed ion implantation of a relatively low energy, 60 keV. When implanting at such a low energy into insulators, positive ions are subjected to surface charging via secondary-electron emission, and the specimen will be charged up to the acceleration voltage or the breakdown voltage [3], resulting in inaccurate implantation or material damage. Consequently, we have employed negative Cu ions of the low energy. One of the important aspects for the colloid performance is compatibility between atomic-scale control and reduction of internal and interfacial defects. Although negative ions may cause intra-solid processes identical to positive ions, high-current phenomena in themselves are not well understood. Dose-rate dependence of plasmon absorption was observed even at the higher energy, 160 keV of Cu [4], where the absorption greatly increased with current density, up to 7.5 VA/cm 2. Previously, we observed in 60 keV-Cu- implantation2 that optical absorption was unusually dependent on the current density, up to 260 itA/cm [5,6]. The colloid formation processes have both features of high-tempera
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