Theoretical Study of Bonding for Au Adatoms on Ge(110) and its Implications for the Observed Motion of Au-Ge Alloy Ad-Clusters

Using DFT, we explore the microscopic physical origins of experimentally observed motion of Au-Ge alloy droplets on Ge(110). Our earlier studies showed no bonds forming between Au adatoms or between an Au adatom and its nearest neighbor (nn) Ge atom, while a strong covalent bond formed between a Ge adatom and its nn Ge. We use 4 model clusters with 2 to 5 Au adatoms alloying with 1 Ge adatom on a substrate model defined by [001] a axis, [1,-1,0] b axis, and [110] c axis directions of the fcc structure. By using 3 schemes for displacements of ad-clusters at the top plane of the substrate along a, a-b, and b directions, we did 72 sets of calculations. When the displacement is <3.50% of the Ge bond length, the b-direction is preferred. When the displacements increased to 6.88%, the Ge adatom changes its nn as a result of bond-breaking and reforming. The preferred direction for motion changes from b-axis to a-b direction. The surface structure plays a role in the preferred moving direction of clusters, regardless of cluster size and whether or not Au and Ge adatoms move together or separately. The direction change is due to the bond breaking of the Ge adatom and the surface Ge atom.