Deposition energy dependence in cluster-assembled thin film densities
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Deposition energy dependence in cluster-assembled thin film densities K. Meinander, T. Clauß, and K. Nordlund Accelerator Laboratory, P.O. Box 43, FI-00014 University of Helsinki, Finland ABSTRACT Mechanical properties of thin films grown by nanocluster deposition are highly dependent on the energy at which the clusters are deposited. Using molecular dynamics computer simulations we have quantitatively studied variations in the properties of copper thin films grown by deposition of Cu nanoclusters, at energies ranging from 5 meV to 10 eV per cluster atom, on a Cu (100) substrate. INTRODUCTION Nanocluster deposition for the purpose of thin film growth has yielded promising results during the past few decades [1, 2]. High-energy deposition, through techniques such as energetic cluster beams (ECB), results in films with superior mechanical qualities and extremely good adhesion [3]. Clusters deposited at low energies, on the other hand, will retain memory of their original shape and size, resulting in the formation of nanocrystalline structures with exotic properties similar to those of free clusters [4]. Nanocrystalline thin films grown by low-energy deposition unfortunately have a tendency towards being very under-dense, and hence exhibit poor mechanical properties. An increase in the deposition energy will lead to a gradual improvement of these properties, mainly due to a higher packing density between the atoms from individual clusters. As deposition energies are further increased, individual clusters will melt and fuse into larger grains, eventually resulting in completely epitaxial films at sufficiently high energies. Deposition of nanoclusters at some intermediate energy could therefore allow for the growth of thin films where both nanocrystallinity is retained and sufficiently good mechanical properties are exhibited. In this study we have examined the effect of deposition energy on the final properties of cluster-assembled thin films. Our main focus has been on the variation of film densities with varying deposition energy. SIMULATION METHOD Classical molecular dynamics (MD) simulations were used to simulate the growth of copper thin films through deposition of nanoclusters at various energies. Each copper film was grown by sequential deposition of 50 Cu nanoclusters, containing 586 atoms each, on an initially smooth (100) Cu substrate at 300 K. Films were grown at deposition energies ranging from 5 meV per atom in the clusters up to 10 eV per cluster atom. The Cu atom interaction was described with the use of Foiles embedded-atom method (EAM) potential [5]. The Berendsen temperature control algorithm [6] with a time constant of 100 fs was used to stabilize the temperature at the bottom and borders of the Cu substrate, which had
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periodic boundaries and a side length of 23 unit cells. In order to realistically mimic thin film growth, the clusters were deposited with random orientations at random impact points. The random impact points were chosen by translating the substrate a r
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