Morphology of the Au-Si interface formed during solidification of liquid Au/Si(111) islands
(Color online) AFM tapping mode micrograph of the same island from sample 3 both before (a) and after (b) Au dissolution using aqua regia. Au-induced step bands are visible in both (a) and (b) as straight features running from top to bottom. Linescans are compared both before (dashed) and after (solid) the etch along identical horizontal (c) and vertical (d) cross sections through the island center. Note that the etched island displays a crater morphology for which the base extends well below the substrate surface. Also observe the (111) plane at the base of the crater.
Relationship between maximum crater depth below substrate surface and area of island footprint for islands grown or annealed at 600 °C. Crater depth is proportional to the area of the island footprint.
(Color online) AFM micrographs of the same region of sample 7 before (a) and after (b) Au dissolution in aqua regia. Image (c) compares the height along identical horizontal line scans through the center of the arrowed island in (a) and (b). The before etch profile is dashed whereas the after etch profile is solid. Due to the radius of the AFM tip we cannot determine whether the pit extends beneath the substrate surface.
(Color online) (a) AFM image of a crater formed beneath an island from sample 3. The height of the crater along the bold horizontal and vertical linescans indicated in (a) is displayed in (b) and (c), respectively. The breaks in the bold linescans of (a) bound planes we identified as well-developed facets. The leftmost break of the horizontal linescan in (a) corresponds to the dashed region in (b) between x = 40 nm and x = 80 nm. This (111) plane is inclined at ∼3° to the average horizontal plane of the image, which correlates well with the nominal bulk miscut of our wafer. The rightmost break of the horizontal linescan in (a) corresponds to the dashed region in (b) between x = 150 nm and x = 180 nm. The break of the vertical linescan in (a) corresponds to the dashed region in (c). The intersection of the horizontal and vertical linescans is near the center of a (331) plane that is inclined at ∼20.5° with respect to the average horizontal plane of the image.
(a) Post-etch ZSTEM image of an island grown at 600 °C. The entire crater is in view and the bright features are ∼7 nm diameter Au nanocrystals. These Au nanocrystals are magnified in (b), which is a brightfield TEM image. In (b), the dark objects are Au in a lighter contrast Si matrix. Observe the moiré fringe pattern evident in the arrowed Au particle, indicating that these segregated Au particles are crystalline.
Growth and annealing parameters of the samples investigated. Here “Slow I” indicates that the sample was cooled at a rate of 0.7 °C/min through the eutectic temperature whereas “Slow II” indicates that the sample was cooled at a rate of 1 °C/min through the eutectic temperature. All annealed samples were annealed at their growth temperature. The rightmost two columns represent the Au coverage remaining after the aqua regia treatment determined using RBS. Based on the precision of our RBS measurements (0.1 ML), we conclude that more Au is left on the samples than that commonly associated with the 1 ML thick √3 layer. Using island areal densities determined by AFM, we compute the average amount of sub-surface Au associated with each island following the aqua regia etch.
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