Mechanisms of Cu<111> Columns Growth
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Mechanisms of Cu Columns Growth Jian Wang and Hanchen Huang* Department of Mechanical, Aerospace and Nuclear Engineering Rensselaer Polytechnic Institute, Troy, NY 12180 ABSTRACT The Cu columns, which are formed during magnetron sputtering deposition, are faceted on the top and zigzag on sides. Our numerical results of large facet-facet diffusion barriers offer an explanation of the facet dimension. Based on the stacking fault formation energies of various face-centered-cubic metals, we suggest that the zigzag shape of Cu columns is a result of deposition twins. Our molecular dynamics simulations indeed confirm this suggestion. Further, the dynamics simulations reveal the transient role of {100} facets, which facilitate the formation of {111} facets and disappear afterwards. 1. INTRODUCTION Columns, which are undesirable in most thin film deposition processes, are favorable in the emerging nanotechnology. At the nanoscale, these columns may develop into nanosprings or nanopillars. Shadowing effects are responsible, or they are considered responsible, for the nanosprings and nanopillars growth [1, 2, 3]. This correlation may be correct if these structures are amorphous. On the other hand, crystal structures do introduce additional, and likely dominant, features. Our recent studies show that the Cu columns are faceted, and that they are zigzag in shape; as shown in Figure 1. Details of the deposition conditions are available in reference [4]. Three features are worth mentioning. First, the columns are of type; that is, a orientation points upward. In Cu or generally face-centered-cubic (fcc) metals, grains point upward in dense films. In porous form, the columns are usually of type. Second, the Cu columns are faceted, in contrast to rounded Figure 1: Scanning electron surfaces. The facets are on the order of 200 nm in linear microscopy image of Cu dimension, as a result of the large three-dimensional Ehrlich-Schwoebel barriers [5, 6, 7]. Third, the sides of each columns, showing the faceted column are in zigzag shape, in contrast to conventional cone surfaces and zigzag sides. shape. As a result of the zigzag feature, the diameter of the columns does not increase as they grow taller. In this paper, we interpret two of the features – faceted surface and zigzag sides, and reveal the mechanisms that are responsible for the nanostructure evolution, based on molecular statics and dynamics simulations. In Section 2, we propose a scenario of zigzag formation based on experimental data of stacking formation energies. Then, we briefly describe the molecular dynamics (MD) simulation methods and elaborate the results in Section 3, to confirm the *
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suggested scenario and to reveal the mechanisms that are responsible for the small dimension of facets and the zigzag shape. Finally, we conclude in Section 4. 2. SCENARIO OF ZIGZAG SHAPE FORMATION Since facets are the common result of surface energy minimization, our starting assumption is that facets do form. Their d
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