Initial growth of Ba on $\mathrm{Ge}\left(001\right)$: An STM and DFT study

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}\left(001\right)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}\left(001\right)$ could play a similar role as on structurally identical $\mathrm{Si}\left(001\right)$, overcoming known limitations of the $\mathrm{Ge}\left(001\right)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}\left(001\right)$ surface as a function of temperature $(270-770\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15-0.4\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}\left(001\right)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}\left(001\right)$ with increasing coverage.

Figure 1

Filled-state (a) and empty-state (b) experimental (imaged at $100\mathrm{K}$) and simulated STM images. The experimental images: $10\times 10\mathrm{n}{\mathrm{m}}^2,I=1\mathrm{nA},U=-2.0\mathrm{V}\mathrm{and}1.8\mathrm{V}$, respectively. Insets: (a) $2\times 2\mathrm{n}{\mathrm{m}}^2,I=1\mathrm{nA},U=-2\mathrm{V}$ and (b) $2\times 2\mathrm{n}{\mathrm{m}}^2,I=20\mathrm{pA},U=2\mathrm{V}$. The simulated images (black-and-white insets) were obtained within the Tersoff-Hamann approximation, the simulations using a $4\times 4$ surface unit cell with integration of the LDOS over $\pm 1.0\mathrm{eV}$ from the Fermi energy and isosurface value of $2.35\times {10}^{-4}\mathrm{e}/{\AA }^3$.

Figure 2

Ba structures formed after deposition of 1.0 ML on the $\mathrm{Ge}\left(001\right)$ substrate (a) at $\mathrm{RT}(I=40\mathrm{pA},U=-2\mathrm{V})$, and after subsequent annealing at (b) $470\mathrm{K}(I=80\mathrm{pA},U=-2\mathrm{V})$, (c) $570\mathrm{K}(I=120\mathrm{pA},U=2\mathrm{V})$, (d) $670\mathrm{K}(I=30\mathrm{pA},U=2\mathrm{V})$, (e) $770\mathrm{K}(I=120\mathrm{pA},U=2\mathrm{V})$, and (f) after deposition of $0.4\mathrm{ML}$ of Ba on $\mathrm{Ge}\left(001\right)$ at $\mathrm{RT}\left(I=200\mathrm{pA},U=2\mathrm{V}\right)$. Images size $50\times 50\mathrm{n}{\mathrm{m}}^2$.

Figure 3

(a) Typical large-scale $(400\times 400\mathrm{n}{\mathrm{m}}^2)$ filled-state STM image of the $\mathrm{Ge}\left(001\right)$ surface covered with Ba $0.15\mathrm{ML}$, deposited at room temperature $(I=20\mathrm{pA},U=-2\mathrm{V})$; (b) magnified section of Fig. (a) $(100\times 100\mathrm{n}{\mathrm{m}}^2)$; (c) asymmetric and (d) symmetric isolated features $(5\times 5\mathrm{n}{\mathrm{m}}^2,I=160\mathrm{pA},U=-2\mathrm{V})$; (e) filled- and (f) empty-state images of a chain $(20\times 20\mathrm{n}{\mathrm{m}}^2,I=160\mathrm{pA},U=\pm 2\mathrm{V})$.

Figure 4

(a, b) Empty- $(+2\mathrm{V},I=20\mathrm{pA})$ and (c, d) filled- $(-1.6\mathrm{V},I=20\mathrm{pA})$ state STM images of the same surface $(50\times 50\mathrm{n}{\mathrm{m}}^2)$, scanned from bottom to top (a, c) and top to bottom (b,d). Some chains are observed to be rearranged on the surface after scanning. Black arrows in (b) and (d) highlight parts of the dimer chains which have been seen to be mobile (with respect (a) and (c), respectively) due to the STM tip influence.

Figure 5

Schematics of the initial atomic structures of $\mathrm{Ba}/\mathrm{Ge}\left(001\right)$ with a single Ba atom per $4\times 4$ unit cell ($1/16$-ML coverage). The large and small white spheres denote the up and down Ge dimer atoms, respectively. Green spheres denote Ba adatoms.

Figure 6

Schematics of initial configurations of $\mathrm{Ba}/\mathrm{Ge}\left(001\right)$ with two Ba atom per $4\times 4$ unit cell ($1/8$-ML coverage). The large and small white spheres denote up and down Ge dimer atoms, respectively. Green spheres denote Ba adatoms.

Figure 7

Considered initial configurations of $\mathrm{Ba}/\mathrm{Ge}\left(001\right)$ with four Ba atoms per $4\times 4$ unit cell ($1/4$-ML coverage). The large and small white spheres denote up and down Ge dimer atoms, respectively. Green spheres denote Ba adatoms.

Figure 8

Top views of the most stable structures of $\mathrm{Ba}/\mathrm{Ge}\left(001\right)$ with (a) one $(1/16\mathrm{ML})$, (b) two $(1/8\mathrm{ML})$, and (c) four $(1/4\mathrm{ML})$ Ba adatoms per $4\times 4$ unit cell. The structural parameters are also shown. The Ba-Ge bond lengths (indicated by the lines between Ba and Ge atoms) are in angstroms and the dimer buckling angles are in degrees. A positive sign of the latter indicates that the dimer atom on the left is higher than the one on the right. Only first layer Ge atoms are shown.

Figure 9

(a) Filled- and (b) empty-state STM images (taken at RT) of the same $5\times 5\mathrm{n}{\mathrm{m}}^2$ region of a $\mathrm{Ge}\left(001\right)$ surface lightly dosed with Ba; an isolated asymmetric feature [see also Fig. 3] is clearly visible. (c) Scheme indicating the position of the Ba adatom (yellow circle) with respect neighboring Ge surface atoms in the reconstruction (gray circles). Simulated filled- (d) and empty- (e, f) state STM images of the isolated Ba adatom on $\mathrm{Ge}\left(001\right)$ within the $4\times 4$ unit cell. Pink and blue arrows indicates equivalent Ge dimers rows in the STM and simulated images.

Figure 10

Simulated filled- and empty-state STM images of the isolated Ba dimer on $\mathrm{Ge}\left(001\right)$ of Fig. 6 [six identical $(4\times 4)$ unit cells reproduced in each image]. Inset in the center, scheme indicating the position of Ba adatoms (yellow circles) with respect to neighboring Ge surface atoms in the reconstruction (gray circles). Red hexagons indicate position of the adsorbed Ba dimers.

Figure 11

Simulated (a–d) STM images of filled and empty state of the structure of Fig. 7, obtained for different values of the integration range [six identical $(4\times 4)$ unit cells are reproduced in each image]. Inset in the center scheme indicating the position of Ba adatoms (yellow circles) with respect to neighboring Ge surface atoms in the reconstructions (gray balls). Red hexagons indicated position of the adsorbed Ba dimers. (e–f) Representative experimental STM images $(10\times 5\mathrm{n}{\mathrm{m}}^2)$ for both bias polarities illustrating the correlation between simulations and experiment.

Figure 12

Surface free energy per unit area (vertical axis) as a function of Ba chemical potential (horizontal axis) for Ba-covered $\mathrm{Ge}\left(001\right)$ surfaces. The most stable configurations are represented by the lines with the lowest (most negative) surface energies. Chains 1 and 2 represent the chain configurations of Figs. 7 and 7, respectively.