Dimer Reconstruction at Metal-Silicide/Silicon Interfaces: A First-Principles Study
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density functional theory (DFT). This theory takes the quantum mechanical properties and the chemistry of interatomic interactions properly into account. The stablest interface structure is determined by calculating the energy ordering of the total energy with full lattice relaxation. Our total energy calculations revealed a new structural model that is more stable than previously proposed models of interface structures and well explains experimental HREM images.
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CALCULATION METHOD All calculations were carried out using the DFT and norm-conserving, fully separable pseudopotentials [15] the generalized gradient within M (C) approximation (GGA). We used the exfunctionals of Fig. 1 Atomic models of the MSi 2/Si(001) (M = change-correlation Ni,Co) interface: (a) sixfold model; (b) eightfold Perdew et al. [16] for the GGA calculations. The MSi 2/Si(001) (M = model; (c) eightfold model with interfacial Si dimers (LGY model); (d) BJV model; (e) sevenfold model; Ni,Co) interface was simulated by a (f) sevenfold model with interfacial Si dimers. The repeating slab structure consisting of five large and small spheres represent metal M and Si Si layers, three MSi 2 layers and a 7.3-A atoms, respectively. vacuum region, and the periodicity in the lateral direction was set to be 2x 1. We used a theoretical GGA lattice constant a0 = 5.46 A of bulk Si for the Si slab. The bottom of the slab was terminated with H atoms in order to passivate the dangling bonds of Si. We used a plane-wave basis set with a kinetic energy cutoff of 60 Ry and a uniform mesh of 32 k points in the surface Brillouin zone (SBZ) of the 2x I unit cell for the k-space integration. The total energy and forces were calculated using the momentum space representation [17]. We relaxed all atoms in the slab except for the bottom-most Si and H atoms. The optimized geometry is identified by the requirement that the remaining forces acting on the atoms are smaller than 0.05 eV/A. Details of the computational technique are described in Refs. [18] and [19]. Table I Structural properties of fluorite MSi 2 (M = Ni,Co) obtained from our plane-wave pseudopotential (PP) calculations in the GGA. Experimental data are given for comparison. ao (A) Bo (Mbar) bulk modulus lattice constant 1.27 NiSi 2 PP-GGA 5.49 CoSi 2
Expt. a
5.41
PP-GGA
5.40
1.69
5.37 b Reference [211
1.72
Expt. a Reference [20]
b
104
To test the accuracy of the pseudopotentials, we calculated the ground-state properties of bulk fluorite nickel and cobalt disilicides, using 12 k points in the irreducible part of the bulk Brillouin zone. The calculated results are given in Table I, together with those of experiments. The calculated values are in good agreement with the experimental data. RESULTS AND DISCUSSIONS We first discuss the energetics and the stablest atomic structure of the NiSi 2/Si(001) and CoSi2 /Si(001) interfaces. Three straightforward interface models, with interfacial metal atoms sixfold, sevenfold and eightfold coordinated, can be constructed by combining bulk-t
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