Scanning tunneling microscopy study of Pd growth on Ge(001)

Abstract

The interaction of Pd with the Ge(001) surface was studied as a function of Pd coverage and temperature using scanning tunneling microscopy. At 300K, initial Pd adsorption displaces Ge from the surface which then combines with additional Pd to form tetramers. At least some of the Pd moves subsurface inducing dimer vacancy complexes. As the temperature was increased, Pd–Ge alloy phases were observed on the surface. At 475K, these formed small three-dimensional clusters that were seen together with two-dimensional Ge islands created from Ge ejected from the surrounding vacancy rich terraces. When several Pd monolayers were deposited at 675K, larger faceted clusters were observed on the surface. High-resolution images revealed a hexagonal structure on the facets with the periodicity of a (3×3)R30° reconstruction of Pd2Ge(0001). This reconstruction was assigned to Pd adtrimers that make the Pd2Ge(0001) surface stoichiometric. At higher temperatures most of the Pd tended to move subsurface. After depositing 100 ML Pd at 300K and annealing to 1030K, however, very large three-dimensional clusters were observed. Atomic resolution images of the surfaces of these clusters revealed a nearly rectangular surface unit cell consistent with a (121−2) reconstruction of the PdGe(110) surface. Unlike recent results for Au and Pt on Ge(001), under no conditions were ordered metallic chains observed. The results were more similar to Ag on Ge(001) where three-dimensional cluster formation was also favored. The results reinforce recent suggestions that relativistic effects in the electronic structure of the 5d metals make them behave more like each other in low coordination than the corresponding 4d metals.