Deciphering the atomic structure of a complex Sr/Ge (100) phase via scanning tunneling microscopy and first-principles calculations

The details of a Sr-induced ($3\times 4$) reconstruction on Ge(100) were examined using scanning tunneling microscopy (STM) and density functional theory. At 1/6 ML of Sr, this reconstruction is similar to the 1/6 ML ($3\times 2$) Sr phase previously observed on Si. In contrast to Si, however, atomic-resolution images of the Sr-Ge phase exhibit more dramatic and unusual bias dependence in STM that could be explained with the help of first-principles calculations of minimum energy structures. Simulated STM images are in excellent agreement with the experimental data and allow the ($3\times 2$) Sr-Si double dimer vacancy alloy model to be extended to the Ge surface through a more complex ($3\times 4$) arrangement of its building blocks. The difference between Si and Ge is interpreted in terms of the lower Ge-Ge binding energy and differences in the interatomic bond lengths.

Figure 1

(a) Filled state ($-$2 V) and (b) empty state (2 V) STM images of 1/6 ML Sr on Ge(100). The x indicates the same spot on the surface, while the dotted line highlights the same line of defects.

Figure 2

(a) Filled state and (b),(c) empty state closeup STM images of areas with $3\times 4$ symmetry and a line of defects (dashed line). Arrows point to the same defect in all three images. Sample biases were $-$2 V (a), 1 V (b), 1.75 V (c).

Figure 3

(a) Top view and (b) side view of the $3\times 4$ structure: Sr-large yellow, outermost Ge dimers-cyan, exposed surface Ge-green, and bulk Ge-dark blue. (c) Partial density of states at different sites. (d)–(g) Simulated filled (d) and empty state (e)–(g) STM images.

Figure 4

Structure (a) and schematic STM images (b)–(d) of 3/16 ML Sr $c(8\times 4)$ phase. The shaded areas highlight features predicted to appear in STM images, with whiter shading highlighting brighter features. Color scheme same as in Fig. 3.