Au-induced atomic wires on stepped Ge(hhk) surfaces

Au adsorption on high-index Si surfaces is known to form quasi-one-dimensional surface reconstructions with interesting physical properties such as Rashba-split bands and ordered arrays of local magnetic moments. Here we report on a novel family of Au-induced chain systems, hosted on Ge(hhk) substrates. Our study includes Ge(553), Ge(557), and Ge(335) for both bare and Au-adsorbed surfaces, respectively. We employ scanning tunneling microscopy and low-energy electron diffraction to characterize the topography and the occurrence of superstructures. Stable bare surfaces with regularly distributed steps are found for the (553) and (335) orientations. For nominal Ge(557) substrates a refaceting to Ge(223) is observed. Addition of Au tends to promote a change in the surface index which creates Ge(221)-Au, Ge(557)-Au, and Ge(335)-Au surfaces. Their STM images show elements that are reminiscent of the well-understood Si(hhk)-Au surfaces but also reveal important differences.

Figure 1

(a) LEED image of the clean Ge(553) surface with the $(1\times 1)$ unit cell of the unreconstructed Ge(111) surface showing split main spots. (b) After Au evaporation, LEED shows intense spot rows of a regularly stepped surface and spots of a ${(\sqrt{3}\times \sqrt{3})}_{111}$ surface. The reconstructed Au-induced surface itself shows a rectangular ${(1\times 1)}_{221}$ unit cell. (c) STM topography image ($50\mathrm{pA}$) shows a refaceted surface with alternating Ge(221)-Au and Ge(111)-Au domains. (d) Detailed STM image ($50\mathrm{pA}$) of chainlike features on the Ge(221) surface with bright protrusions in between showing a horizontal alignment (green dots). (e) Schematic of the refaceting of the Ge(553) surface to the Au-covered (221) and (111) oriented domains. (f) Side and top view of a structural atomic model based on the atomic models of the Ge(111)-M [23] and Si(hhk)-Au surfaces [6]. In contrast to all Si(hhk)-Au surface reconstructions the ${(1\times 1)}_{221}$ unit cell has a rectangular shape.

Figure 2

(a) LEED and (b) STM ($50\mathrm{pA}$) images of the clean Ge(557) substrate show the formation of a Ge(223) surface with a $\times 5$ superstructure along the step edges. (c) An STM bias series of a small surface area without major defects in the surface reconstruction reveals four chainlike features per Ge(223) terrace (see black/white arrows). Three different shapes of topographic features can be distinguished: (i) paired protrusions, (ii) a spadelike shape, and (iii) a butterflylike shape. The paired protrusions (i) are not only arranged in a regular $\times 5$ superstructure along a single terrace but also order perpendicular to the step edge direction with the paired protrusions located on neighboring terraces (see red rectangle).

Figure 3

(a) LEED pattern of the regularly stepped Ge(557)-Au surface with main spot rows and a streak of a $\times 2$ superstructure along the step edge direction. (b) In STM, areas with regularly distributed Ge(557)-Au steps show many defects and adatoms. (c) A bias series of a small surface area reveals three chainlike structures per terrace: (cyan stars) pearl-like chain with $\times 2$ superstructure, (yellow dots) pearl-like chain with $\times 3$ superstructure, (red dashed line) irregular, cloudy chain. Small insets: STM images of the Si(557)-Au surface from Sauter et al. [35], also taken at $77\mathrm{K}$, bear great resemblance to the Ge(557)-Au surface. (d) Atomic model of Ge(557)-Au with a ${(1\times 2)}_{557}$ unit cell in side and top view adapted from the Si(hhk)-Au model from Crain et al. [6].

Figure 4

(a) LEED and (b) STM ($50\mathrm{pA}$) image of the Ge(335)-Au surface. Besides the main spot rows, the LEED pattern shows streaks of a $\times 3$ superstructure. In STM, large Ge(335)-Au areas with a regular step array up to $100\mathrm{nm}$ in width can be observed. (c) Detailed STM images of the Ge(335)-Au surface (left side) reveal small patches with a $\times 3$ superstructure along the step edge direction (green dots). The intensities invert between images of the occupied and unoccupied states, similar to the Si HC step edge of the Si(335)-Au surface (right side) [40]. (d) Atomic structure model of Ge(335)-Au based on the Si(hhk)-Au atomic model, proposed by Crain et al. [6], overlayed with the corresponding ${(1\times 1)}_{335}$ unit cell.