MEAM molecular dynamics study of a gold thin film on a silicon substrate

Abstract

Modified embedded atom method (MEAM) models have been used in conjunction with molecular dynamics simulations to study the interface between a silicon substrate and a thin overlayer of gold. Due to the absence of stable compound formation in the Au–Si phase diagram, we computed ground state properties for a representative Au–Si structure, namely the B1(NaCl) structure within the local density approximation (LDA), and fitted MEAM parameters to this data. The MEAM potentials were tested for their ability to reproduce LDA calculations such as the predictions of properties for Au/Si in the L1 2 structure, the binding energies and bond lengths for two isolated clusters, the energy barrier for an Au atom migrating in the (1 1 0) channel of c-Si, and some fundamental physical properties for elemental Au. All errors are within 10%. The molecular dynamics simulations focussed mainly on investigating three critical issues where experimental data conflict: (1) the critical coverage of Au atoms needed to induce intermixing at the interface, (2) the composition of the topmost layer and the surface region under different Au coverages, and (3) the influence of temperature on the chemical composition of the Au–Si interface. NVT molecular dynamics simulations at room temperature showed that the formation of silicides at the Au–Si interface is very fast and can be formed at very low coverage (0.11 monolayer). We observed a structural transition from disordered silicides at low coverage to a crystalline Au top layer as the number of monolayers increased. The thickness of the silicide layer formed at the interface initially increased rapidly but reached a constant value after a certain amount of Au atoms were deposited. The interface between an Au thin film and an Si substrate consisted of two alloyed regions separated by an 80-at.% Au-rich layer. In contrast, the interface between an Au island and an Si substrate involved a 50–50 alloy, formed near the top layer of the Si substrate. Annealing at 500 K altered the distribution of Au and Si atoms in the alloyed region, even though the average composition of the intermixed layer remained the same as that before annealing. Between 500 and 800 K, we observed rapid growth of a silicide layer and a transition from an Au-rich to an Si-rich surface region due to a dramatic increase of Si outdiffusion. Annealing to 1000 K did not alter the boundary between the Au silicide and the Si substrate, implying that Au indiffusion had diminished and the growth of Au silicides was dominated by the outdiffusion of Si atoms within this temperature range.