Dislocation Nucleation and Propagation During Deposition of Cubic Metal Thin Films
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Dislocation Nucleation and Propagation During Deposition of Cubic Metal Thin Films W. C. Liu, Y. X. Wang, C. H. Woo, and Hanchen Huang @ Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong
ABSTRACT In this paper we present three-dimensional molecular dynamics simulations of dislocation nucleation and propagation during thin film deposition. Aiming to identify mechanisms of dislocation nucleation in polycrystalline thin films, we choose the film material to be the same as the substrate – which is stressed. Tungsten and aluminum are taken as representatives of BCC and FCC metals, respectively, in the molecular dynamics simulations. Our studies show that both glissile and sessile dislocations are nucleated during the deposition, and surface steps are preferential nucleation sites of dislocations. Further, the results indicate that dislocations nucleated on slip systems with large Schmid factors more likely survive and propagate into the film. When a glissile dislocation is nucleated, it propagates much faster horizontally than vertically into the film. The mechanisms and criteria of dislocation nucleation are essential to the implementation of the atomistic simulator ADEPT. INTRODUCTION Thin films, in particular polycrystalline thin films, are most ubiquitous to modern engineering, because of their important applications and their complexity. The performance of thin films is directly affected or controlled by their microstructures. Starting from the BCF theory in 1950’s [1] to the atomistic simulator [2] developed nowadays, the microstructure evolution of thin films has always been a focus of intensive investigations. The atomistic simulator ADEPT [2], which has been extended to simulate competition of two textures [3] and that of multiple textures [4], provides a tool to study the texture evolution during thin film deposition at the atomic level. Recent development and applications of this simulation are discussed in details in references [5-7]. So far, all the simulations using ADEPT have ignored direct contributions of stress to the microstructure development. Under stress, a thin film tends to generate dislocations so as to relax the stress and minimize the energy. The analytical formulation of Frank and van der Merwe [8], based on a balance of elastic energy due to stress and energy associated with a dislocation. In this theory, a simple model of one-dimensional springs on a periodically modulated substrate was used. A dislocation will be generated – nucleated and propagated to the film-substrate interface – if the dislocation’s presence leads to the reduction of the total energy. However, the theory tells nothing about how a dislocation nucleates and how it propagates. The mechanisms of dislocation nucleation and propagation would become clear if details of atomic trajectories are known during the deposition of thin films. Molecular dynamics simulations are idea for this purpose, except that the maximum physical time accessible is only nano-seconds. This limitation is also part
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