Molecular-Dynamics Simulation of the Initial Period of Cluster Deposition
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Molecular-Dynamics Simulation of the Initial Period of Cluster Deposition K. Shintani, T. Nakajima, and Y. Taniguchi Dept of ME & Intelligent Sys, Univ of Electro-Comm, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan E-mail: [email protected], URL: http://www.shintani.mce.uec.ac.jp/ ABSTRACT The initial periods of deposition process of metal clusters in the soft-landing regime are investigated by the molecular-dynamics simulation. The embedded-atom method potential is adopted for calculation of the interaction between metallic atoms. The predictor-corrector method for second-order differential equations is employed for integration of the equations of motion. A simulation begins with equilibration of clusters and a substrate at a specified temperature. The lowest atomic layer in the substrate is fixed and the next few atomic layers are set to be velocity-scaling layers during the deposition process. The periodic boundary conditions are imposed in the horizontal directions. A single cluster with no velocity is deposited on the substrate. The simulations are performed at different temperatures of the clusters and substrate and for different sizes of clusters. How the morphological transition of the deposited nanostructures is affected by these parameters is discussed. INTRODUCTION Cluster deposition is a promising way to grow thin film structures and to fabricate nanostructures [1]. This deposition method was developed in order to enhance the controllability and efficiency in the deposition process of molecular beam epitaxy [2]. Although cluster formation could not be verified in the first attempt with the ionized cluster beam deposition, the successors [3, 4] confirmed its formation and succeeded in fabricating thin films of smooth surfaces. Cluster deposition is classified into the three categories in terms of cluster energy: soft-landing, LECBD (low-energy cluster beam deposition), and ECI (energetic cluster impact). The first and second categories are applicable to thin film growth and fabrication of nanostructures. The third category is suited to thin film growth. Recent researches have been directed towards keeping and utilizing the properties of free neutral clusters after their soft-landing on the substrate [5-12]. The morphology of deposited nanostructures depends on the temperature, velocity, mean-size, density, crystallinity, and morphology of clusters. If these parameters can be controlled, it is probable nanostructures of required shapes will be deposited at the desired positions on the substrates. In this study, the classical MD (molecular-dynamics) simulation is performed to know the dependence of the morphology of the deposited clusters on the temperatures of the free clusters and substrate and on the morphology of the free clusters. The deposition is assumed to occur in the soft-landing mode. Prior to deposition simulation, the two-parameter space of the size and temperature of a cluster is divided into the two regimes: in one regime, a free cluster takes a crystalline structure while, in
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