Scanning tunneling microscopy study of Au growth on Ge(0 0 1): Bulk migration, self-organization, and clustering

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Gold growth on Ge(0 0 1) was studied as a function of coverage and growth temperature using scanning tunneling microscopy (STM). Initial Au deposition at 475 K resulted only in two-dimensional Ge islands and numerous vacancies on the surface. The Ge islands were attributed to Ge atoms ejected onto the surface by Au moving sub-surface. At higher temperatures the vacancies ordered into (1 + 2 + 1) dimer vacancy complexes, and step structures not seen on clean Ge surfaces were observed. When more Au was deposited between 475 and 800 K, alternating bright and dim chains appeared on the surface that were attributed to Au–Au and Au–Ge dimer rows, respectively. An increase in the density of double-height steps was observed as the chains formed suggesting that the Ge atoms incorporated into the mixed dimer rows were extracted from step edges. Increasing the Au coverage further caused the chains to organize into well-ordered (4 × 2) domains. After depositing 1.5 ML of Au at 675 K the surface was completely covered by the chains. Similar chains have been observed during Pt growth on Ge but not during Ag growth, supporting the idea that the 5d metals favor the formation of one-dimensional chains. After the surface was covered with chains, additional Au aggregated into three-dimensional clusters. The smaller of these clusters exhibited a rectangular shape consistent with the formation of Au(1 1 0) clusters. The [1 1 0]-orientation was confirmed through atomic resolution imaging of the tops of larger clusters. Larger clusters, however, adopted a different shape—an asymmetric octagon—and had their top surfaces tilted with respect to the Ge(0 0 1) surface. The tilt was attributed to stress relief due to the lattice mismatch, which broke the equivalence between the surface energies of inclined facets on opposite sides of the cluster, accounting for the asymmetric shape. Annealing the Au-covered surfaces to 1000 K caused all of the Au to move into the bulk leaving a defective Ge surface.