The Structure and Evolution of Ge Nanoscale Structures on Si(111) -- Observations and Theory

We report scanning tunneling microscopy (STM) observations and first-principles calculations for the evolution of self-organized Ge nanostructures formed on Si(111)-$7{\times}7$ for Ge coverages up to 0.5~ML and operating temperatures from room temperature to 300$^{\circ}$C. STM measurements show that, depending on coverage and temperature, Ge atoms form various structures ranging from single-atom correlated patterns, 2-D ordered nanoscale domains, and 3-D disordered and ordered nanoclusters. First-principles theory focuses on the single-atom patterns and 2-D ordered nanostructures. We show that Ge atoms replace the Si adatoms in the initial adsorption stage. We also show that annealing of the Ge/Si(111) surface results in a partial transformation of ($7{\times}7$) reconstructed unit cells to unreconstructed Si(111) configurations on which the Ge adatoms reside at the $T_{4}$ sites and form a $({\sqrt{3}}{\times}{\sqrt{3}})R30^{\circ}$ reconstruction.