Molecular Dynamics Studies of Thin-Films of Sn On Cu
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energy electron diffraction (LEED) studies of thin films of Sn on Cu(100) have found evidence of formation of a commensurate two-dimensional phase with a structure strongly-dependent on anneal temperature and Sn concentration [4]. An earlier study of deposition of Sn on Cu(100) reported no evidence of alloy formation for depositions up to one monolayer (ML) of Sn [5]. Thus, although the study of thin overlayers on Sn on Cu surfaces has helped understand the kinetics and early stages of Cu-Sn alloys formation, there remain many questions in regard to the mobility of Cu and Sn atoms at the interface. For instance, one should expect that for coverages of less than 1 ML, the diffusion of Cu atoms onto the Sn overlayer should be less than the dilute alloy value due to the large reduction in the number of interstitial sites available to the Cu atoms. We present here preliminary results of Molecular Dynamics (MD) simulations of Sn overlayers on Cu(100) and Cu( 111) surfaces. We looked at Sn coverages of up to 2 ML over a wide temperature range: 500-1300 K. Our initial aim was to investigate the kinetics of Sn-Cu alloy formation and to see whether coverages below 1 ML modify the diffusion of Cu atoms into the overlayer via the same interstitial mechanism which occurs in bulk. The atomic interactions are described by modified embedded atom method (MEAD)potentials [6]. The MEAM formalism is a modification of the well-known embedded atom method (EAM) and includes the angular dependence of the electron density in order to describe bond bending forces necessary to model covalent materials. It has been applied successfully to fcc, bcc, hcp [7] and diamond cubic materials and has been 43 Mat. Res. Soc. Symp. Proc. Vol. 492 ©1998 Materials Research Society
shown to reproduce quite well the mechanical and thermodynamic properties of the diamond a, f3 and liquid phases of Sn [8]. Using these potentials we calculated the diffusion activation energies of Sn in Cu and Cu in Sn and made some modifications to the previously published parameters to obtain better agreement with experiment. We will discuss these changes and will present our results on the structure and dynamics of the Sn overlayer with temperature and the correlations between Cu and Sn atoms at the interface. METHODOLOGY AND POTENTIALS We utilized slabs of 1800-2000 Cu atoms with 1 and 2 ML of Sn periodic in the plane of the surface (XY). The Z-direction is defined as normal to the interface. The Cu side of the slab consisted of 9 layers of which the bottom 3 were kept rigid in order to simulate a semi-infinite slab. The starting configurations were created by placing 3 layers of Sn in a simple cubic crystal geometry onto the (111) and (100) Cu slabs. These configurations were then annealed at 400 K for 10 ps in order to obtain equilibrium densities at the interface. From these relaxed slabs, the 1 and 2 ML Sn samples were obtained by removing 2 and 1 layers respectively from the annealed configurations. The resulting slabs were annealed further at 400 and 500 K for 5 ps.
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