Amorphous Alloy Formation in Immiscible Cu-Ta and Cu-W Systems by Atomistic Modeling and Ion-Beam Mixing
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Amorphous Alloy Formation in Immiscible Cu-Ta and Cu-W Systems by Atomistic Modeling and Ion-Beam Mixing H.R. Gong, L.T. Kong, and B.X. Liu Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, CHINA ABSTRACT For the immiscible Cu-Ta and Cu-W systems, realistic n-body potentials are derived under an embedded-atom method through fitting the cross potentials to some physical properties obtained from ab initio calculations for a few possible metastable Cu-Ta and Cu-W crystalline phases, respectively. Based on the derived potentials, molecular dynamics simulations reveal that in the Cu-Ta system, 30 at. % of Ta in Cu is the critical composition for the crystal-to-amorphous transition in the Cu-rich Cu-Ta solid solutions, and that in the Cu-W system, amorphous alloys can be formed within the composition range of 20-65 at. % of W. Interestingly, amorphous alloys are indeed obtained by ion-beam mixing in properly designed Cu70Ta30, Cu65Ta35, Cu60Ta40, and Cu50Ta50 multilayered films, while crystalline Cu and Ta remain in Cu75Ta25 multilayered sample, which matches well with the critical composition of 30 at. % of Ta predicted by simulation. Moreover, there have been experimental data, which are in support of the predicted composition range of the Cu-W system by simulations.
INTRODUCTION In 1960, Duwez et al. obtained the first amorphous alloy by liquid melt quenching (LMQ) technique in the Au-Si system [1]. Since early 1980s, two other schemes, i.e., ion-beam mixing (IBM) and solid-state reaction (SSR), have been introduced to produce amorphous alloys and a great number of new amorphous alloys have been obtained in the binary metal systems, even in the equilibrium immiscible systems characterized by a positive heat of formation [2-4]. Moreover, molecular dynamics (MD) simulation has been proven to be a powerful means to monitor the process of amorphization at an atomistic level and ion-beam mixing (IBM) has been regarded as a powerful means to form amorphous alloys in the binary metallic systems, as its effective cooling speed was estimated, based on a thermal spike model, to be as high as 1013-14 K/sec [4]. In the present study, MD simulation and IBM experiment are combined to study amorphous alloy formation in the immiscible binary systems with an emphasis on finding out the critical composition for the crystalline-to-amorphous transition. Among the immiscible copper-refractory metal systems, the Cu-Ta and Cu-W systems have the heats of formation of + 3 and +33 kJ/mol [5], respectively, and are selected as model systems in the present study, partly due to the potential application of Ta or W as a diffusion barrier between Cu and Si/SiO2 in very large scale integration (VLSI) devices. Accordingly, we first construct n-body Cu-Ta and Cu-W
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potentials under an embedded-atom method (EAM) by fitting the cross potentials to some physical properties, which are obtained from first-principles calculations. Applying the constructed potentials, MD simulations are then performed
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