Formation of metastable two-dimensional structures during Ag growth on Ge(100)

Abstract

Silver growth on Ge(100) between 330 and 570 K was studied using scanning tunneling microscopy. At 330 K, three-dimensional (3D) clusters along with two types of two-dimensional (2D) islands were observed: one elongated perpendicular to the substrate dimer rows, the other parallel to the dimer rows. The spacing and registry with the substrate of the former could be explained by Ag adatoms attaching across the substrate dimer bond, while the latter could be attributed to Ag ad-dimers. Annealing caused the larger 3D clusters to grow at the expense of the smaller 3D clusters and 2D islands; at 470 K the 2D islands were completely eliminated. Low-energy electron diffraction indicated that the 3D clusters were oriented with their (110) planes parallel to the surface. After annealing, the 3D clusters became elongated parallel to the Ge dimer rows indicating that the $\mathrm{Ag}\left[1¯10\right]$ direction aligns parallel to the Ge 1× direction. Although this suggested that the 3D clusters were lower in energy, Ag growth at 470 K led to two types of 2D islands along with pitting of the Ge surface; the pitting was taken as an indication that the islands were intermixed. Images of the islands revealed $2\times 3$ and $3\times 2$ periodicities. Images of the $3\times 2$ structure showed a single maximum per unit cell, while images of domain boundaries showed that the $3\times 2$ structure changed the outermost Ge layer. A model based on Ag-Ge mixed ad-dimers and Ag substitution into the Ge surface can explain these features. The $2\times 3$ and $3\times 2$ structures could only be seen after Ag deposition at ∼470 K; both annealing films deposited at lower temperatures and higher growth temperatures created only 3D clusters. Thus formation of these intermixed structures was attributed to growth of metastable Ag-Ge nuclei at 470 K that were kinetically inhibited from forming at lower temperatures and whose lifetimes were too short to grow at higher temperatures.