Formation of Irregular Al Islands by Room-Temperature Deposition on NiAl(110)
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Formation of Irregular Al Islands by Room-Temperature Deposition on NiAl(110) Dapeng Jing1,6, Yong Han5, BariúhQDO2,6,*, J. W. Evans3,4,6 and P. A. Thiel1,2,6 Departments of 1Chemistry, 2Materials Science & Engineering, 3Mathematics, and 4 Physics & Astronomy, Iowa State University, Ames, Iowa 50011 5 Institute of Physical Research and Technology, Iowa State University, Ames, Iowa 50011 6 Ames Laboratory - USDOE, Iowa State University, Ames, Iowa 50011 ABSTRACT STM studies reveal that irregular non-equilibrium two-dimensional Al islands form during deposition of Al on NiAl(110) at 300 K. These structures reflect the multiple adsorption sites and diffusion paths available for Al adatoms on the binary alloy surface, as well as the details of inhibited edge diffusion and detachment-attachment kinetics of Al adatoms for numerous distinct step edge configurations. We attempt to capture these features by multi-site lattice-gas modeling incorporating DFT energetics for adatoms both at adsorption sites and transition states. This formulation enables description and elucidation of the observed island growth shapes. INTRODUCTION A rich variety of complex far-from-equilibrium morphologies are known to form in homoepitaxial growth of single-component metal films (A on A) at lower deposition temperatures, T. This behavior occurs despite the feature that these systems have very simple equilibrium states [1,2]. The complex morphologies result from kinetic limitations on step edge diffusion or interlayer diffusion on the time-scale of film growth. Similar behavior is observed in heteroepitaxy (A on B) at least for lower lattice misfit, but distinct strain-induced features generally occur for higher misfit [3]. Rather than the single-component substrates employed in the above studies, our focus here is in the use of binary alloy substrates where a variety of farfrom-equilibrium structures should also form under low-T deposition. We are motivated by the potential for these systems to provide new opportunities to guide the creation of thin film nanostructures with desired properties. Interestingly, despite this potential, well-controlled ultrahigh-vacuum (UHV) analyses of film growth on binary alloys (A or B on BC) are rare [4-7]. In this paper, we consider submonolayer deposition of Al on NiAl(110) at low T = 300 K, corresponding to far-from-equilibrium growth with effectively frozen substrate dynamics. However, it is appropriate to comment on a previous study of Al deposition on NiAl(110) at high T | 900 K, where the NiAl substrate is actually quite dynamic or “reactive” [4]. In the initial stages of Al deposition at around 900 K, half the deposited Al replaces excess Ni residing at Al sites in the bulk. This displaced Ni diffuses to the surface and combines with the other half of the depositing Al to form a new stoichiometric NiAl(110) alloy adlayer [4]. Once the excess Ni concentration in the substrate is sufficiently reduced, the system turns to another mechanism involving bulk defects to bring Ni to the surface to perpetuate al
