Temperature-dependence of Ni+Al co-deposition on NiAl(110): Atomistic-level modeling of deviations from perfect alloy ordering

Stoichiometric co-deposition of Ni and Al on NiAl(110) for high enough temperatures (below the order-disorder transition for NiAl) must produce near-perfect alloy islands and overlayers. However, at 300K, island structure is far from perfectly ordered and depends strongly on the deposition protocol (e.g., simultaneous vs. sequential). Realistic atomistic-level modeling of this non-equilibrium behavior must provide an accurate description of not just alloy thermodynamics (through adatom adsorption and interaction energies), but also of diffusion kinetics (for adatom attachment-detachment at and transport along island edges). This is achieved by multi-site lattice gas modeling with DFT input for adsorption and interaction energies for adatoms both at adsorption sites and at transition states for hopping [T. Duguet, Y. Han et al., Proc. Nat. Acad. Sci. 107 (2010) Special Issue on Surface Chemistry; Y. Han et al., submitted (2010)]. Model analysis by KMC simulation shows a transition from poor alloy order at 300K to almost perfect order at 600K.