The formation of nano-structured surface alloys Project in collaboration with Miroslav Kotrla, Academy of Sciences, Prague.

In hetero-epitaxial growth of metals, The formation of surface alloys is of particular interest. Frequently, adsorbate and substrate would intermix and form a thin film of alloy. Another interesting phenomenon is observed in ternary systems, where two metals, say "A" and "B" are co-deposited on a suitable substrate "S". This can result in the formation of a two-dimensional A-B alloy, even if these materials are bulk-immiscible.
Furthermore, non-trivial nanoscale structures of alternating domains of A and B atoms can be found in some systems. Alloy islands form in a dendritic shape, and a substructure in the shape of stripes or veins is observed. The formation of stripes with a width of nanometers has been observed in a variety of AB/S systems, including CoAg/Ru(0001), CoAg/Mo(110), FeAg/Mo(11), or PdAu/Ru(0001) [see our publications for references]. Potential technical applications could arise in the future development of novel magnetic storage devices.
The key property of the above material combinations is that the bulk lattice constant of A (B) is smaller (larger) than that of the substrate S, respectively.

Two mechanisms have been suggested to be responsible for the strip formation:
· Strain relaxation
As the misfit of A/B particles is positive/negative with respect to the substrate, it is favorable to achieve a low effective misfit by alternate arrangement of the materials.
· Kinetic seggregation It is plausible to assume that the inter-species binding A-B is weaker than that of A-A and B-B. Hence, the system would seperate the elements with an A-B boundary as short as possible in thermal equilibrium. The different binding energies should also have a strong effect under growth conditions and trigger at least a partial separation of A and B.

We have studied the influence of these effects on the growth of an A/B adsorbate in the first atomic layer. In the framework of a ternary model system with pair-wise interactions that favor a cubic symmetry, we haver performed off-lattice KMC simulations. In addition, we have performed equilibrium simulations of a competely filled monolayer.
Our results clearly show, that both effects are required to fully understand the qualitative features observed in experiment. In addition, this example demonstrates how long range elastic effects emerge from particle-particle interactions and determine the surface configuration. Their influence on the growth dynamics cannot be modelled in a lattice gas description in a straightforward fashion.

(example island for misfit +4% and -4%)

We have extended our studies to the investigation of an fcc lattice structure (S. Weber, Ph.D. project, Würzburg).

The following (selected) publications deal with the self-organized formation of islands in Stranski-Krastanov growth:

T. Volkmann, F. Much, M. Biehl, M. Kotrla
Interplay of strain relaxation and chemically induced diffusion barriers: nanostructure formation in 2D alloys
Surface Science 586 (2005) 157-173
preprint version available at the cond-mat archive: (direct link to the paper)

M. Kotrla, F. Much, T. Volkmann, M. Biehl
Mechanism of formation of self-assembled nanostructures in heteroepitaxy
in: Proc. of Int. Conference NANO'03, Brno/Cz 2003, ed. P. Sandera, FSI VUT Brno, 98-103

M. Biehl, F. Much, and C. Vey

Off-lattice Kinetic Monte Carlo simulations of strained heteroepitaxial growth
preprint version of an invited contribution to an MFO Mini-Workshop (Oberwolfach, 2004),
in: Multiscale Modeling in Epitaxial Growth, ed. A. Voigt, Int. Series of Numerical Mathematics 149 (Birkhaeuser, 2005), 41-57

S. Weber, M. Biehl, M. Kotrla, W. Kinzel
Simulation of self-assembled nanopatterns in strained 2D alloys on the fcc(111) surface
J. Phys.: Cond. Matter 20: 265004 (2008)
preprint version: (PDF)