Morphology and composition of Au catalysts on Ge(111) obtained by thermal dewetting

We investigate the chemical and morphological structure of the Au nanodots on Ge(111), which serve as catalysts for the formation of epitaxial Ge nanowires. We show that dewetting of an Au film on Ge(111) gives rise to a thin Au-Ge wetting layer and Au-Ge dots. These dots are crystallized but not with a single crystallographic orientation. Thanks to the spatially resolved x-ray and transmission electron microscopy measurements, a chemical characterization of both binary Au-Ge catalysts and wetting layer is obtained at the nanoscale. We show that Ge vertical growth is achieved even without an external Ge supply.

Figure 1

(a) STM image of 0.8-nm Au deposited onto a room-temperature (RT) clean Ge(111). (b) Au 4$f_{7/2}$ XPD profiles versus polar angle θ along the [11-2], [10-1], and [$-$1-12] direction of the Ge(111) substrate.

Figure 2

(a) STM images of 0.8-nm Au deposited onto a room-temperature (RT) clean Ge(111) annealed for 10 min at 300 °C. Also shown in this image is a line profile across islands. (b) Au 4$f_{7/2}$ XPD profile versus polar angle θ along the [11-2] direction of the Ge(111) substrate measured on the Au deposit annealed at 300 °C (i) and on the as-deposited Au layer (ii).

Figure 3

STM images of 0.8-nm Au deposited onto a room-temperature (RT) clean Ge(111) annealed for 10 min at 325 °C. Also shown in this image is a line profile, which clearly distinguishes flat and domelike islands.

Figure 4

STM images of 0.8-nm Au deposited onto a room-temperature (RT) clean Ge(111) annealed for 10 min at 350 °C. Also shown in this image is a line profile across a domelike island.

Figure 5

(A) STM image of 0.8-nm Au deposited after 12 h annealing at 350 °C and (B) a detail of a droplet. (C) Au 4$f_{7/2}$ XPD profiles versus polar angle θ along the [11-2] direction of the Ge(111) substrate measured at room temperature (RT) (a), on the Au deposit annealed at 300 °C (b), and on the Au deposit annealed at 350 °C (c).

Figure 6

(a) STM images (2 μm × 2 μm) of an Au deposit of 0.4 nm, 0.8 nm, and 1.2 nm annealed at 350 °C for 10 min. Also shown for a 0.8-nm Au deposit annealed at 350 °C. (b) Droplets’ diameter versus the annealing time. (c) Droplets’ height versus annealing time. (d) Proportion of the surface covered by droplets versus annealing time.

Figure 7

RHEED pattern measured on a 1.2-nm-thick Au layer annealed at 300 °C (a) and 350 °C (b). The primary energy is 30 keV and the angle of incidence is <0.77° from the surface.

Figure 8

SEM images collected at room temperature for a 1.2-nm Au deposit after anneal at 350 °C (a) and 400 °C, (b)–(d), for 1 h. The topmost images (a) and (b) are 45° tilted images to enhance the observation of the pedestal. The beam energy is 10 keV.

Figure 9

(a) XPEEM image of the sample surface for 1.2-nm Au deposit annealed at 350 °C for 12 h. This image is acquired in the x-ray-absorption mode, at the Ge $L_{2,3}$ edge, at photon energy of 30 eV. The field of view is 4 μm. (b) XPEEM image acquired in the XPS mode. The sample is illuminated at photon energy of 201 eV and the lateral distribution of the Au 4$f$ intensity is taken at a kinetic energy of $E_c$ $=$ 113 eV, thus at the Au 4$f_{7/2}$ line maximum. (c) Normalized Au 4$f$ lines measured on droplets (i) and surface in between (ii).

Figure 10

Cross-section TEM images for a 1.2-nm Au deposit annealed at 400 °C.

Figure 11

Cross-section TEM images for a 1.2-nm Au deposit annealed at 350 °C, (a)–(e) and 400 °C, (c) and (f). This figure also shows the Ge $L$, Ge $K$, and Au $M$α lines intensity across the yellow line of the cross-section images at selected points.