Density functional study of copper segregation in aluminum

Journal of Electroanalytical Chemistry, 2002

Using the embedded-atom method, the structure of small copper clusters on Au(111) electrodes has been investigated both by static and dynamic calculations. By varying the size of roughly circular clusters, the edge energy per atom is obtained; it agrees quite well with estimates based on experimental results. Small three-dimensional clusters tend to have the shape of a pyramid, whose sides are oriented in the directions of small surface energy. The presence of a cluster is found to distort the underlying lattice of adsorbed copper atoms.

Surface Sci, 2005

A comparative study of the interaction between single copper, silver and gold atoms and the regular Al-terminated a-Al 2 O 3 (0 0 0 1) surface is reported. The surface was simulated by means of periodic slabs and the calculations were undertaken using the spin-polarized density functional theory within the generalized gradient approach. For metal adsorption, five different sites were considered, two on top of Al and O atoms, and three on hollow positions in which the transition metal binds three surface O atoms. Metal-surface interaction is relatively weak with adsorption energies in the range 0.7-1.1 eV, following the order Cu > Au > Ag. The preferred site for Cu and Ag are the three fold hollow oxygen sites while Au atoms prefer to bind the surface on top of the oxygen atoms. Examination of the electron density plots shows that some charge transfer toward the surface takes place according to the sequence Cu > Ag > Au. An analysis of the bond mechanism based on model cluster calculations indicates that for Cu and Ag the largest contribution to the interaction energy arise from the charge transfer from the adsorbed atom to the surface while in the case of Au, the interaction is dominated by the polarization of the transition metal.

Applied Surface Science, 2010

The surface relaxation and surface energy of both the mixed AuCu and pure Cu terminated Cu 3 Au (0 0 1) surfaces are simulated and calculated by using the modified analytical embedded-atom method. We find that the mixed AuCu termination is energetically preferred over the pure Cu termination thereby the mono-vacancy diffusion is also investigated in the topmost few layers of the mixed AuCu terminated Cu 3 Au (0 0 1) surface. In the mixed AuCu terminated surface the relaxed Au atoms are raised above Cu atoms for 0.13 Å in the topmost layer. All the surface atoms displace outwards, this effect occurs in the first three layers and changes the first two inter-layer spacing. For mono-vacancy migration in the first layer, the migration energies of Au and Cu mono-vacancy via two-type in-plane displace: the nearest neighbor jump (NNJ) and the second nearest neighbor jump (2NNJ), are calculated and the results show that the NNJ requires a much lower energy than 2NNJ. For the evolution of the energy requirements for successive nearest neighbor jumps (SNNJ) along three different paths: circularity, zigzag and beeline, we find that the circularity path is preferred over the other two paths due to its minimum energy barriers and final energies. In the second layer, the NN jumps in intra-and inter-layer of the Cu mono-vacancy are investigated. The calculated energy barriers and final energies show that the vacancy prefer jump up to a proximate Cu site. This replacement between the Cu vacancy in the second layer and Cu atom in the first layer is remunerative for the Au atoms enrichment in the topmost layer.

Surface Science, 2003

Using an effective potential model in analogy to the tight-binding scheme in the second-moment approximation, we investigated the transport processes and the structural properties of the Cu 3 Au and Ni 3 Al(1 1 1) surfaces by molecular dynamics simulations. We found that in the case of the Cu 3 Au(1 1 1) surface both Cu and Au adatoms are unstable and penetrating into the bulk (already at room temperature) induce local surface disorder that is propagating into the substrate. It is interesting to note that the interlayer diffusion takes place exclusively via Cu atoms and therefore, only Au adatoms stimulate local disorder through this process. Unlike, in the Ni 3 Al(1 1 1) surface the adatoms are stable, they diffuse via a multitude of hopping mechanisms, the Ni being more active than Al adatom. Below T s ¼ 900 K exchange mechanisms are lacking for both adatoms, while above this temperature exchange diffusion of the Al adatom with Ni surface atoms is present inducing local disorder in the surface layer. Interestingly, no exchange events were detected for the Ni adatom. Consequently, below T s the presence of adatoms on this surface does not affect the surface order, in agreement with the available experimental observations referring to Ni 3 Al. In addition, contrary to the behaviour found in the cases of the (0 0 1) and (1 1 0) surfaces, the presence of surface vacancies has no serious effect on the surface order in either of the alloys. This is essentially attributed to the structural difference of this surface, the second layer (Au or Al) atoms appearing to be an effective restraining element of any surface vacancy hopping mechanism.

Surface Science, 2008

Results of a theoretical study of collision processes are reported. The relation between the impact energy for the deposition of copper clusters (with N = 13, 18, 38, and 55 atoms) on the Cu(1 1 1) surface and the structural and energetic stability of the products is investigated by means of molecular dynamics simulations. The interatomic interactions are described with a potential from the embedded atom method (EAM) family. The roles of the impact energy and cluster size are studied. It is shown that larger clusters change their structure less than smaller clusters, whereas the smaller (magic) Cu 13 and (non-magic) Cu 18 clusters lose rapidly their similarity to the original clusters for not too small impact energies. Moreover, for an impact energy of 0.5 eV/atom the structure of these clusters shows the lowest similarity to the original structures. In this case, the Cu 18 cluster forms a monolayer on the surface, with one atom of the surface substituted by an atom from the cluster, while the Cu 13 icosahedron forms a slightly deformed monolayer. Only at this impact energy monolayers can be formed. Instead, increasing the impact energy leads to a symmetrical pyramidal product for Cu 13 and to a double-layered cluster for Cu 18 . On the other hand, even at an impact energy of 0.9 eV/atom the final products of the larger fcc Cu 38 and the icosahedral Cu 55 clusters contain two, and three layers, respectively, on the surface.

Physical Review B, 1995

We report on experimental and calculated occupied and unoccupied electronic distributions of crystalline A12Cu and various Al-Cu-Fe alloys. The experiments have been carried out by means of soft-x-rayspectroscopy and photoelectron-spectroscopy techniques. The densities of states have been calculated using either the linear-muffin-tin-orbital atomic-sphere-approximation or the augmented-plane-wave methods. The comparison between experiment and calculations is discussed. We pay special attention to the effect of Fe in the various alloys and emphasize the role of Al atom neighbors. We propose that changes in Al site occupancy that induce changes in Al p-d hybridization near the Fermi level could be responsible for the modifications of the densities of states observed in quasicrystalline Al-Cu-Fe phases with respect to the crystalline Al-Cu2Fe alloy.

MRS Proceedings, 1998

ABSTRACTPreparation of Cu-0.3 at.% Al alloy films by homogenization anneals of metallic bilayers consisting of Cu/Cu-AI on SiO2 is studied to produce advanced copper based metallization for future ICs. The bilayer films were annealed at temperature in the range of 200°C- 400°C and the depth profiles were obtained using Rutherford Backscattering technique. It is found that Al diffuses through the undoped copper layer to surface - leading to lower Al concentration in the original Al doped Cu layer and apparently increased concentration of Al in undoped Cu (not detectable by RBS). The resistivity of these annealed films ranged from 2.1 to 2.4 μΩ-cm depending on final Al concentration achieved. Electrical stability of such MOS capacitors was also investigated. All these results will be presented and discussed showing that Al-doped copper layers can be used as diffusion barrier/adhesion promoter between the oxide ILD and copper metal.

Surface and Interface …, 2003

Solid State Communications, 1991

A new method, involving a series of coupled electronic structure calculations and free energy minimizations, has been developed to determine surface compositions in binary alloys. Layer-and concentration-dependent point and pair interactions are calculated selfconsistently by means of a configurational averaging procedure within the tight-binding recursion method and the Bragg-Williams approximation. Results for the segregation profile of TicRhl-c above the bulk order-disorder temperature are discussed and found to be m good agreement with experiment.